Date: | 2024-07-20 |
Project: | Programming Language C++ |
Reference: | ISO/IEC IS 14882:2020 |
Reply to: | Jens Maurer |
jens.maurer@gmx.net |
This document contains the C++ core language issues on which the Committee (INCITS PL22.16 + WG21) has not yet acted, that is, issues with status "Ready," "Tentatively Ready," "Review," "Drafting," and "Open." (See Issue Status below.)
This document is part of a group of related documents that together describe the issues that have been raised regarding the C++ Standard. The other documents in the group are:
Section references in this document reflect the section numbering of document WG21 N4986.
The purpose of these documents is to record the disposition of issues that have come before the Core Language Working Group of the ANSI (INCITS PL22.16) and ISO (WG21) C++ Standard Committee.
Some issues represent potential defects in the ISO/IEC IS 14882:2020 document and corrected defects in the earlier 2017, 2014, 2011, 2003, and 1998 documents; others refer to text in the working draft for the next revision of the C++ language and not to any Standard text. Issues are not necessarily formal ISO Defect Reports (DRs). While some issues will eventually be elevated to DR status, others will be disposed of in other ways.
The most current public version of this document can be found at http://www.open-std.org/jtc1/sc22/wg21. Requests for further information about these documents should include the document number, reference ISO/IEC 14882:2020, and be submitted to the InterNational Committee for Information Technology Standards (INCITS), 1250 Eye Street NW, Suite 200, Washington, DC 20005, USA.
Information regarding C++ standardization can be found at http://isocpp.org/std.
Revision 115, 2024-07-20:
Added new issues 2880, 2881, 2882, 2883, 2884, 2885, 2886, 2887, 2888, 2889, 2890, 2891, 2892, 2893, 2894, 2895, 2896, 2897, 2898, 2899, 2900, 2901, 2902, 2903, 2904, 2905, 2906, 2907, 2908, 2909, 2910, 2911, and 2912. Reflected the decisions of the June, 2024 plenary meeting. Reflected the deliberations of the teleconferences up to 2024-06-14.
Revision 114, 2024-04-18:
Added new issues 2842, 2843, 2844, 2845, 2846, 2847, 2848, 2849, 2850, 2851, 2852, 2853, 2854, 2855, 2856, 2857, 2858, 2859, 2860, 2861, 2862, 2863, 2864, 2865, 2866, 2867, 2868, 2869, 2870, 2871, 2872, 2873, 2874, 2875, 2876, 2877, 2878, and 2879. Reflected the decisions of the March, 2024 plenary meeting. Reflected the deliberations of the teleconferences up to 2024-04-05.
Revision 113, 2023-12-19:
Added new issues 2781, 2782, 2783, 2784, 2785, 2786, 2787, 2788, 2789, 2790, 2791, 2792, 2793, 2794, 2795, 2796, 2797, 2798, 2799, 2800, 2801, 2802, 2803, 2804, 2805, 2806, 2807, 2808, 2809, 2810, 2811, 2812, 2813, 2814, 2815, 2816, 2817, 2818, 2819, 2820, 2821, 2822, 2823, 2824, 2825, 2826, 2827, 2828, 2829, 2830, 2831, 2832, 2833, 2834, 2835, 2836, 2837, 2838, 2839, 2840, and 2841. Reflected the decisions of the November, 2023 plenary meeting. Reflected the deliberations of the teleconferences up to 2023-12-15.
Revision 112, 2023-08-18:
Added new issues 2711, 2712, 2713, 2714, 2715, 2716, 2717, 2718, 2719, 2720, 2721, 2722, 2723, 2724, 2725, 2726, 2727, 2728, 2729, 2730, 2731, 2732, 2733, 2734, 2735, 2736, 2737, 2738, 2739, 2740, 2741, 2742, 2743, 2744, 2745, 2746, 2747, 2748, 2749, 2750, 2751, 2752, 2753, 2754, 2755, 2756, 2757, 2758, 2759, 2760, 2761, 2762, 2763, 2764, 2765, 2766, 2767, 2768, 2769, 2770, 2771, 2772, 2773, 2774, 2775, 2776, 2777, 2778, 2779, and 2780. Closed issues 6, 504, 528, and 640. Reflected the decisions of the June, 2023 plenary meeting. Reflected the deliberations of the teleconferences up to 2023-07-14.
Revision 111, 2023-03-24:
Added new issues 2655, 2656, 2657, 2658, 2659, 2660, 2661, 2662, 2663, 2664, 2665, 2666, 2667, 2668, 2669, 2670, 2671, 2672, 2673, 2674, 2675, 2676, 2677, 2678, 2679, 2680, 2681, 2682, 2683, 2684, 2685, 2686, 2687, 2688, 2689, 2690, 2691, 2692, 2693, 2694, 2695, 2696, 2697, 2698, 2699, 2700, 2701, 2702, 2703, 2704, 2705, 2706, 2707, 2708, 2709, and 2710. Reflected the deliberations of the teleconferences up to 2023-03-03. Reflected the decisions of the February, 2023 plenary meeting.
Revision 110, 2022-11-27:
Reflected publication of the C++23 CD N4917. Closed issue 578. Added new issues 2610, 2611, 2612, 2613, 2614, 2615, 2616, 2617, 2618, 2619, 2620, 2621, 2622, 2623, 2624, 2625, 2626, 2627, 2628, 2629, 2630, 2631, 2632, 2633, 2634, 2635, 2636, 2637, 2638, 2641, Created issues for the handling of NB comments on the Committee Draft: 2639, 2640, 2642, 2643, 2644, 2645, 2646, 2647, 2648, 2649, 2650, 2651, 2652, 2653, 2654. Reflected the decisions of the November, 2022 plenary meeting.
Revision 109, 2022-08-07:
Added links for section references. Reset status of issues assigned to people no longer active on the committee. Reflected the deliberations of the teleconferences. Reflected the decisions of the July, 2022 plenary meeting. Added detailed descriptions to issues 2187, 2188, 2189, 2190, 2193, 2195, 2196, 2197, 2200, 2203, 2204, 2211, 2212, 2216, 2221, 2225, 2228, 2230, 2231, 2236, 2245, 2246, 2247, 2248, 2250, 2251, 2258, 2263, 2268, 2269, 2270, 2272, 2274, 2275, 2276, 2279, 2283, 2284, 2286, 2288, 2311, 2314, 2319, and 2320. Closed issue 1326 and 2420 as duplicates. Closed issues 476, 687, 689, 728, 916, 944, 1326, 1393, 1469, 1561, 1742, 1912, 1931, 1972, 2212, and 2341. Dissolved the "concurrency" status in favor of liaison indications, affecting issues 1842 and 2298. Reopened issues 2169 and 2355 after obtaining EWG guidance. Added new issues 2531, 2532, 2533, 2534, 2535, 2536, 2537, 2538, 2539, 2540, 2541, 2542, 2543, 2544, 2545, 2546, 2547, 2548, 2549, 2550, 2551, 2552, 2553, 2554, 2555, 2556, 2557, 2558, 2559, 2560, 2561, 2562, 2563, 2564, 2565, 2566, 2567, 2568, 2569, 2570, 2571, 2572, 2573, 2574, 2575, 2576, 2577, 2578, 2579, 2580, 2581, 2582, 2583, 2584, 2585, 2586, 2587, 2588, 2589, 2591, 2592, 2593, 2594, 2595, 2596, 2597, 2598, 2599, 2600, 2601, 2602, 2603, 2604, 2605, 2606, 2607, 2608, and 2609.
Revision 108, 2022-01-25:
Reflected the deliberations of the January, 2022 teleconferences and the decisions of the February, 2022 plenary meeting. Added new issues 2518, 2519, 2520, 2521, 2522, 2523, 2524, 2525, 2526, 2527, 2528, 2529, and 2530.
Revision 107, 2021-12-09:
Updated the status of issues 2482 and 2493. Reflected the deliberations of the December, 2021 CWG teleconference. Added new issues 2506, 2507, 2508, 2509, 2510, 2511, 2512, 2513, 2514, 2515, 2516, and 2517.
Revision 106, 2021-11-22:
Reflected the deliberations of CWG teleconferences from September through November, 2021, and the decisions of the October, 2021 plenary meeting. Added new issues 2502, 2503, 2504, and 2505.
Revision 105, 2021-06-08:
Reflected the deliberations of CWG teleconferences from March through August, 2021, and the decisions of the June, 2021 plenary meeting. Added new issues 2481, 2482, 2483, 2484, 2485, 2486, 2487, 2488, 2489, 2490, 2491, 2492, 2493, 2494, 2495, 2496, 2497, 2498, 2499, 2500, and 2501.
Revision 104, 2021-02-24:
Reflected the decisions of the February, 2021 meeting. Updated the status of issues 1332 and 2402, which were previously resolved by editorial action and adoption of a paper, respectively. Added new issue 2480.
Revision 103, 2021-02-17:
Updated the status of a number of issues that were resolved or rendered moot by other issue resolutions or papers, including issues 156, 192, 278, 297, 555, 560, 617, 670, 1230, 1285, 1331, 1426, 1451, 1452, 1545, 1640, 1644, 1646, 1713, 1801, 1857, 1859, 1880, 1917, 1943, 1974, 2045, 2080, 2112, 2121, 2215, 2295, 2343, 2364, 2367, 2371, and 2411. Reflected the deliberations of the February, 2021 teleconference. Added new issues 2470, 2471, 2472, 2473, 2474, 2475, 2476, 2477, 2478, and 2479.
Revision 102, 2020-12-15: Changed issue 2331 back to "drafting" status after problems were found with the proposed resolution. Fixed transcription errors in the proposed resolution for issue 2020. Corrected the date for resolutions approved at the February, 2019 meeting (they previously indicated approval at the “November, 2019” meeting). Reflected actions at committee meetings and teleconferences. Added new issues 2403, 2404, 2405, 2406, 2407, 2408, 2409, 2410, 2411, 2412, 2413, 2414, 2415, 2416, 2417, 2418, 2419, 2420, 2421, 2422, 2423, 2424, 2425, 2426, 2427, 2428, 2429, 2430, 2431, 2432, 2433, 2434, 2435, 2436, 2437, 2438, 2439, 2440, 2441, 2442, 2443, 2444, 2445, 2446, 2447, 2448, 2449, 2450, 2451, 2452, 2453, 2454, 2455, 2456, 2457, 2458, 2459, 2460, 2461, 2462, 2463, 2464, 2465, 2466, 2467, 2468, and 2469.
Revision 101, 2019-02-27: Reflected actions at committee meetings and teleconferences. Added new issues 2366, 2367, 2368, 2369, 2370, 2371, 2372, 2373, 2374, 2375, 2376, 2377, 2378, 2379, 2380, 2381, 2382, 2383, 2384, 2385, 2386, 2387, 2388, 2389, 2390, 2391, 2392, 2393, 2394, 2395, 2396, 2397, 2398, 2399, 2400, 2401, and 2402,.
Revision 100, 2018-04-11: Reflected actions at the March, 2018 committee meeting and the April, 2018 teleconference. Added new issues 2357, 2358, 2359, 2360, 2361, 2362, 2363, 2364, and 2365.
Revision 99, 2018-02-27: Reflected actions of committee meetings and teleconferences. Added new C++17 status and moved issues incorporated in that IS to have that status. Added new issues 2278, 2279, 2280, 2281, 2282, 2283, 2284, 2285, 2286, 2287, 2288, 2289, 2290, 2291, 2292, 2293, 2294, 2295, 2296, 2297, 2298, 2299, 2300, 2301, 2302, 2303, 2304, 2305, 2306, 2307, 2308, 2309, 2310, 2311, 2312, 2313, 2314, 2315, 2316, 2317, 2318, 2319, 2320, 2321, 2322, 2323, 2324, 2325, 2326, 2327, 2328, 2329, 2330, 2331, 2332, 2333, 2334, 2335, 2336, 2337, 2338, 2339, 2340, 2341, 2342, 2343, 2344, 2345, 2346, 2347, 2348, 2349, 2350, 2351, 2352, 2353, 2354, 2355, and 2356.
Revision 98, 2017-03-20: Reflected the deliberations of the February-March, 2017 Committee meeting.
Revision 97, 2017-02-07: Reflected the deliberations of the 2016 Committee meetings and teleconferences. Added new issues 2123, 2124, 2125, 2126, 2127, 2128, 2129, 2130, 2131, 2132, 2133, 2134, 2135, 2136, 2137, 2138, 2139, 2140, 2141, 2142, 2143, 2144, 2145, 2146, 2147, 2148, 2149, 2150, 2151, 2152, 2153, 2154, 2155, 2156, 2157, 2158, 2159, 2160, 2161, 2162, 2163, 2164, 2165, 2166, 2167, 2168, 2169, 2170, 2171, 2172, 2173, 2174, 2175, 2176, 2177, 2178, 2179, 2180, 2181, 2182, 2183, 2184, 2185, 2186, 2187, 2188, 2189, 2190, 2191, 2192, 2193, 2194, 2195, 2196, 2197, 2198, 2199, 2200, 2201, 2202, 2203, 2204, 2205, 2206, 2207, 2208, 2209, 2210, 2211, 2212, 2213, 2214, 2215, 2216, 2217, 2218, 2219, 2220, 2221, 2222, 2223, 2224, 2225, 2226, 2227, 2228, 2229, 2230, 2231, 2232, 2233, 2234, 2235, 2236, 2237, 2238, 2239, 2240, 2241, 2242, 2243, 2244, 2245, 2246, 2247, 2248, 2249, 2250, 2251, 2252, 2253, 2254, 2255, 2256, 2257, 2258, 2259, 2260, 2261, 2262, 2263, 2264, 2265, 2266, 2267, 2268, 2269, 2270, 2271, 2272, 2273, 2274, 2275, 2276, and 2277. (Note that a number of the new issues have only titles at this point; full descriptions will be added in the next revision of the list.)
Revision 96, 2016-02-15: Changed the status of issue 2047 from "ready" to "tentatively ready" to reflect revisions to accommodate changes in the underlying wording after the October, 2015 meeting. Reflected the results of drafting review teleconferences held 2016-01-11 and 2016-02-08.
Revision 95, 2015-11-10: Reflected the deliberations of the October, 2015 Committee meeting. Moved issue 1893 back to "drafting" status; it was incorrectly inadvertently accepted as a DR at the November, 2014 meeting.XS Issue 1992 was moved back to drafting to allow application to additional text. Added new issues 2123, 2124, 2125, 2126, 2127, 2128, 2129, 2130, 2131, 2132, 2133, 2134, 2135, 2136, 2137, 2138, 2139, 2140, 2141, 2142, 2143, 2144, 2145, 2146, 2147, 2148, 2149, 2150, 2151, 2152, 2153, 2154, 2155, 2156, 2157, 2158, 2159, 2160, 2161, 2162, 2163, 2164, 2165, 2166, 2167, 2168, 2169, 2170, 2171, 2172, 2173, 2174, 2175, 2176, 2177, 2178, 2179, 2180, 2181, 2182, 2183, 2184, 2185, and 2186.
Revision 94, 2015-05-25: Reflected deliberations of the May, 2015 meeting. Moved issues 1734 and 1928 to "extension" status, reflecting EWG's request to develop a position on trivial special functions before CWG addresses them. Added new issues 2036, 2037, 2038, 2039, 2040, 2041, 2042, 2043, 2044, 2045, 2046, 2047, 2048, 2049, 2050, 2051, 2052, 2053, 2054, 2055, 2056, 2057, 2058, 2059, 2060, 2061, 2062, 2063, 2064, 2065, 2066, 2067, 2068, 2069, 2070, 2071, 2072, 2073, 2074, 2075, 2076, 2077, 2078, 2079, 2080, 2081, 2082, 2083, 2084, 2085, 2086, 2087, 2088, 2089, 2090, 2091, 2092, 2093, 2094, 2095, 2096, 2097, 2098, 2099, 2100, 2101, 2102, 2103, 2104, 2105, 2106, 2107, 2108, 2109, 2110, 2111, 2112, 2113, 2114, 2115, 2116, 2117, 2118, 2119, 2120, 2121, and 2122.
Revision 93, 2015-04-13: Incorporated EWG decisions on issues 92, 203, 622, 822, 1077, 1259, 1272, 1564, 1577, 1582, 1586, 1643, 1754, 1798, 1826, and 1833. Moved issue 1657 to "accepted" status, reflecting adoption of paper N4266 at the Urbana meeting. Moved issue 1683 to "DRWP" status, reflecting approval of N4140 as the working paper at the November, 2014 meeting. Incorporated resolutions from the drafting review teleconference held 2015-04-06.
Revision 92, 2014-11-24: Changed all issues approved by the Committee since the April, 2013 Committee Draft to "C++14" status, to reflect the adoption of ISO/IEC 14882:2014. Moved issues 1299, 1651, 1893, and 1817 to "drafting" status, 1584 to "open" status, and 314, 343, 1710, 1794, and 1812 to "review" status in light of concerns that were raised with their proposed resolutions. Moved issue 1048 to "CD3" status to reflect the fact that it had been addressed by paper N3638, adopted at the April, 2013 (Bristol) meeting. Closed issues 550 and 1855 as duplicates. Reflected the actions of the November, 2014 (Urbana) meeting. Added new issues 1948, 1949, 1950, 1951, 1952, 1953, 1954, 1955, 1956, 1957, 1958, 1959, 1960, 1961, 1962, 1963, 1964, 1965, 1966, 1967, 1968, 1969, 1970, 1971, 1972, 1973, 1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023, 2024, 2025, 2026, 2027, 2028, 2029, 2030, 2031, 2032, 2033, 2034, and 2035.
Revision 91, 2014-10-13: Incorporated deliberations of drafting review teleconferences held 2014-07-14 and 2014-10-06. Added new issue 1947.
Revision 90, 2014-07-07: Issue 1715 was returned to "drafting" status in light of an alternative suggestion for its resolution. Issue 1927 was closed as a duplicate of issue 1695. Reflected the deliberations of CWG at the 2014-06 (Rapperswil) meeting. Added new issues 1932, 1933, 1934, 1935, 1936, 1937, 1938, 1939, 1940, 1941, 1942, 1943, 1944, 1945, and 1946.
Revision 89, 2014-05-27: Issues 1351 1356 1465, 1590, 1639 1708, and 1810 were returned to "review" status for further discussion. Restored issue 1397 to "ready"; it had incorrectly been moved back to "drafting" because of a misunderstood comment. Added new issues 1866, 1867, 1868, 1869, 1870, 1871, 1872, 1873, 1874, 1875, 1876, 1877, 1878, 1879, 1880, 1881, 1882, 1883, 1884, 1885, 1886, 1887, 1888, 1889, 1890, 1891, 1892, 1893, 1894, 1895, 1896, 1897, 1898, 1899, 1900, 1901, 1902, 1903, 1904, 1905, 1906, 1907, 1908, 1909, 1910, 1911, 1912, 1913, 1914, 1915, 1916, 1917, 1918, 1919, 1920, 1921, 1922, 1923, 1924, 1925, 1926, 1927, 1928, 1929, 1930, and 1931.
Issues progress through various statuses as the Core Language Working Group and, ultimately, the full PL22.16 and WG21 committees deliberate and act. For ease of reference, issues are grouped in these documents by their status. Issues have one of the following statuses:
Open: The issue is new or the working group has not yet formed an opinion on the issue. If a Suggested Resolution is given, it reflects the opinion of the issue's submitter, not necessarily that of the working group or the Committee as a whole.
Drafting: Informal consensus has been reached in the working group and is described in rough terms in a Tentative Resolution, although precise wording for the change is not yet available.
Review: Exact wording of a Proposed Resolution is now available for an issue on which the working group previously reached informal consensus.
Ready: The working group has reached consensus that a change in the working draft is required, the Proposed Resolution is correct, and the issue is ready to forward to the full Committee for ratification.
Tentatively Ready: Like "ready" except that the resolution was produced and approved by a subset of the working group membership between meetings. Persons not participating in these between-meeting activities are encouraged to review such resolutions carefully and to alert the working group with any problems that may be found.
DR: The full Committee has approved the item as a proposed defect report. The Proposed Resolution in an issue with this status reflects the best judgment of the Committee at this time regarding the action that will be taken to remedy the defect; however, the current wording of the Standard remains in effect until such time as a Technical Corrigendum or a revision of the Standard is issued by ISO.
accepted: Like a DR except that the issue concerns the wording of the current Working Paper rather than that of the current International Standard.
TC1: A DR issue included in Technical Corrigendum 1. TC1 is a revision of the Standard issued in 2003.
CD1: A DR issue not resolved in TC1 but included in Committee Draft 1. CD1 was advanced for balloting at the September, 2008 WG21 meeting.
CD2: A DR issue not resolved in CD1 but included in the Final Committee Draft advanced for balloting at the March, 2010 WG21 meeting.
C++11: A DR issue not resolved in CD2 but included in ISO/IEC 14882:2011.
CD3: A DR/DRWP or Accepted/WP issue not resolved in C++11 but included in the Committee Draft advanceed for balloting at the April, 2013 WG21 meeting.
C++14: A DR/DRWP or Accepted/WP issue not resolved in CD3 but included in ISO/IEC 14882:2014.
CD4: A DR/DRWP or Accepted/WP issue not resolved in C++14 but included in the Committee Draft advanced for balloting at the June, 2016 WG21 meeting.
C++17: a DR/DRWP or Accepted/WP issue not resolved in CD4 but included in ISO/IEC 14882:2017.
CD5: A DR/DRWP or Accepted/WP issue not resolved in C++17 but included in the Committee Draft advanced for balloting at the July, 2019 WG21 meeting.
C++20: a DR/DRWP or Accepted/WP issue not resolved in CD5 but included in ISO/IEC 14882:2020.
CD6: A DR/DRWP or Accepted/WP issue not resolved in C++20 but included in the Committee Draft advanced for balloting at the July, 2022 WG21 meeting.
C++23: a DR/DRWP or Accepted/WP issue not resolved in CD6 but included in ISO/IEC 14882:2023.
DRWP: A DR issue whose resolution is reflected in the current Working Paper. The Working Paper is a draft for a future version of the Standard.
WP: An accepted issue whose resolution is reflected in the current Working Paper.
Dup: The issue is identical to or a subset of another issue, identified in a Rationale statement.
NAD: The working group has reached consensus that the issue is not a defect in the Standard. A Rationale statement describes the working group's reasoning.
Extension: The working group has reached consensus that the issue is not a defect in the Standard but is a request for an extension to the language. The working group expresses no opinion on the merits of an issue with this status; however, the issue will be maintained on the list for possible future consideration as an extension proposal.
Concepts: The issue relates to the “Concepts” proposal that was removed from the working paper at the Frankfurt (July, 2009) meeting and hence is no longer under consideration.
Section | Issue | Status | Liaison | Title |
2 intro.refs | 2843 | review | LWG, EWG | Undated reference to Unicode makes C++ a moving target |
3 intro.defs | 783 | open | Definition of “argument” | |
3 intro.defs | 2632 | drafting | 'user-declared' is not defined | |
4.1 intro.compliance | 949 | open | Requirements for freestanding implementations | |
4.1.1 intro.compliance.general | 2776 | open | EWG | Substitution failure and implementation limits |
5.3 lex.charset | 2779 | open | Restrictions on the ordinary literal encoding | |
5.4 lex.pptoken | 369 | drafting | Are new/delete identifiers or preprocessing-op-or-punc? | |
5.4 lex.pptoken | 1655 | drafting | Line endings in raw string literals | |
5.5 lex.digraph | 2726 | review | EWG | Alternative tokens appearing as attribute-tokens |
5.6 lex.token | 1901 | drafting | punctuator referenced but not defined | |
5.12 lex.operators | 189 | drafting | Definition of operator and punctuator | |
5.13 lex.literal | 1924 | review | editor | Definition of “literal” and kinds of literals |
5.13.4 lex.fcon | 2752 | open | EWG | Excess-precision floating-point literals |
5.13.5 lex.string | 2766 | open | Repeated evaluation of a string-literal may yield different objects | |
5.13.9 lex.ext | 1266 | open | user-defined-integer-literal overflow | |
5.13.9 lex.ext | 1723 | drafting | Multicharacter user-defined character literals | |
5.13.9 lex.ext | 1735 | drafting | Out-of-range literals in user-defined-literals | |
6.1 basic.pre | 1529 | drafting | Nomenclature for variable vs reference non-static data member | |
6.3 basic.def.odr | 1209 | open | Is a potentially-evaluated expression in a template definition a “use?” | |
6.3 basic.def.odr | 1897 | review | ODR vs alternative tokens | |
6.3 basic.def.odr | 2781 | open | Unclear recursion in the one-definition rule | |
6.3 basic.def.odr | 2782 | open | Treatment of closure types in the one-definition rule | |
6.3 basic.def.odr | 2910 | open | Effect of requirement-parameter-lists on odr-usability | |
6.4.1 basic.scope.scope | 2488 | open | Overloading virtual functions and functions with trailing requires-clauses | |
6.4.1 basic.scope.scope | 2788 | open | Correspondence and redeclarations | |
6.4.1 basic.scope.scope | 2835 | open | Name-independent declarations | |
6.4.3 basic.scope.block | 2838 | open | Declaration conflicts in lambda-expressions | |
6.5.1 basic.lookup.general | 2480 | drafting | Lookup for enumerators in modules | |
6.5.2 class.member.lookup | 380 | open | Definition of "ambiguous base class" missing | |
6.5.4 basic.lookup.argdep | 2888 | review | Missing cases for reference and array types for argument-dependent lookup | |
6.5.5.1 basic.lookup.qual.general | 1089 | open | Template parameters in member selections | |
6.6 basic.link | 2670 | open | Programs and translation units | |
6.6 basic.link | 2706 | open | Repeated structured binding declarations | |
6.7.1 intro.memory | 1953 | open | Data races and common initial sequence | |
6.7.2 intro.object | 2324 | drafting | Size of base class subobject | |
6.7.2 intro.object | 2325 | drafting | std::launder and reuse of character buffers | |
6.7.2 intro.object | 2334 | open | Creation of objects by typeid | |
6.7.2 intro.object | 2469 | drafting | Implicit object creation vs constant expressions | |
6.7.2 intro.object | 2744 | open | Multiple objects of the same type at the same address | |
6.7.2 intro.object | 2765 | open | Address comparisons between potentially non-unique objects during constant evaluation | |
6.7.3 basic.life | 419 | open | Can cast to virtual base class be done on partially-constructed object? | |
6.7.3 basic.life | 1027 | review | Type consistency and reallocation of scalar types | |
6.7.3 basic.life | 1530 | drafting | Member access in out-of-lifetime objects | |
6.7.3 basic.life | 2258 | open | Storage deallocation during period of destruction | |
6.7.3 basic.life | 2514 | open | SG12 | Modifying const subobjects |
6.7.3 basic.life | 2551 | review | "Refers to allocated storage" has no meaning | |
6.7.3 basic.life | 2676 | open | Replacing a complete object having base subobjects | |
6.7.3 basic.life | 2677 | review | Replacing union subobjects | |
6.7.3 basic.life | 2821 | review | Lifetime, zero-initialization, and dynamic initialization | |
6.7.3 basic.life | 2863 | drafting | Unclear synchronization requirements for object lifetime rules | |
6.7.4 basic.indet | 1997 | drafting | Placement new and previous initialization | |
6.7.5 basic.stc | 365 | open | Storage duration and temporaries | |
6.7.5 basic.stc | 1634 | drafting | Temporary storage duration | |
6.7.5.5.2 basic.stc.dynamic.allocation | 1676 | drafting | auto return type for allocation and deallocation functions | |
6.7.5.5.2 basic.stc.dynamic.allocation | 1682 | open | Overly-restrictive rules on function templates as allocation functions | |
6.7.5.5.2 basic.stc.dynamic.allocation | 2073 | drafting | Allocating memory for exception objects | |
6.7.5.5.3 basic.stc.dynamic.deallocation | 523 | open | Can a one-past-the-end pointer be invalidated by deleting an adjacent object? | |
6.7.5.5.3 basic.stc.dynamic.deallocation | 2042 | drafting | Exceptions and deallocation functions | |
6.7.6 basic.align | 1211 | drafting | Misaligned lvalues | |
6.7.6 basic.align | 2840 | open | Missing requirements for fundamental alignments | |
6.7.7 class.temporary | 2434 | review | Mandatory copy elision vs non-class objects | |
6.7.7 class.temporary | 2666 | open | Lifetime extension through static_cast | |
6.7.7 class.temporary | 2826 | drafting | Missing definition of "temporary expression" | |
6.7.7 class.temporary | 2832 | open | Invented temporary variables and temporary objects | |
6.7.7 class.temporary | 2868 | open | Self-references in trivially copyable objects as function return values | |
6.8 basic.types | 350 | open | WG14 | signed char underlying representation for objects |
6.8 basic.types | 1701 | drafting | Array vs sequence in object representation | |
6.8.2 basic.fundamental | 146 | open | Floating-point zero | |
6.8.2 basic.fundamental | 251 | open | How many signed integer types are there? | |
6.8.2 basic.fundamental | 2827 | review | Representation of unsigned integral types | |
6.8.4 basic.compound | 2544 | open | Address of past-the-end of a potentially-overlapping subobject | |
6.9.1 intro.execution | 698 | open | The definition of “sequenced before” is too narrow | |
6.9.2 intro.multithread | 1842 | open | SG1 | Unevaluated operands and “carries a dependency” |
6.9.2.2 intro.races | 2297 | open | Unclear specification of atomic operations | |
6.9.2.2 intro.races | 2298 | open | SG1 | Actions and expression evaluation |
6.9.2.2 intro.races | 2587 | review | Visible side effects and initial value of an object | |
6.9.2.3 intro.progress | 2816 | review | Unclear phrasing "may assume ... eventually" | |
6.9.3.2 basic.start.static | 371 | open | Interleaving of constructor calls | |
6.9.3.2 basic.start.static | 1294 | open | Side effects in dynamic/static initialization | |
6.9.3.2 basic.start.static | 1659 | open | Initialization order of thread_local template static data members | |
6.9.3.2 basic.start.static | 1986 | drafting | odr-use and delayed initialization | |
6.9.3.2 basic.start.static | 2148 | drafting | Thread storage duration and order of initialization | |
6.9.3.3 basic.start.dynamic | 2444 | drafting | Constant expressions in initialization odr-use | |
6.9.3.3 basic.start.dynamic | 2684 | open | thread_local dynamic initialization | |
6.9.3.3 basic.start.dynamic | 2833 | review | Evaluation of odr-use | |
7.2.1 basic.lval | 2901 | open | Unclear semantics for near-match aliased access | |
7.3.2 conv.lval | 2899 | open | Bad value representations should cause undefined behavior | |
7.3.6 conv.qual | 2438 | open | Problems in the specification of qualification conversions | |
7.5.5 expr.prim.id | 2503 | drafting | Unclear relationship among name, qualified name, and unqualified name | |
7.5.5.1 expr.prim.id.general | 2902 | review | Implicit this transformation outside of permitted contexts | |
7.5.5.2 expr.prim.id.unqual | 2738 | review | "denotes a destructor" is missing specification | |
7.5.5.3 expr.prim.id.qual | 2549 | review | Implicitly moving the operand of a throw-expression in unevaluated contexts | |
7.5.5.5 expr.prim.id.dtor | 2473 | drafting | Parentheses in pseudo-destructor calls | |
7.5.6.3 expr.prim.lambda.capture | 2086 | drafting | Reference odr-use vs implicit capture | |
7.5.6.3 expr.prim.lambda.capture | 2737 | review | Temporary lifetime extension for reference init-captures | |
7.5.8.1 expr.prim.req.general | 2565 | open | EWG | Invalid types in the parameter-declaration-clause of a requires-expression |
7.5.8.1 expr.prim.req.general | 2911 | open | Unclear meaning of expressions "appearing within" subexpressions | |
7.5.8.5 expr.prim.req.nested | 2739 | open | Nested requirement not a constant expression | |
7.6.1.3 expr.call | 2284 | open | Sequencing of braced-init-list arguments | |
7.6.1.3 expr.call | 2515 | open | Result of a function call | |
7.6.1.3 expr.call | 2660 | open | Confusing term "this parameter" | |
7.6.1.3 expr.call | 2688 | open | Calling explicit object member functions | |
7.6.1.4 expr.type.conv | 914 | open | EWG | Value-initialization of array types |
7.6.1.4 expr.type.conv | 1521 | drafting | T{expr} with reference types | |
7.6.1.4 expr.type.conv | 2283 | drafting | Missing complete type requirements | |
7.6.1.4 expr.type.conv | 2894 | review | Functional casts create prvalues of reference type | |
7.6.1.5 expr.ref | 2557 | drafting | Class member access referring to an unrelated class | |
7.6.1.5 expr.ref | 2705 | open | Accessing ambiguous subobjects | |
7.6.1.6 expr.post.incr | 742 | open | Postfix increment/decrement with long bit-field operands | |
7.6.1.7 expr.dynamic.cast | 1965 | drafting | Explicit casts to reference types | |
7.6.1.8 expr.typeid | 282 | open | Namespace for extended_type_info | |
7.6.1.9 expr.static.cast | 2048 | open | C-style casts that cast away constness vs static_cast | |
7.6.1.9 expr.static.cast | 2243 | drafting | Incorrect use of implicit conversion sequence | |
7.6.1.11 expr.const.cast | 2879 | drafting | Undesired outcomes with const_cast | |
7.6.2.5 expr.sizeof | 2609 | open | Padding in class types | |
7.6.2.5 expr.sizeof | 2817 | open | sizeof(abstract class) is underspecified | |
7.6.2.8 expr.new | 267 | open | Alignment requirement for new-expressions | |
7.6.2.8 expr.new | 901 | drafting | Deleted operator delete | |
7.6.2.8 expr.new | 1628 | open | Deallocation function templates | |
7.6.2.8 expr.new | 2281 | drafting | Consistency of aligned operator delete replacement | |
7.6.2.8 expr.new | 2532 | open | Kind of pointer value returned by new T[0] | |
7.6.2.8 expr.new | 2566 | review | Matching deallocation for uncaught exception | |
7.6.2.8 expr.new | 2592 | open | Missing definition for placement allocation/deallocation function | |
7.6.2.8 expr.new | 2623 | drafting | Invoking destroying operator delete for constructor failure | |
7.6.2.8 expr.new | 2812 | open | Allocation with explicit alignment | |
7.6.2.8 expr.new | 2912 | open | Too-large value for size in array new | |
7.6.2.9 expr.delete | 196 | open | Arguments to deallocation functions | |
7.6.2.9 expr.delete | 2805 | open | Underspecified selection of deallocation function | |
7.6.2.9 expr.delete | 2880 | open | Accessibility check for destructor of incomplete class type | |
7.6.2.9 expr.delete | 2889 | open | Requiring an accessible destructor for destroying operator delete | |
7.6.3 expr.cast | 2878 | open | EWG | C-style casts to reference types |
7.6.4 expr.mptr.oper | 2593 | review | Insufficient base class restriction for pointer-to-member expression | |
7.6.6 expr.add | 2013 | drafting | Pointer subtraction in large array | |
7.6.6 expr.add | 2182 | drafting | Pointer arithmetic in array-like containers | |
7.6.10 expr.eq | 2786 | open | Comparing pointers to complete objects | |
7.6.16 expr.cond | 2023 | drafting | Composite reference result type of conditional operator | |
7.6.16 expr.cond | 2316 | drafting | Simplifying class conversions in conditional expressions | |
7.6.16 expr.cond | 2906 | tentatively ready | Lvalue-to-rvalue conversion of class types for conditional operator | |
7.6.19 expr.ass | 1542 | drafting | Compound assignment of braced-init-list | |
7.7 expr.const | 1255 | drafting | Definition problems with constexpr functions | |
7.7 expr.const | 1256 | open | Unevaluated operands are not necessarily constant expressions | |
7.7 expr.const | 2166 | drafting | Unclear meaning of “undefined constexpr function” | |
7.7 expr.const | 2186 | drafting | Unclear point that “preceding initialization” must precede | |
7.7 expr.const | 2192 | open | Constant expressions and order-of-eval undefined behavior | |
7.7 expr.const | 2301 | open | Value-initialization and constexpr constructor evaluation | |
7.7 expr.const | 2456 | open | Viable user-defined conversions in converted constant expressions | |
7.7 expr.const | 2536 | open | EWG | Partially initialized variables during constant initialization |
7.7 expr.const | 2545 | open | Transparently replacing objects in constant expressions | |
7.7 expr.const | 2559 | open | Defaulted consteval functions | |
7.7 expr.const | 2633 | open | typeid of constexpr-unknown dynamic type | |
7.7 expr.const | 2656 | drafting | Converting consteval lambda to function pointer in non-immediate context | |
7.7 expr.const | 2702 | open | Constant destruction of reference members | |
7.7 expr.const | 2734 | open | Immediate forward-declared function templates | |
7.7 expr.const | 2740 | open | Too many objects have constexpr-unknown type | |
7.7 expr.const | 2778 | review | Trivial destructor does not imply constant destruction | |
7.7 expr.const | 2800 | review | Instantiating constexpr variables for potential constant evaluation | |
7.7 expr.const | 2907 | open | Constant lvalue-to-rvalue conversion on uninitialized std::nullptr_t | |
7.7 expr.const | 2909 | open | Subtle difference between constant-initialized and constexpr | |
8.6.5 stmt.ranged | 1680 | drafting | Including <initializer_list> for range-based for | |
8.7 stmt.jump | 2115 | drafting | Order of implicit destruction vs release of automatic storage | |
8.7.4 stmt.return | 2495 | open | Glvalue result of a function call | |
8.8 stmt.dcl | 2123 | open | Omitted constant initialization of local static variables | |
8.9 stmt.ambig | 1223 | drafting | Syntactic disambiguation and trailing-return-types | |
9.1 dcl.pre | 157 | open | Omitted typedef declarator | |
9.2.2 dcl.stc | 498 | open | Storage class specifiers in definitions of class members | |
9.2.2 dcl.stc | 2232 | open | thread_local anonymous unions | |
9.2.6 dcl.constexpr | 2117 | drafting | Explicit specializations and constexpr function templates | |
9.2.9.2 dcl.type.cv | 2195 | open | Unsolicited reading of trailing volatile members | |
9.2.9.5 dcl.type.elab | 144 | open | Position of friend specifier | |
9.2.9.7 dcl.spec.auto | 1348 | drafting | Use of auto in a trailing-return-type | |
9.2.9.7 dcl.spec.auto | 1670 | drafting | auto as conversion-type-id | |
9.2.9.7 dcl.spec.auto | 1868 | drafting | Meaning of “placeholder type” | |
9.2.9.7 dcl.spec.auto | 2412 | review | SFINAE vs undeduced placeholder type | |
9.3.2 dcl.name | 1488 | drafting | abstract-pack-declarators in type-ids | |
9.3.3 dcl.ambig.res | 2228 | open | EWG | Ambiguity resolution for cast to function type |
9.3.4.1 dcl.meaning.general | 2671 | open | friend named by a template-id | |
9.3.4.6 dcl.fct | 1001 | review | Parameter type adjustment in dependent parameter types | |
9.3.4.6 dcl.fct | 1668 | drafting | Parameter type determination still not clear enough | |
9.3.4.6 dcl.fct | 1790 | open | EWG | Ellipsis following function parameter pack |
9.3.4.6 dcl.fct | 2537 | drafting | Overbroad grammar for parameter-declaration | |
9.3.4.6 dcl.fct | 2553 | review | Restrictions on explicit object member functions | |
9.3.4.6 dcl.fct | 2802 | open | Constrained auto and redeclaration with non-abbreviated syntax | |
9.3.4.7 dcl.fct.default | 325 | drafting | When are default arguments parsed? | |
9.3.4.7 dcl.fct.default | 361 | open | Forward reference to default argument | |
9.3.4.7 dcl.fct.default | 1580 | drafting | Default arguments in explicit instantiations | |
9.3.4.7 dcl.fct.default | 1609 | open | Default arguments and function parameter packs | |
9.3.4.7 dcl.fct.default | 2701 | open | Default arguments in multiple scopes / inheritance of array bounds in the same scope | |
9.4 dcl.init | 2327 | drafting | Copy elision for direct-initialization with a conversion function | |
9.4.2 dcl.init.aggr | 2128 | drafting | Imprecise rule for reference member initializer | |
9.4.3 dcl.init.string | 1304 | drafting | Omitted array bound with string initialization | |
9.4.4 dcl.init.ref | 1414 | drafting | Binding an rvalue reference to a reference-unrelated lvalue | |
9.4.4 dcl.init.ref | 1827 | drafting | Reference binding with ambiguous conversions | |
9.4.4 dcl.init.ref | 2704 | open | Clarify meaning of "bind directly" | |
9.4.5 dcl.init.list | 1996 | drafting | Reference list-initialization ignores conversion functions | |
9.4.5 dcl.init.list | 2168 | open | Narrowing conversions and +/- infinity | |
9.4.5 dcl.init.list | 2742 | drafting | Guaranteed copy elision for brace-initialization from prvalue | |
9.5.1 dcl.fct.def.general | 1962 | open | EWG | Type of __func__ |
9.5.1 dcl.fct.def.general | 2362 | open | EWG | __func__ should be constexpr |
9.5.2 dcl.fct.def.default | 1854 | drafting | Disallowing use of implicitly-deleted functions | |
9.5.4 dcl.fct.def.coroutine | 2562 | open | Exceptions thrown during coroutine startup | |
9.5.4 dcl.fct.def.coroutine | 2563 | drafting | EWG | Initialization of coroutine result object |
9.6 dcl.struct.bind | 2340 | open | Reference collapsing and structured bindings | |
9.7.1 dcl.enum | 1485 | drafting | Out-of-class definition of member unscoped opaque enumeration | |
9.7.1 dcl.enum | 2131 | drafting | Ambiguity with opaque-enum-declaration | |
9.8.2.2 namespace.unnamed | 2505 | drafting | Nested unnamed namespace of inline unnamed namespace | |
9.9 namespace.udecl | 813 | open | typename in a using-declaration with a non-dependent name | |
9.9 namespace.udecl | 2555 | drafting | Ineffective redeclaration prevention for using-declarators | |
9.11 dcl.link | 1817 | drafting | Linkage specifications and nested scopes | |
9.12 dcl.attr | 2866 | open | EWG | Observing the effects of [[no_unique_address]] |
9.12.1 dcl.attr.grammar | 1706 | drafting | alignas pack expansion syntax | |
9.12.2 dcl.align | 1617 | open | alignas and non-defining declarations | |
9.12.2 dcl.align | 2223 | drafting | Multiple alignas specifiers | |
10.1 module.unit | 2541 | open | Linkage specifications, module purview, and module attachment | |
10.2 module.interface | 2607 | drafting | Visibility of enumerator names | |
10.3 module.import | 2727 | open | Importing header units synthesized from source files | |
11.2 class.prop | 511 | open | POD-structs with template assignment operators | |
11.2 class.prop | 2463 | open | EWG | Trivial copyability and unions with non-trivial members |
11.2 class.prop | 2736 | open | Standard layout class with empty base class also in first member | |
11.4 class.mem | 1890 | drafting | Member type depending on definition of member function | |
11.4.1 class.mem.general | 2188 | open | empty-declaration grammar ambiguity | |
11.4.1 class.mem.general | 2852 | open | Complete-class contexts and class-scope lambdas | |
11.4.4 special | 2787 | open | Kind of explicit object copy/move assignment function | |
11.4.5 class.ctor | 1623 | drafting | Deleted default union constructor and member initializers | |
11.4.5 class.ctor | 1808 | drafting | Constructor templates vs default constructors | |
11.4.5.1 class.ctor.general | 2841 | open | When do const objects start being const? | |
11.4.5.2 class.default.ctor | 2799 | drafting | Inheriting default constructors | |
11.4.5.2 class.default.ctor | 2885 | review | LWG | Non-eligible trivial default constructors |
11.4.5.3 class.copy.ctor | 1092 | drafting | Cycles in overload resolution during instantiation | |
11.4.5.3 class.copy.ctor | 1548 | drafting | Copy/move construction and conversion functions | |
11.4.5.3 class.copy.ctor | 1594 | drafting | Lazy declaration of special members vs overload errors | |
11.4.5.3 class.copy.ctor | 2203 | drafting | Defaulted copy/move constructors and UDCs | |
11.4.5.3 class.copy.ctor | 2264 | drafting | Memberwise copying with indeterminate value | |
11.4.5.3 class.copy.ctor | 2743 | open | Copying non-trivial objects nested within a union | |
11.4.5.3 class.copy.ctor | 2837 | open | Instantiating and inheriting by-value copy constructors | |
11.4.6 class.copy.assign | 1499 | drafting | Missing case for deleted move assignment operator | |
11.4.6 class.copy.assign | 2329 | drafting | Virtual base classes and generated assignment operators | |
11.4.6 class.copy.assign | 2897 | open | Copying potentially-overlapping union subobjects | |
11.4.7 class.dtor | 1977 | drafting | Contradictory results of failed destructor lookup | |
11.4.7 class.dtor | 2158 | drafting | Polymorphic behavior during destruction | |
11.4.7 class.dtor | 2839 | open | Explicit destruction of base classes | |
11.4.8.3 class.conv.fct | 2513 | open | Ambiguity with requires-clause and operator-function-id | |
11.4.9.3 class.static.data | 1283 | drafting | Static data members of classes with typedef name for linkage purposes | |
11.4.9.3 class.static.data | 1721 | review | Diagnosing ODR violations for static data members | |
11.4.9.3 class.static.data | 2335 | drafting | Deduced return types vs member types | |
11.5 class.union | 57 | open | Empty unions | |
11.5 class.union | 1404 | drafting | Object reallocation in unions | |
11.5 class.union | 1702 | drafting | Rephrasing the definition of “anonymous union” | |
11.5.1 class.union.general | 2675 | open | start_lifetime_as, placement-new, and active union members | |
11.5.2 class.union.anon | 2767 | open | Non-defining declarations of anonymous unions | |
11.5.2 class.union.anon | 2773 | open | Naming anonymous union members as class members | |
11.6 class.local | 2890 | review | Defining members of local classes | |
11.7.3 class.virtual | 2554 | review | Overriding virtual functions, also with explicit object parameters | |
11.8.3 class.access.base | 2246 | drafting | Access of indirect virtual base class constructors | |
11.8.4 class.friend | 1699 | open | EWG | Does befriending a class befriend its friends? |
11.8.5 class.protected | 472 | drafting | Casting across protected inheritance | |
11.8.5 class.protected | 1883 | drafting | Protected access to constructors in mem-initializers | |
11.8.5 class.protected | 2187 | drafting | Protected members and access via qualified-id | |
11.8.5 class.protected | 2244 | open | Base class access in aggregate initialization | |
11.9 class.init | 2756 | review | Completion of initialization by delegating constructor | |
11.9.3 class.base.init | 1915 | open | EWG | Potentially-invoked destructors in non-throwing constructors |
11.9.3 class.base.init | 2056 | drafting | Member function calls in partially-initialized class objects | |
11.9.3 class.base.init | 2403 | drafting | Temporary materialization and base/member initialization | |
11.9.3 class.base.init | 2669 | open | EWG | Lifetime extension for aggregate initialization |
11.9.5 class.cdtor | 1517 | drafting | Unclear/missing description of behavior during construction/destruction | |
11.9.5 class.cdtor | 2757 | review | Deleting or deallocating storage of an object during its construction | |
11.9.6 class.copy.elision | 1049 | open | Copy elision through reference parameters of inline functions | |
11.10.3 class.spaceship | 2703 | review | Three-way comparison requiring strong ordering for floating-point types, take 2 | |
12.2.2.2.2 over.call.func | 1278 | drafting | Incorrect treatment of contrived object | |
12.2.2.2.3 over.call.object | 2189 | open | Surrogate call template | |
12.2.2.2.3 over.call.object | 2564 | drafting | Conversion to function pointer with an explicit object parameter | |
12.2.2.3 over.match.oper | 545 | open | User-defined conversions and built-in operator overload resolution | |
12.2.2.3 over.match.oper | 1919 | open | Overload resolution for ! with explicit conversion operator | |
12.2.2.3 over.match.oper | 2089 | drafting | Restricting selection of builtin overloaded operators | |
12.2.2.3 over.match.oper | 2730 | open | Comparison templates on enumeration types | |
12.2.2.3 over.match.oper | 2797 | review | EWG | Meaning of "corresponds" for rewritten operator candidates |
12.2.2.3 over.match.oper | 2804 | open | EWG | Lookup for determining rewrite targets |
12.2.2.3 over.match.oper | 2844 | open | Enumerating a finite set of built-in candidates | |
12.2.2.7 over.match.ref | 2028 | drafting | Converting constructors in rvalue reference initialization | |
12.2.2.7 over.match.ref | 2108 | drafting | Conversions to non-class prvalues in reference initialization | |
12.2.2.8 over.match.list | 2194 | drafting | Impossible case in list initialization | |
12.2.2.8 over.match.list | 2311 | open | Missed case for guaranteed copy elision | |
12.2.2.9 over.match.class.deduct | 2425 | open | Confusing wording for deduction from a type | |
12.2.2.9 over.match.class.deduct | 2467 | drafting | CTAD for alias templates and the deducible check | |
12.2.2.9 over.match.class.deduct | 2471 | drafting | Nested class template argument deduction | |
12.2.2.9 over.match.class.deduct | 2680 | open | Class template argument deduction for aggregates with designated initializers | |
12.2.4 over.match.best | 2735 | open | List-initialization and conversions in overload resolution | |
12.2.4.2 over.best.ics | 2319 | drafting | Nested brace initialization from same type | |
12.2.4.2.1 over.best.ics.general | 2525 | drafting | Incorrect definition of implicit conversion sequence | |
12.2.4.2.1 over.best.ics.general | 2679 | open | Implicit conversion sequence with a null pointer constant | |
12.2.4.2.1 over.best.ics.general | 2829 | open | Redundant case in restricting user-defined conversion sequences | |
12.2.4.2.1 over.best.ics.general | 2898 | open | Clarify implicit conversion sequence from cv T to T | |
12.2.4.2.3 over.ics.user | 2731 | open | List-initialization sequence with a user-defined conversion | |
12.2.4.2.5 over.ics.ref | 2077 | drafting | Overload resolution and invalid rvalue-reference initialization | |
12.2.4.2.6 over.ics.list | 1536 | drafting | Overload resolution with temporary from initializer list | |
12.2.4.2.6 over.ics.list | 2169 | open | Narrowing conversions and overload resolution | |
12.2.4.2.6 over.ics.list | 2492 | drafting | Comparing user-defined conversion sequences in list-initialization | |
12.2.4.2.6 over.ics.list | 2741 | open | Implicit conversion sequence from empty list to array of unknown bound | |
12.2.4.2.6 over.ics.list | 2790 | open | Aggregate initialization and user-defined conversion sequence | |
12.2.4.3 over.ics.rank | 1459 | open | Reference-binding tiebreakers in overload resolution | |
12.2.4.3 over.ics.rank | 1789 | open | Array reference vs array decay in overload resolution | |
12.2.4.3 over.ics.rank | 2110 | drafting | Overload resolution for base class conversion and reference/non-reference | |
12.2.4.3 over.ics.rank | 2337 | open | Incorrect implication of logic ladder for conversion sequence tiebreakers | |
12.2.4.3 over.ics.rank | 2815 | drafting | Overload resolution for references/pointers to noexcept functions | |
12.2.4.3 over.ics.rank | 2842 | open | Preferring an initializer_list over a single value | |
12.3 over.over | 2572 | review | Address of overloaded function with no target | |
12.3 over.over | 2873 | open | Taking the address of a function involving template argument deduction | |
12.4 over.oper | 1989 | drafting | Insufficient restrictions on parameters of postfix operators | |
12.4.3 over.binary | 1549 | open | Overloaded comma operator with void operand | |
12.5 over.built | 260 | open | User-defined conversions and built-in operator= | |
12.5 over.built | 954 | open | Overload resolution of conversion operator templates with built-in types | |
12.6 over.literal | 1620 | open | User-defined literals and extended integer types | |
13 temp | 205 | drafting | Templates and static data members | |
13.1 temp.pre | 1463 | drafting | EWG | extern "C" alias templates |
13.1 temp.pre | 2862 | drafting | Unclear boundaries of template declarations | |
13.1 temp.pre | 2904 | open | Introducing template-names | |
13.2 temp.param | 1444 | drafting | Type adjustments of non-type template parameters | |
13.2 temp.param | 1635 | drafting | How similar are template default arguments to function default arguments? | |
13.2 temp.param | 2395 | drafting | Parameters following a pack expansion | |
13.2 temp.param | 2617 | review | Default template arguments for template members of non-template classes | |
13.3 temp.names | 579 | open | What is a “nested” > or >>? | |
13.3 temp.names | 2903 | tentatively ready | Can we omit the template disambiguator in nested-name-specifiers in type-only contexts? | |
13.4 temp.arg | 2105 | open | When do the arguments for a parameter pack end? | |
13.4.3 temp.arg.nontype | 2043 | drafting | Generalized template arguments and array-to-pointer decay | |
13.4.3 temp.arg.nontype | 2401 | drafting | Array decay vs prohibition of subobject non-type arguments | |
13.4.4 temp.arg.template | 2057 | drafting | Template template arguments with default arguments | |
13.4.4 temp.arg.template | 2398 | drafting | Template template parameter matching and deduction | |
13.5.2 temp.constr.constr | 2686 | open | Pack expansion into a non-pack parameter of a concept | |
13.5.2.3 temp.constr.atomic | 2589 | review | Context of access checks during constraint satisfaction checking | |
13.6 temp.type | 2037 | drafting | Alias templates and template declaration matching | |
13.7 temp.decls | 1730 | drafting | Can a variable template have an unnamed type? | |
13.7.5 temp.friend | 2118 | open | Stateful metaprogramming via friend injection | |
13.7.6 temp.spec.partial | 708 | open | Partial specialization of member templates of class templates | |
13.7.6 temp.spec.partial | 1647 | drafting | Type agreement of non-type template arguments in partial specializations | |
13.7.6 temp.spec.partial | 2127 | drafting | Partial specialization and nullptr | |
13.7.6 temp.spec.partial | 2173 | open | Partial specialization with non-deduced contexts | |
13.7.6.1 temp.spec.partial.general | 2179 | drafting | Required diagnostic for partial specialization after first use | |
13.7.6.2 temp.spec.partial.match | 549 | drafting | Non-deducible parameters in partial specializations | |
13.7.6.4 temp.spec.partial.member | 1755 | drafting | Out-of-class partial specializations of member templates | |
13.7.7.2 temp.over.link | 310 | open | Can function templates differing only in parameter cv-qualifiers be overloaded? | |
13.7.7.2 temp.over.link | 2584 | open | Equivalent types in function template declarations | |
13.7.7.3 temp.func.order | 402 | open | More on partial ordering of function templates | |
13.7.7.3 temp.func.order | 1157 | open | Partial ordering of function templates is still underspecified | |
13.7.7.3 temp.func.order | 2160 | open | Issues with partial ordering | |
13.7.7.3 temp.func.order | 2834 | review | Partial ordering and explicit object parameters | |
13.7.8 temp.alias | 1286 | drafting | Equivalence of alias templates | |
13.7.8 temp.alias | 1430 | open | Pack expansion into fixed alias template parameter list | |
13.7.8 temp.alias | 1554 | drafting | Access and alias templates | |
13.7.8 temp.alias | 1979 | drafting | Alias template specialization in template member definition | |
13.7.8 temp.alias | 1980 | drafting | Equivalent but not functionally-equivalent redeclarations | |
13.7.8 temp.alias | 2236 | drafting | When is an alias template specialization dependent? | |
13.7.8 temp.alias | 2794 | open | Uniqueness of lambdas in alias templates | |
13.8 temp.res | 1257 | open | Instantiation via non-dependent references in uninstantiated templates | |
13.8 temp.res | 2067 | open | Generated variadic templates requiring empty pack | |
13.8.1 temp.res.general | 2462 | drafting | Problems with the omission of the typename keyword | |
13.8.1 temp.res.general | 2468 | drafting | Omission of the typename keyword in a member template parameter list | |
13.8.2 temp.local | 186 | open | Name hiding and template template-parameters | |
13.8.3.2 temp.dep.type | 1390 | drafting | Dependency of alias template specializations | |
13.8.3.2 temp.dep.type | 1524 | drafting | Incompletely-defined class template base | |
13.8.3.2 temp.dep.type | 1619 | open | Definition of current instantiation | |
13.8.3.2 temp.dep.type | 2074 | drafting | Type-dependence of local class of function template | |
13.8.3.3 temp.dep.expr | 2275 | drafting | Type-dependence of function template | |
13.8.3.3 temp.dep.expr | 2487 | drafting | Type dependence of function-style cast to incomplete array type | |
13.8.3.4 temp.dep.constexpr | 2774 | open | Value-dependence of requires-expressions | |
13.8.3.4 temp.dep.constexpr | 2905 | tentatively ready | Value-dependence of noexcept-expression | |
13.8.3.5 temp.dep.temp | 2090 | drafting | Dependency via non-dependent base class | |
13.8.4 temp.dep.res | 2 | drafting | How can dependent names be used in member declarations that appear outside of the class template definition? | |
13.8.4.1 temp.point | 287 | drafting | Order dependencies in template instantiation | |
13.8.4.1 temp.point | 1845 | drafting | Point of instantiation of a variable template specialization | |
13.8.4.1 temp.point | 2245 | drafting | Point of instantiation of incomplete class template | |
13.8.4.1 temp.point | 2250 | open | Implicit instantiation, destruction, and TUs | |
13.8.4.1 temp.point | 2497 | drafting | Points of instantiation for constexpr function templates | |
13.9 temp.spec | 2435 | open | Alias template specializations | |
13.9.2 temp.inst | 1602 | review | Linkage of specialization vs linkage of template arguments | |
13.9.2 temp.inst | 1856 | open | Indirect nested classes of class templates | |
13.9.2 temp.inst | 2202 | drafting | When does default argument instantiation occur? | |
13.9.2 temp.inst | 2222 | drafting | Additional contexts where instantiation is not required | |
13.9.2 temp.inst | 2263 | drafting | Default argument instantiation for friends | |
13.9.2 temp.inst | 2265 | drafting | Delayed pack expansion and member redeclarations | |
13.9.2 temp.inst | 2596 | drafting | Instantiation of constrained non-template friends | |
13.9.2 temp.inst | 2808 | review | Explicit specialization of defaulted special member function | |
13.9.3 temp.explicit | 293 | open | Syntax of explicit instantiation/specialization too permissive | |
13.9.3 temp.explicit | 1046 | open | What is a “use” of a class specialization? | |
13.9.3 temp.explicit | 1665 | drafting | Declaration matching in explicit instantiations | |
13.9.3 temp.explicit | 2421 | drafting | Explicit instantiation of constrained member functions | |
13.9.3 temp.explicit | 2501 | drafting | Explicit instantiation and trailing requires-clauses | |
13.9.4 temp.expl.spec | 529 | drafting | Use of template<> with “explicitly-specialized” class templates | |
13.9.4 temp.expl.spec | 1840 | drafting | Non-deleted explicit specialization of deleted function template | |
13.9.4 temp.expl.spec | 1993 | drafting | Use of template<> defining member of explicit specialization | |
13.9.4 temp.expl.spec | 2409 | drafting | Explicit specializations of constexpr static data members | |
13.9.4 temp.expl.spec | 2847 | review | Constrained explicit specializations of function templates at class scope | |
13.10.2 temp.arg.explicit | 264 | open | Unusable template constructors and conversion functions | |
13.10.2 temp.arg.explicit | 2055 | drafting | Explicitly-specified non-deduced parameter packs | |
13.10.3 temp.deduct | 697 | open | Deduction rules apply to more than functions | |
13.10.3 temp.deduct | 1172 | drafting | “instantiation-dependent” constructs | |
13.10.3 temp.deduct | 1322 | drafting | Function parameter type decay in templates | |
13.10.3 temp.deduct | 1582 | drafting | Template default arguments and deduction failure | |
13.10.3 temp.deduct | 1844 | open | EWG | Defining “immediate context” |
13.10.3 temp.deduct | 2296 | open | EWG | Are default argument instantiation failures in the “immediate context”? |
13.10.3 temp.deduct | 2896 | review | Template argument deduction involving exception specifications | |
13.10.3.1 temp.deduct.general | 2498 | open | Partial specialization failure and the immediate context | |
13.10.3.1 temp.deduct.general | 2769 | open | Substitution into template parameters and default template arguments should be interleaved | |
13.10.3.1 temp.deduct.general | 2770 | open | Trailing requires-clause can refer to function parameters before they are substituted into | |
13.10.3.2 temp.deduct.call | 503 | open | Cv-qualified function types in template argument deduction | |
13.10.3.2 temp.deduct.call | 1513 | drafting | initializer_list deduction failure | |
13.10.3.2 temp.deduct.call | 1584 | drafting | Deducing function types from cv-qualified types | |
13.10.3.2 temp.deduct.call | 1939 | open | Argument conversions to nondeduced parameter types revisited | |
13.10.3.3 temp.deduct.funcaddr | 1486 | drafting | Base-derived conversion in member pointer deduction | |
13.10.3.5 temp.deduct.partial | 1221 | open | Partial ordering and reference collapsing | |
13.10.3.5 temp.deduct.partial | 1610 | drafting | Cv-qualification in deduction of reference to array | |
13.10.3.6 temp.deduct.type | 1763 | open | Length mismatch in template type deduction | |
13.10.3.6 temp.deduct.type | 2328 | drafting | Unclear presentation style of template argument deduction rules | |
13.10.3.6 temp.deduct.type | 2900 | open | Deduction of non-type template arguments with placeholder types | |
14.4 except.handle | 2172 | drafting | Multiple exceptions with one exception object | |
14.4 except.handle | 2219 | drafting | Dynamically-unreachable handlers | |
14.5 except.spec | 2417 | open | Explicit instantiation and exception specifications | |
15 cpp | 2002 | open | WG14 | White space within preprocessing directives |
15.2 cpp.cond | 925 | open | Type of character literals in preprocessor expressions | |
15.2 cpp.cond | 1436 | open | Interaction of constant expression changes with preprocessor expressions | |
15.2 cpp.cond | 2190 | open | Insufficient specification of __has_include | |
15.2 cpp.cond | 2575 | open | SG12 | Undefined behavior when macro-replacing "defined" operator |
15.3 cpp.include | 2576 | open | SG12 | Undefined behavior with macro-expanded #include directives |
15.6 cpp.replace | 1718 | drafting | WG14 | Macro invocation spanning end-of-file |
15.6 cpp.replace | 2003 | drafting | Zero-argument macros incorrectly specified | |
15.6.1 cpp.replace.general | 2577 | open | SG12 | Undefined behavior for preprocessing directives in macro arguments |
15.6.3 cpp.stringize | 1625 | open | WG14 | Adding spaces between tokens in stringizing |
15.6.3 cpp.stringize | 1709 | drafting | Stringizing raw string literals containing newline | |
15.6.3 cpp.stringize | 2578 | open | SG12 | Undefined behavior when creating an invalid string literal via stringizing |
15.6.4 cpp.concat | 2522 | open | WG14 | Removing placemarker tokens and retention of whitespace |
15.6.4 cpp.concat | 2579 | open | SG12 | Undefined behavior when token pasting does not create a preprocessing token |
15.6.5 cpp.rescan | 268 | open | WG14 | Macro name suppression in rescanned replacement text |
15.7 cpp.line | 2580 | open | SG12 | Undefined behavior with #line |
15.7 cpp.line | 2693 | open | WG14 | Escape sequences for the string-literal of #line |
15.7 cpp.line | 2908 | open | Counting physical source lines for __LINE__ | |
15.9 cpp.pragma | 1889 | drafting | Unclear effect of #pragma on conformance | |
15.11 cpp.predefined | 2581 | open | SG12 | Undefined behavior for predefined macros |
15.12 cpp.pragma.op | 897 | open | _Pragma and extended string-literals | |
15.12 cpp.pragma.op | 2694 | open | WG14 | string-literals of the _Pragma operator |
17.2.4 support.types.layout | 2784 | open | EWG | Unclear definition of member-designator for offsetof |
17.13.3 csetjmp.syn | 2361 | open | Unclear description of longjmp undefined behavior | |
Annex C diff | 1944 | open | New C incompatibilities | |
C.6 diff.cpp03 | 1279 | drafting | Additional differences between C++ 2003 and C++ 2011 | |
C.7 diff.iso | 1248 | open | Updating Annex C to C99 | |
C.7.4 diff.expr | 2875 | review | Missing support for round-tripping null pointer values through indirection/address operators |
(From submission #550.)
There are two known situations where the lvalue-to-rvalue conversion is applied to class types, which can be non-constexpr even if the resulting copy constructor invocation would be constexpr (7.7 [expr.const] bullet 5.9). The other such situation is 7.6.1.3 [expr.call] paragraph 11. Here, the concern is with 7.6.16 [expr.cond] paragraph 7, which can be invoked for class types; for example:
struct S {}; S a; constexpr S b = a; // OK, call to implicitly-declared copy constructor constexpr S d = false ? S{} : a; // error: lvalue-to-rvalue conversion of 'a' is not a constant expression
Major implementations disagree with the ill-formed outcome.
Proposed resolution (approved by CWG 2024-06-26):
Change in 7.6.16 [expr.cond] paragraph 7 as follows:
Otherwise, the result is a prvalue. If the second and third operands do not have the same type, ...
Lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointerArray-to-pointer (7.3.3 [conv.array]),and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the second and third operands. After those conversions, one of the following shall hold:
- The second and third operands have the same type; the result is of that type and the result
objectisinitializedcopy-initialized using the selected operand.- The second and third operands have arithmetic or enumeration type; the usual arithmetic conversions (7.4 [expr.arith.conv]) are performed to bring them to a common type, and the result is of that type.
- One or both of the second and third operands have pointer type; lvalue-to-rvalue (7.3.2 [conv.lval]), pointer
conversions(7.3.12 [conv.ptr]), function pointerconversions(7.3.14 [conv.fctptr]), and qualification conversions (7.3.6 [conv.qual]) are performed to bring them to their composite pointer type (7.2.2 [expr.type]). The result is of the composite pointer type.- One or both of the second and third operands have pointer-to-member type; lvalue-to-rvalue (7.3.2 [conv.lval]), pointer to member
conversions(7.3.13 [conv.mem]), function pointerconversions(7.3.14 [conv.fctptr]), and qualification conversions (7.3.6 [conv.qual]) are performed to bring them to their composite pointer type (7.2.2 [expr.type]). The result is of the composite pointer type.- Both the second and third operands have type std::nullptr_t or one has that type and the other is a null pointer constant. The result is of type std::nullptr_t.
Are the following examples well-formed? Note the absence of a template disambiguator:
template<typename T> class C { T::X<int> g1(); // #1 T::X<int>::Y g2(); // #2 }; template<typename T> T::X<int> h1(); // #3 template<typename T> T::X<int>::Y h2(); // #4
The return type is a type-only context per 13.8.1 [temp.res.general] paragraph 4. However, 13.3 [temp.names] paragraph 3 excludes nested-name-specifiers from the set of situations where template can be omitted. That means that #1 and #3 are valid; X is not part of a nested-name-specifier. However, #2 and #4 are invalid; the template disambiguator is missing. Those examples ought to be valid, too.
Proposed resolution (approved by CWG 2024-06-14):
Change in 13.3 [temp.names] paragraph 3 as follows:
A name is in a transitive type-only context ifA < is interpreted as the delimiter of a template-argument-list if it follows a name that is not a conversion-function-id and
- it is in a type-only context (13.8.1 [temp.res.general]) and is not the terminal name of a nested-name-specifier, or
- it is the terminal name of a nested-name-specifier that precedes a name in a transitive type-only context (possibly with an intervening template keyword).
- that follows the keyword template or a ~ after a nested-name-specifier or in a class member access expression, or
- for which name lookup finds the injected-class-name of a class template or finds any declaration of a template, or
- that is an unqualified name for which name lookup either finds one or more functions or finds nothing, or
- that is a terminal name in a using-declarator (9.9 [namespace.udecl]), in a declarator-id (9.3.4 [dcl.meaning]), or in a transitive type-only context
other than a nested-name-specifier (13.8 [temp.res]).
(From submission #554.)
The following examples of noexcept-expressions are not specified to be value-dependent, but ought to be, because value-initialization of T might throw an exception.
template<typename T> void f() { noexcept(static_cast<int>(T{})); noexcept(typeid(*T{})); noexcept(delete T{}); }
Proposed resolution (approved by CWG 2024-06-26):
Change in 13.8.3.4 [temp.dep.constexpr] paragraph 2 as follows:
Expressions of the following form are value-dependent if the unary-expression or expression is type-dependent or the type-id is dependent:sizeof unary-expression sizeof ( type-id ) typeid ( expression ) typeid ( type-id ) alignof ( type-id )noexcept ( expression )
Change in 13.8.3.4 [temp.dep.constexpr] paragraph 3 as follows:
Expressions of the following form are value-dependent if either the type-idor, simple-type-specifier, or typename-specifier is dependent or the expression or cast-expression is value-dependent:simple-type-specifier ( expression-listopt ) typename-specifier ( expression-listopt ) simple-type-specifier braced-init-list typename-specifier braced-init-list static_cast < type-id > ( expression ) const_cast < type-id > ( expression ) reinterpret_cast < type-id > ( expression ) dynamic_cast < type-id > ( expression ) ( type-id ) cast-expression
Add a new paragraph before 13.8.3.4 [temp.dep.constexpr] paragraph 5 as follows:
A noexcept-expression (7.6.2.7 [expr.unary.noexcept]) is value-dependent if its expression involves a template parameter.
An expression of the form &qualified-id where ...
Clause 2 [intro.refs] paragraph 1.10 uses an undated reference for the Unicode standard, meaning that a published (and otherwise frozen) standard for C++ (e.g. C++23) will implicitly refer to a new revision of the Unicode standard the moment such is issued:
... For undated references, the latest edition of the referenced document (including any amendments) applies.
- ...
- The Unicode Consortium. The Unicode Standard. Available from: https://www.unicode.org/versions/latest/
This situation is strictly worse than the lack of support for certain scripts or languages, which can be rectified by updating the reference to Unicode in the next revision of the C++ standard, as is regularly done with any other missing language feature deemed worth addressing.
Possible resolution [SUPERSEDED]:
Change in Clause 2 [intro.refs] paragraph 1.10 as follows:
Additional notes (January, 2024)
Forwarded to SG16 and LWG by decision of the CWG chair, via paper issue 1736.
SG16 2024-01-10
SG16 has consensus to have a dated reference to Unicode in the "Normative references", indicating a minimum version, and add permission to implement an implementation-defined later version.
Possible resolution [SUPERSEDED]:
Change in Clause 2 [intro.refs] paragraph 1.10 as follows:
Add a paragraph before 4.1.1 [intro.compliance.general] paragraph 8 as follows:
A conforming implementation may implement an implementation-defined version of the Unicode Standard that is a later version than the one referenced in Clause 2 [intro.refs].
A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any well-formed program.
CWG 2024-01-19
CWG took note of the issue. No objections were raised regarding the suggested direction.
EWG 2024-03-18
Allow implementation-defined Unicode version, but require at least version 15.1.
Possible resolution:
Change in Clause 2 [intro.refs] paragraph 1.10 as follows:
Add a paragraph before 4.1.1 [intro.compliance.general] paragraph 8 as follows:
A conforming implementation may implement an implementation-defined version of the Unicode Standard that is a later version than the one referenced in Clause 2 [intro.refs].
A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any well-formed program.
Subclause 5.5 [lex.digraph] paragraph 2 specifies:
In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling. [ Footnote: ... ]
However, 9.12.1 [dcl.attr.grammar] paragraph 4 specifies:
... If a keyword (5.11 [lex.key]) or an alternative token (5.5 [lex.digraph]) that satisfies the syntactic requirements of an identifier (5.10 [lex.name]) is contained in an attribute-token, it is considered an identifier. ...
It seems an alternative token (say, or) is treated the same as the operator ||, yet only the alternative token is considered an identifier, not the operator, when appearing in an attribute-token. That seems contradictory.
Proposed resolution (approved by CWG 2023-05-12):
Change in 5.2 [lex.phases] paragraph 2 as follows:
Whitespace characters separating tokens are no longer significant. Each preprocessing token is converted into a token (5.6 [lex.token]), replacing each alternative token by its corresponding primary token (5.5 [lex.digraph]). The resulting tokens constitute a translation unit and are syntactically and semantically analyzed and translated.
Change in 9.12.1 [dcl.attr.grammar] paragraph 4 as follows:
... If a keyword (5.11 [lex.key])or an alternative token (5.5 [lex.digraph])that satisfies the syntactic requirements of an identifier (5.10 [lex.name]) is contained in an attribute-token, it is considered an identifier. ...
This resolution also addresses issue 1897.
Additional notes (May, 2023)
During additional discussion on the EWG reflector, Alisdair Meredith expressed that he intends to propose that some alternative tokens be treated as full keywords in phase 7, to prevent and from being used as an rvalue reference and compl from being used for naming destructors. That would reverse the above direction.
Furthermore, an idea was floated to treat all alternative tokens as full keywords in phase 7 and as identifiers in phase 4, amending the grammar productions for expressions as necessary. This removes the special treatment of alternative tokens entirely, however the treatment of examples such as #define and blah would change from ill-formed to well-formed. Some opposition was voiced against changing the phase 4 treatment of alternative tokens.
CWG 2023-06-16
Forwarded to EWG with paper issue #1581.
The term “literal” is used without definition except the implicit connection with the syntactic nonterminal literal. The relationships of English terms to syntactic nonterminals (such as “integer literal” and integer-literal) should be examined throughout 5.13 [lex.literal] and its subsections.
Notes from the November, 2016 meeting:
This issue will be handled editorially. It is being placed in "review" status until that point.
According to 5.5 [lex.digraph] paragraph 2,
In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling.
However, the primary and alternative tokens are different tokens, which runs afoul of the ODR requirement in 6.3 [basic.def.odr] paragraph 6 that the definitions consist of the “same sequence of tokens.” This wording should be amended to allow for use of primary and alternative tokens.
CWG 2023-05-12
Addressed by issue 2726.
The specification for associated entities in argument-dependent lookup recurses into template argument types, but misses handling reference and array cases. Those cannot appear for function arguments. For example:
namespace N { template<typename T> struct A { operator T() const noexcept; }; } namespace M { struct B {}; void f(B&); } void caller() { N::A<M::B&> arg; f(arg); }
Possible resolution:
Change in 6.5.4 [basic.lookup.argdep] paragraph 3 as follows:
For each argumenttype Tin the function call, there is a set of zero or more associated entities to be considered. The set of entities is determined entirely by the types of the function arguments (and any template template arguments). Any typedef-names and using-declarations used to specify the types do not contribute to this set. The set of associated entities for a type cv T is determined in the following way:
- If T is a fundamental type, its
associatedset of associated entities is empty.- If T is a class type (including unions), its associated entities are: the class itself; the class of which it is a member, if any; and, if it is a complete type, its direct and indirect base classes. Furthermore, if T is a class template specialization, its associated entities also include: the entities associated with the types of the template arguments provided for template type parameters; the templates used as template template arguments; and the classes of which any member templates used as template template arguments are members. [Note 2: Non-type template arguments do not contribute to the set of associated entities. —end note]
- If T is an enumeration type, its associated entities are T and, if it is a class member, the member's class.
- If T is a pointer to U, a reference to U, or an array of U, its associated entities are those associated with U.
- If T is a function type, its associated entities are those associated with the function parameter types and those associated with the return type.
- If T is a pointer to a member function of a class X, its associated entities are those associated with the function parameter types and return type, together with those associated with X.
- If T is a pointer to a data member of class X, its associated entities are those associated with the member type together with those associated with X.
In addition, if the argument is an overload set or the address of such a set, its associated entities are the union of those associated with each of the members of the set, i.e., the entities associated with its parameter types and return type. Additionally,The associated entities of a function call are the associated entities of each argument A of the call, consisting of
- if A is an id-expression whose terminal name refers to an overload set O, the union of the associated entities of the function types of the members of O, and
- if
the aforementioned overload set is named withA is a template-id I whose terminal name refers to an overload set,its associated entities also include itsthe template template-arguments of I andthosethe associatedwith itsentities of each typetemplate-arguments.template-argument of I, and- otherwise, if A has a type, the associated entities of the type of A,
- otherwise, A has no associated entities.
Is the following well-formed?
int f() { int i = 3; new (&i) float(1.2); return i; }
The wording that is intended to prevent such shenanigans, 6.7.3 [basic.life] paragraphs 7-9, doesn't quite apply here. In particular, paragraph 7 reads,
If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if:
the storage for the new object exactly overlays the storage location which the original object occupied, and
the new object is of the same type as the original object (ignoring the top-level cv-qualifiers), and...
The problem here is that this wording only applies “after the lifetime of an object has ended and before the storage which the object occupied is reused;” for an object of a scalar type, its lifetime only ends when the storage is reused or released (paragraph 1), so it appears that these restrictions cannot apply to such objects.
(See also issues 1116 and 1338.)
Proposed resolution (August, 2010) [SUPERSEDED]:
This issue is resolved by the resolution of issue 1116.
Possible resolution:
Change in 6.7.3 [basic.life] paragraph 6 as follows:
Before the lifetime of an object has started but after the storage which the object will occupy has been allocated [ Footnote: ... ], or,after the lifetime of an object has ended and before the storage which the object occupied isreused orreleased, any pointer that represents the address of the storage location where the object will be or was located may be used but only in limited ways. ...
Change in 6.7.3 [basic.life] paragraph 7 as follows:
Similarly, before the lifetime of an object has started but after the storage which the object will occupy has been allocated, or,after the lifetime of an object has ended and before the storage which the object occupied isreused orreleased, any glvalue that refers to the original object may be used but only in limited ways. ...
6.7.3 [basic.life] paragraph 6 specifies:
Before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that represents the address of the storage location where the object will be or was located may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise, such a pointer refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), and using the pointer as if the pointer were of type void* is well-defined.
Similarly, 6.7.3 [basic.life] paragraph 7 specifies:
Similarly, before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any glvalue that refers to the original object may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise, such a glvalue refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), and using the properties of the glvalue that do not depend on its value is well-defined.
In either case, it is unclear what "refers to allocated storage" means, beyond the properties ascribed to an object in 6.7.2 [intro.object].
See also issue 1853.
Proposed resolution:
Change in 6.7.3 [basic.life] paragraph 6 as follows:
For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise,such a pointer refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), andusingthesuch a pointer as if the pointer were of type void* is well-defined.
Change in 6.7.3 [basic.life] paragraph 7 as follows:
For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise,such a glvalue refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), andusing the properties ofthesuch a glvalue that do not depend on its value is well-defined.
The resolution to NB comment US 041 (C++20 CD) does not seem to have fully addressed the original issue, allowing:
union U { int i, j; }; U u; new (&u) U{.i = 5}; int k = u.j; // OK! New U::i transparently replaces existing u.j!
The suggestion is to allow a newly-created complete object to transparently replace any object of the same type in the same storage, except for a potentially-overlapping subobject or a const complete object, and to allow a newly-created subobject to transparently replace only a corresponding subobject of an existing object.
Suggested resolution [SUPERSEDED]:
Change in 6.7.3 [basic.life] paragraph 8 as follows:
... An object o1 is transparently replaceable by an object o2 if:
- o2 is a complete object and:
- the storage that o2 occupies exactly overlays the storage that o1 occupied, and
- o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and
- o1 is not a const, complete object, and
neithero1nor o2is not a potentially-overlapping subobject (6.7.2 [intro.object]),andoreither o1 and o2 are both complete objects, oro1 and o2 are corresponding direct subobjects of objects p1 and p2, respectively, and p1 is transparently replaceable by p2.
Additional notes (February, 2023)
The suggested resolution above does not handle the additional example in issue 2676.
Suggested resolution:
Change in 6.7.2 [intro.object] paragraph 2 as follows:
Objects can contain other objects, called subobjects. A subobject can be a member subobject (11.4 [class.mem]), a base class subobject (11.7 [class.derived]), or an array element. An object that is not a subobject of any other object is called a complete object. If an object is created in storage associated with amember subobject or array elementsubobject e (which may or may not be within its lifetime), the created object is a subobject of e's containing object if:In this case, e and the created object are corresponding direct subobjects.
- the lifetime of e's containing object has begun and not ended, and
- the storage for the new object exactly overlays the storage location associated with e, and
- e is not a potentially-overlapping subobject, and
- the new object is of the same type as e (ignoring cv-qualification).
Change in 6.7.3 [basic.life] paragraph 8 as follows:
... An object o1 is transparently replaceable by an object o2 if either
- o1 and o2 are complete objects for which:
- o1 is not const,
- the storage that o2 occupies exactly overlays the storage that o1 occupied, and
- o1 and o2 are of the same type (ignoring the top-level cv-qualifiers),
andoro1 is not a const, complete object, andneither o1 nor o2 is a potentially-overlapping subobject (6.7.2 [intro.object]), andeither o1 and o2 are both complete objects, oro1 and o2 are corresponding direct subobjectsof objects p1 and p2, respectively, and p1 is transparently replaceable by p2(6.7.2 [intro.object]) for which:
- the complete object of o1 is not const or
- o1 is a mutable member subobject or a subobject thereof.
Consider:
int i = i;
According to 6.9.3.2 [basic.start.static] paragraph 2, i is zero-initialized and then dynamic initialization is applied. However, it is unclear whether the lifetime of i has started at the point when the dynamic initialization occurs. According to 6.7.3 [basic.life] paragraph 1:
... The lifetime of an object of type T begins when:except ...
- storage with the proper alignment and size for type T is obtained, and
- its initialization (if any) is complete (including vacuous initialization) (9.4 [dcl.init]),
Possible directions:
The example in 6.9.3.2 [basic.start.static] paragraph 3 suggests the first option.
CWG 2024-01-19
The first option is is intended.
Possible resolution:
Split and change 6.7.3 [basic.life] paragraph 1 as follows:
... The lifetime of an object of type T begins when:
except that if the object is a union member or subobject thereof, its lifetime only begins if that union member is the initialized member in the union (9.4.2 [dcl.init.aggr], 11.9.3 [class.base.init]), or as described in 11.5 [class.union], 11.4.5.3 [class.copy.ctor], and 11.4.6 [class.copy.assign], and except as described in 20.2.10.2 [allocator.members]. [ Note: In particular, the lifetime of a complete class object begins when the non-delegating constructor returns (11.9.3 [class.base.init]). The lifetime of a complete object with static or thread storage duration begins when static initialization is complete (6.9.3.2 [basic.start.static]). -- end note ]
- storage with the proper alignment and size for type T is obtained, and
itsany initialization(if any)is complete (including vacuous initialization) (9.4 [dcl.init]),The lifetime of an object o of type T ends when: ...
In the following example,
int f() { X x; return 4; } int a = f();
a must be directly initialized in the return statement of f() because the exception permitting temporaries for function arguments and return types in 6.7.7 [class.temporary] paragraph 3 applies only to certain class types:
When an object of class type X is passed to or returned from a function, if X has at least one eligible copy or move constructor (11.4.4 [special]), each such constructor is trivial, and the destructor of X is either trivial or deleted, implementations are permitted to create a temporary object to hold the function parameter or result object. The temporary object is constructed from the function argument or return value, respectively, and the function's parameter or return object is initialized as if by using the eligible trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object). [Note: This latitude is granted to allow objects of class type to be passed to or returned from functions in registers. —end note]
This requirement is observable, since the destructor of X in the example could inspect the value of a.
The permissions in this paragraph should also apply to all non-class types.
Possible resolution:
Change in 6.7.7 [class.temporary] paragraph 3 as follows:
When an object of class type X is passed to or returned from a function, if X has at least one eligible copy or move constructor (11.4.4 [special]), each such constructor is trivial, and the destructor of X is either trivial or deleted, implementations are permitted to create a temporary object to hold the function parameter or result object. The temporary object is constructed from the function argument or return value, respectively, and the function's parameter or return object is initialized as if by using the eligible trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object). Similarly, implementations are permitted to create a temporary object of scalar type to hold a function parameter or result object. [Note 4: This latitude is granted to allow objects of scalar or class type to be passed to or returned from functions in registers. —end note]
(From editorial issue 4893.)
It is unclear whether the representation of unsigned integral types is unspecified or implementation-defined.
Proposed resolution [SUPERSEDED]:
Change in 6.8.2 [basic.fundamental] paragraph 17 as follows:
The types described in this subclause are called fundamental types. The representation of a fundamental type is unspecified except as stated in this subclause.
CWG 2023-12-01
Since there is no requirement for "implementation-defined" in the specification, the representation is unspecified by omission. It was noted that "unspecified behavior" has well-defined (and narrow) meaning, whereas the not-specified property here is static throughout the instance of the abstract machine. In that sense, it is similar to implementation-defined, abent the documentation requirement. It might be worthwhile to have a defined term for such properties.
Proposed resolution:
Change in 6.8.2 [basic.fundamental] paragraph 17 as follows:
The types described in this subclause are called fundamental types. The representation of a fundamental type is implementation-defined, subject to the constraints in this subclause.
Subclause 6.9.2.2 [intro.races] paragraph 13 specifies:
A visible side effect A on a scalar object or bit-field M with respect to a value computation B of M satisfies the conditions:The value of a non-atomic scalar object or bit-field M, as determined by evaluation B, shall be the value stored by the visible side effect A.
- A happens before B and
- there is no other side effect X to M such that A happens before X and X happens before B.
However, a side effect is defined as 6.9.1 [intro.execution] paragraph 7:
Reading an object designated by a volatile glvalue (7.2.1 [basic.lval]), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.
It seems that initialization of an object is not a side effect, and thus the value of an scalar object can never be the value obtained during initialization.
Proposed resolution:
Change in 6.9.1 [intro.execution] paragraph 7 as follows:
Reading an object designated by a volatile glvalue (7.2.1 [basic.lval]), modifying an object (including initialization), calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment. ...
Subclause 6.9.2.3 [intro.progress] paragraph 1 specifies:
The implementation may assume that any thread will eventually do one of the following:
- terminate,
- make a call to a library I/O function,
- perform an access through a volatile glvalue, or
- perform a synchronization operation or an atomic operation.
This formulation has two issues. First, what are the consequences of the user violation the assumption? It should be clearly specified that undefined behavior results. Second, what does "eventually" mean? The wording gives the impression that the requirement is satisfied if a thread performs some I/O operation at its beginning and then goes into an empty infinite loop.
It was also noted that that 6.9.2.3 [intro.progress] paragraph 3 is a near-duplicate of paragraph 1.
Possible resolution:
Remove 6.9.2.3 [intro.progress] paragraph 1:
The implementation may assume that any thread will eventually do one of the following:[Note 1: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven. —end note]
- terminate,
- make a call to a library I/O function,
- perform an access through a volatile glvalue, or
- perform a synchronization operation or an atomic operation.
Change in 6.9.2.3 [intro.progress] paragraph 5 as follows:
The behavior is undefined if a thread of execution that has not terminated stops making execution steps. [Note: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven.Because of this and the preceding requirement regarding what threads of execution have to perform eventually, itIt follows that no thread of execution can execute forever without an execution step occurring. —end note]
The concept of odr-use is a static program property, independent of runtime control flow. For example,
void f() {
if (false)
g(); // unconditionally odr-uses g
}
Yet, some parts of the standard talk about an odr-use participating in the happens-before relation, which is a category error.
Proposed resolution [SUPERSEDED]:
Change in 6.3 [basic.def.odr] paragraph 6 through 8 as follows:
A structured binding is odr-used
if it appears asby a potentially-evaluated expression E if it is named by E.*this is odr-used if this appears as a potentially-evaluated expression (including as the result of the implicit transformation in the body of a non-static member function (11.4.3 [class.mfct.non.static])).
A virtual member function is odr-used if it is not pure. A function is odr-used by
if it is named bya potentially-evaluated expression or conversion E if it is named by E. A non-placement allocation or deallocation function for a class is odr-used by the definition of a constructor of that class. A non-placement deallocation function for a class is odr-used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor (11.4.7 [class.dtor]). [ Footnote: ... ]
Change in 6.9.3.3 [basic.start.dynamic] paragraph 4 as follows:
A non-initialization odr-use of an entity is the evaluation of an expression that odr-usesan odr-use(6.3 [basic.def.odr]) the entity where the evaluation is not caused directly or indirectly by the initialization of a non-block static or thread storage duration variable.
CWG 2024-01-19
Since "odr-use" is a category error here, use the term "non-initialization use" instead.
Possible resolution:
Change in 6.3 [basic.def.odr] paragraph 6 through 8 as follows:
A structured binding is odr-used
if it appears asby a potentially-evaluated expression E if it is named by E.*this is odr-used if this appears as a potentially-evaluated expression (including as the result of the implicit transformation in the body of a non-static member function (11.4.3 [class.mfct.non.static])).
A virtual member function is odr-used if it is not pure. A function is odr-used by
if it is named bya potentially-evaluated expression or conversion E if it is named by E. A non-placement allocation or deallocation function for a class is odr-used by the definition of a constructor of that class. A non-placement deallocation function for a class is odr-used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor (11.4.7 [class.dtor]). [ Footnote: ... ]
Change in 6.9.3.3 [basic.start.dynamic] paragraph 4 through 7 as follows:
A non-initialization
odr-useuse of an entity is the evaluation of an expression that odr-usesan odr-use(6.3 [basic.def.odr]) the entity where the evaluation is not caused directly or indirectly by the initialization of a non-block static or thread storage duration variable....If it is deferred, it strongly happens before any non-initialization
odr-useuse of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized. [ Footnote: ... ] ... [ Example: ... It is implementation-defined whether either a or b is initialized before main is entered or whether the initializations are delayed until a is firstodr-usedused in main. In particular, if a is initialized before main is entered, it is not guaranteed that b will be initialized before it isodr-usedused by the initialization of a, that is, before A::A is called. If, however, a is initialized at some point after the first statement of main, b will be initialized prior to its use in A::A. -- end example ]... If it is deferred, it strongly happens before any non-initialization
odr-useuse of that variable. ...... If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization
odr-useuse by t of any non-inline variable with thread storage duration ...
(From submission #553.)
Subclause 7.5.5.1 [expr.prim.id.general] paragraph 2 specifies:
If an id-expression E denotes a non-static non-type member of some class C at a point where the current class (7.5.3 [expr.prim.this]) is X andthe id-expression is transformed into a class member access expression using (*this) as the object expression.
- E is potentially evaluated or C is X or a base class of X, and
- E is not the id-expression of a class member access expression (7.6.1.5 [expr.ref]), and
- if E is a qualified-id, E is not the un-parenthesized operand of the unary & operator (7.6.2.2 [expr.unary.op]),
This rule transforms the following valid code (according to the example in 7.5.3 [expr.prim.this] paragraph 5) into invalid code:
struct A { int x; int a[sizeof(x)]; decltype(x) f(); };
This is a regression introduced by P1787R6 (Declarations and where to find them).
Proposed resolution (reviewed by CWG 2024-06-14) [SUPERSEDED]:
Change in 7.5.5.1 [expr.prim.id.general] paragraph 2 as follows:
If an id-expression E denotes a non-static non-type member of some class C at a point where the current class (7.5.3 [expr.prim.this]) is X andthe id-expression is transformed into a class member access expression using (*this) as the object expression.
- the point is in a context where 7.5.3 [expr.prim.this] specifies a type for this, and
- E is potentially evaluated or C is X or a base class of X, and
- E is not the id-expression of a class member access expression (7.6.1.5 [expr.ref]), and
- if E is a qualified-id, E is not the un-parenthesized operand of the unary & operator (7.6.2.2 [expr.unary.op]),
Additional notes (June, 2024)
Unevaluated uses in static member functions are valid (and should not be transformed). However, the resolution above does not address the status quo which does perform the transformation.
Possible resolution:
Change in 7.5.5.1 [expr.prim.id.general] paragraph 2 through 4 as follows:
If an id-expression E denotes a non-static non-type member of some class Cat a point where the current class (7.5.3 [expr.prim.this]) is X and, the following rules apply. E is said to refer to a local member of some class X if E appears at a point where X is the current class and C is X or a base class of X. E has the following associated cv-qualification relative to some class X:
- If E appears in a function-definition, member-declarator, or declarator that declares a member function or member function template of X, at a point after the optional cv-qualifier-seq, the associated cv-qualification is the cv-qualifier-seq, if present, and otherwise empty.
- Otherwise, if E appears in a member-declarator declaring a non-static data member of X, at a point within the optional default member initializer, the associated cv-qualification is empty.
- Otherwise, E has no associated cv-qualification.
E has the following meaning:
- If E is the id-expression of a class member access, see 7.6.1.5 [expr.ref].
- Otherwise, if E is a qualified-id that is the un-parenthesized operand of the unary & operator, see 7.6.2.2 [expr.unary.op].
- Otherwise, if E denotes a non-static data member with declared type T and E is not potentially evaluated, the result is an lvalue of type cv T where cv is empty unless E refers to a local member of some class X, in which case cv is the associated cv-qualification relative to X (if any). [ Example:
template<int n> class R { }; struct S { int m; char g(int&); // #1 int g(const int&); // #2 auto f() const -> R<sizeof(g(m))>; // OK, return type is R<sizeof(int)> }; int j = sizeof(S::m + 42); // OK-- end example ]- Otherwise, if E is refers to a local member of some class X:
- If E appears in an implicit object member function, is potentially evaluated, and has a (possibly empty) associated cv-qualification relative to X, E is transformed into a class member access expression using (*this) as the object expression. This transformation does not apply in the template definition context (13.8.3.2 [temp.dep.type]).
- Otherwise, E shall not be potentially evaluated; the type and value category of the result is as specified in 7.5.5.2 [expr.prim.id.unqual] and 7.5.5.3 [expr.prim.id.qual].
- Otherwise, if E denotes an explicit object member funtion or a member enumerator, the result is as specified in 7.5.5.2 [expr.prim.id.unqual] and 7.5.5.3 [expr.prim.id.qual].
- Otherwise, the program is ill-formed.
- E is potentially evaluated or C is X or a base class of X, and
- E is not the id-expression of a class member access expression (7.6.1.5 [expr.ref]), and
- if E is a qualified-id, E is not the un-parenthesized operand of the unary & operator (7.6.2.2 [expr.unary.op]),
the id-expression is transformed into a class member access expression using (*this) as the object expression. [Note 2: If C is not X or a base class of X, the class member access expression is ill-formed. Also, if the id-expression occurs within a static or explicit object member function, the class member access is ill-formed. —end note] This transformation does not apply in the template definition context (13.8.3.2 [temp.dep.type]).
If an id-expression E denotes a member M of an anonymous union (11.5.2 [class.union.anon]) U:
- If U is a non-static data member, E refers to M as a member of the lookup context of the terminal name of E (after any implicit transformation to a class member access expression). [Example 1: o.x is interpreted as o.u.x, where u names the anonymous union member. —end example]
- Otherwise, E is interpreted as a class member access (7.6.1.5 [expr.ref]) that designates the member subobject M of the anonymous union variable for U. [Note 3: Under this interpretation, E no longer denotes a non-static data member. —end note] [Example 2: N::x is interpreted as N::u.x, where u names the anonymous union variable. —end example]
An id-expression that denotes a non-static data member or implicit object member function of a class can only be used:
- as part of a class member access (after any implicit transformation (see above)) in which the object expression refers to the member's class or a class derived from that class, or
- to form a pointer to member (7.6.2.2 [expr.unary.op]), or
- if that id-expression denotes a non-static data member and it appears in an unevaluated operand. [Example 3:
struct S { int m; }; int i = sizeof(S::m); // OK int j = sizeof(S::m + 42); // OK—end example]
Change in 7.6.1.5 [expr.ref] paragraph 5 as follow:
Otherwise, the object expression shall be of class type. The class type shall be complete unless the class member access appears in the definition of that class. E2 shall denote a member of that class. [Note 3: The program is ill-formed if the result differs from that when the class is complete (6.5.2 [class.member.lookup]). —end note] [Note 4: 6.5.5 [basic.lookup.qual] describes how names are looked up after the . and -> operators. —end note]
Subclause 7.5.5.2 [expr.prim.id.unqual] paragraph 1 has a note that lacks a corresponding normative specification:
[Note 1: For operator-function-ids, see 12.4 [over.oper]; for conversion-function-ids, see 11.4.8.3 [class.conv.fct]; for literal-operator-ids, see 12.6 [over.literal]; for template-ids, see 13.3 [temp.names]. A type-name or decltype-specifier prefixed by ~ denotes the destructor of the type so named; see 7.5.5.5 [expr.prim.id.dtor]. Within the definition of a non-static member function, an identifier that names a non-static member is transformed to a class member access expression (11.4.3 [class.mfct.non.static]). —end note]
Proposed resolution:
Change in 7.5.5.2 [expr.prim.id.unqual] paragraph 1 as follows:
A type-name or decltype-specifier prefixed by ~ denotes the destructor of the named type; see 7.5.5.5 [expr.prim.id.dtor]. [Note 1: For operator-function-ids, see 12.4 [over.oper]; for conversion-function-ids, see 11.4.8.3 [class.conv.fct]; for literal-operator-ids, see 12.6 [over.literal]; for template-ids, see 13.3 [temp.names].A type-name or decltype-specifier prefixed by ~ denotes the destructor of the type so named; see 7.5.5.5 [expr.prim.id.dtor].Within the definition of a non-static member function, an identifier that names a non-static member is transformed to a class member access expression (11.4.3 [class.mfct.non.static]). —end note]
Consider:
void f() { X x; // Is x an lvalue or an xvalue here? void g(int n = (decltype((throw x, 0))())); // status quo: x is move-eligible here } void f() { X x; struct A { void g() { try { struct Y { // Is x an lvalue or an xvalue here? void h(int n = (decltype((throw x, 0))())); }; } catch (...) { } } }; }
11.9.6 [class.copy.elision] paragraph 3 specifies:
An implicitly movable entity is a variable of automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type. In the following copy-initialization contexts, a move operation is first considered before attempting a copy operation:
- ...
- if the operand of a throw-expression (7.6.18 [expr.throw]) is a (possibly parenthesized) id-expression that names an implicitly movable entity that belongs to a scope that does not contain the compound-statement of the innermost try-block or function-try-block (if any) whose compound-statement or ctor-initializer contains the throw-expression,
Thus, in the first example above, x is treated as an xvalue, but it is treated as an lvalue in the second example. This outcome is surprising.
(P2266R2 (Simpler implicit move) moved this wording, introduced by P1825R0 (Merged wording for P0527R1 and P1155R3), from 11.9.6 [class.copy.elision] to 7.5.5.2 [expr.prim.id.unqual].)
Proposed resolution:
Change in 7.5.5.2 [expr.prim.id.unqual] paragraph 4:
An implicitly movable entity is a variableofwith automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type.In the following contexts, anAn id-expression is move-eligible:if
- it names an implicitly movable entity declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression and
Iftheid-expression(possibly parenthesized) id-expression is the operand of
- a return or co_return statement
, and names an implicitly movable entity declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expressionorif the id-expression (possibly parenthesized) is the operand ofa potentially-evaluated throw-expression, and names an implicitly movable entity that belongs to a scope that does not contain the compound-statement of the innermost lambda-expression, try-block, or function-try-block (if any) whose compound-statement or ctor-initializer encloses the throw-expressionwhere no try-block or function-try-block intervenes between the declaration of the entity and the innermost enclosing scope of the throw-expression.
Consider:
struct S {}; const S f(); auto x = [&ref = f()] { return ref; }
Subclause 7.5.6.3 [expr.prim.lambda.capture] paragraph 6 specifies:
An init-capture inhabits the lambda scope (6.4.5 [basic.scope.lambda]) of the lambda-expression. An init-capture without ellipsis behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;”, except that:
- if the capture is by copy (see below), the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same object, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and
- if the capture is by reference, the variable's lifetime ends when the closure object's lifetime ends.
It is unclear whether the temporary returned by f() is lifetime-extended by being bound to a reference init-capture.
There is implementation divergence: gcc rejects the example; clang accepts and extends the lifetime of the temporary; MSVC and EDG accept and do not extend the lifetime.
Proposed resolution (reviewed by CWG 2024-03-01):
Change in 7.5.6.3 [expr.prim.lambda.capture] paragraph 6 as follows:
An init-capture inhabits the lambda scope (6.4.5 [basic.scope.lambda]) of the lambda-expression. An init-capture without ellipsis behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;”, except that:
- if the capture is by copy (see below), the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same object, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and
- if the capture is by reference and the reference would bind to an object that is the result of a temporary expression, the program is ill-formed, and
- if the capture is by reference, the variable's lifetime ends when the closure object's lifetime ends.
(From submission #536.)
For T{...}, the rule in 7.6.1.4 [expr.type.conv] paragraph 2 yields a prvalue of reference type if T is a reference type:
... Otherwise, the expression is a prvalue of the specified type whose result object is direct-initialized (9.4) with the initializer. ...
Also, it should be clarified that void(1, 2) and void{1} are ill-formed.
Possible resolution [SUPERSEDED]:
Change in 7.6.1.4 [expr.type.conv] paragraph 1 and 2 as follows:
A simple-type-specifier (9.2.9.3 [dcl.type.simple]) or typename-specifier (13.8 [temp.res]) followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer. If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by overload resolution for class template deduction (12.2.2.9 [over.match.class.deduct]) for the remainder of this subclause. Otherwise, if the type contains a placeholder type, it is replaced by the type determined by placeholder type deduction (9.2.9.7.2 [dcl.type.auto.deduct]). Let T denote the resulting type. [ Example: ... ]
If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression (7.6.3 [expr.cast]). Otherwise, if
the typeT is cv voidand, the initializerisshall be () or {} (after pack expansion, if any), and the expression is a prvalue of type void that performs no initialization. Otherwise, the expressionis a prvalue of the specified type whose result object is direct-initialized (9.4 [dcl.init]) with the initializerhas the same effect as direct-initializing an invented variable t of type T from the initializer and then using t as the result of the expression. The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type, an xvalue if T is an rvalue reference to object type, and a prvalue otherwise. If the initializer is a parenthesized optional expression-list,the specified typeT shall not be an array type.
CWG 2024-06-14
The resolution above introduces an additional variable even for a prvalue, which defeats mandatory copy elision. Use the pattern from 7.6.1.9 [expr.static.cast] paragraph 4 instead.
Possible resolution:
Change in 7.6.1.4 [expr.type.conv] paragraph 1 and 2 as follows:
A simple-type-specifier (9.2.9.3 [dcl.type.simple]) or typename-specifier (13.8 [temp.res]) followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer. If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by overload resolution for class template deduction (12.2.2.9 [over.match.class.deduct]) for the remainder of this subclause. Otherwise, if the type contains a placeholder type, it is replaced by the type determined by placeholder type deduction (9.2.9.7.2 [dcl.type.auto.deduct]). Let T denote the resulting type. [ Example: ... ]
If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression (7.6.3 [expr.cast]). Otherwise, if
the typeT is cv voidand, the initializerisshall be () or {} (after pack expansion, if any), and the expression is a prvalue of type void that performs no initialization. Otherwise, if T is a reference type, the expression has the same effect as direct-initializing an invented variable t of type T from the initializer and then using t as the result of the expression; the result is an lvalue if T is an lvalue reference type or an rvalue reference to function type and an xvalue otherwise. Otherwise, the expression is a prvalue ofthe specifiedtype T whose result object is direct-initialized (9.4 [dcl.init]) with the initializer. If the initializer is a parenthesized optional expression-list,the specified typeT shall not be an array type.
Initialization of an object may terminate via an exception, in which case any dynamically-allocated memory is freed, per 7.6.2.8 [expr.new] paragraph 26:
If any part of the object initialization described above [ Footnote: ... ] terminates by throwing an exception and a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed, after which the exception continues to propagate in the context of the new-expression. If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed.
However, implementations do not consistently support this provision in case the exception remains uncaught:
#include <iostream> struct C { void* operator new(std::size_t n) { std::cout << "malloc\n"; return malloc(n); } void operator delete(void* ptr) { std::cout << "free\n"; free(ptr); } C() { throw 0; } }; int main() { auto ptr = new C; }
Both clang and GCC do not free the memory in this example; they do so if the exception is caught in main.
Maybe a similar provision as used for stack unwinding in 14.4 [except.handle] paragraph 9 is desirable:
If no matching handler is found, the function std::terminate is invoked; whether or not the stack is unwound before this invocation of std::terminate is implementation-defined (14.6.2 [except.terminate]).
Suggested resolution:
Integrate freeing dynamically-allocated memory with stack unwinding (14.3 [except.ctor]), since this is what implementations actually do.
Possible resolution:
Change in 7.6.2.8 [expr.new] paragraph 26, 27, and 28 as follows:
If any part of the object initialization described above [ Footnote: This can include evaluating a new-initializer and/or calling a constructor. ] terminates by throwing an exception and a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed, after which the exception continues to propagate in the context of the new-expression. If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed. [Note 13: This is appropriate when the called allocation function does not allocate memory; otherwise, it is likely to result in a memory leak. —end note]For purposes of stack unwinding (14.3 [except.ctor]), the matching deallocation function is determined as follows: If the new-expression does not begin with a unary :: operator and the allocated type is a class type T or an array thereof, a search is performed for the deallocation function's name in the scope of T. Otherwise, or if nothing is found, the deallocation function's name is looked up by searching for it in the global scope.
A declaration of a placement deallocation function matches the declaration of a placement allocation function if it has the same number of parameters and, after parameter transformations (9.3.4.6 [dcl.fct]), all parameter types except the first are identical. If the lookup finds a single matching deallocation function, that function
will be calledis the matching deallocation function; otherwise,no deallocation function will be calledthere is no matching deallocation function. If the lookup finds a usual deallocation function and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed. For a non-placement allocation function, the normal deallocation function lookup is used to find the matching deallocation function (7.6.2.9 [expr.delete]).
Change in 14.3 [except.ctor] paragraph 1 as follows:
As control passes from the point where an exception is thrown to a handler, objects are destroyed and deallocation functions are invoked by a process, specified in this subclause, called stack unwinding.
Change in 14.3 [except.ctor] paragraph 5 as follows:
[Note 4: If the object was allocated by a new-expression (7.6.2.8 [expr.new]),If the evaluation of a new-expression other than the invocation of the allocation function is terminated by an exception, the matching deallocation function(6.7.5.5.3 [basic.stc.dynamic.deallocation]), if any, is called (7.6.2.8 [expr.new])to free the storage occupied by the object.—end note]
Consider:
struct A {}; struct AA : A { int y; }; struct B : A { int x; }; struct C : AA, B {}; constexpr int f(const A &a) { int A::*mp = static_cast<int A::*>(&B::x); return a.*mp; } extern char x[f(static_cast<const AA &>(C{{{}, 13}, {{}, 42}}))]; extern char x[13];
Subclause 7.6.4 [expr.mptr.oper] paragraph 4 specifies:
Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression. If the dynamic type of E1 does not contain the member to which E2 refers, the behavior is undefined.
In the example, the dynamic type of a is C, which does contain B::x, and the undefined behavior provision does not trigger. Thus the call to f is required to yield 42; however common implementations produce 13. The behavior for this case ought to be undefined.
Suggested resolution:
Change in 7.6.4 [expr.mptr.oper] paragraph 4 as follows:
Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression.If the dynamic type of E1 does not contain the member to which E2 refers,Where the type of E2 is "pointer to member of T", C is the (unique) class of which the member to which E2 refers is a direct member, and B is the object of type T that either is the result of E1 or is the uniquely so-typed base subobject thereof, if B is neither of type C nor a base class subobject of an object of type C, then the behavior is undefined.
Consider:
struct Foo {};
struct Bar {
mutable Foo f;
};
constexpr Bar b{}; // #1
In C++20, Foo and Bar are literal types, and #1 is well-formed. In C++23, the requirement for the variable b changed to "constant destruction". However, Bar does not have constant destruction, because its mutable member violates 7.7 [expr.const] bullet 5.17:
- ...
- an invocation of a destructor (11.4.7 [class.dtor]) or a function call whose postfix-expression names a pseudo-destructor (7.6.1.3 [expr.call]), in either case for an object whose lifetime did not begin within the evaluation of E;
- ...
This is because the mutable member is considered to have its lifetime begin outside of E per 7.7 [expr.const] paragraph 9:
An object a is said to have constant destruction if:
- it is not of class type nor (possibly multidimensional) array thereof, or
- it is of class type or (possibly multidimensional) array thereof, that class type has a constexpr destructor, and for a hypothetical expression E whose only effect is to destroy a, E would be a core constant expression if the lifetime of a and its non-mutable subobjects (but not its mutable subobjects) were considered to start within E.
Proposed resolution (reviewed by CWG 2024-03-01):
Change in 7.7 [expr.const] paragraph 10 as follows:
An object a is said to have constant destruction if:
- it is not of class type nor (possibly multidimensional) array thereof, or
- it is of class type or (possibly multidimensional) array thereof, that class type has a constexpr destructor, and for a hypothetical expression E whose only effect is to destroy a, E would be a core constant expression if any invocation of a trivial destructor for a mutable subobject is ignored and if the lifetime of a and its non-mutable subobjects (but not its mutable subobjects) were considered to start within E.
Consider:
template <typename T> struct A { T data; static const A a; }; template <typename T> inline constexpr A<T> A<T>::a {42}; static_assert(A<int>::a.data == 42);
This ought to be well-formed, but there is no rule that would cause instantiation of A<int>::a.
Also consider:
template<typename T> struct A { static T x; }; template<typename T> T A<T>::x = (std::cout << "constructed", T()); template<typename T> void b(decltype(&A<int>::x));
For the second example, it is undesirable to instantiate A<int>.
Possible resolution [SUPERSEDED]:
Change in 7.7 [expr.const] paragraph 3 as follows:
A variable is potentially-constant if it is constexpr or it has reference or non-volatile const-qualified integral or enumeration type or, for a templated variable, if the definition that would be instantiated if the variable were needed for constant evaluation uses the constexpr decl-specifier (13.7.6.2 [temp.spec.partial.match]).
CWG 2023-10-20
The phrasing suggests that const int variables instantiated from a variable template are no longer potentially-constant.
Possible resolution:
Change in 7.7 [expr.const] paragraph 3 as follows:
A variable is potentially-constant if it isconstexprdeclared with the constexpr decl-specifier or it has reference or non-volatile const-qualified integral or enumeration type. Such a variable may be instantiated from a templated variable, in which case the definition that would be instantiated if the variable were needed for constant evaluation is considered (13.7.6.2 [temp.spec.partial.match]).
The status of the following example is not clear:
template <typename T> auto foo(T); // Not defined
template <typename T> struct FooCallable {
template<class U>
static constexpr bool check_foo_callable(...) { return false; }
template<class U, class = decltype(foo(U{})) >
static constexpr bool check_foo_callable(int) { return true; }
static constexpr bool value = check_foo_callable<T>(0);
};
static_assert(FooCallable<int>::value == false, "");
The static_assert causes the evaluation of the default template argument decltype(foo<int>(int{})). However, foo is not defined, leaving it with an undeduced placeholder return type. This situation could conceivably be handled in two different ways. According to 9.2.9.7 [dcl.spec.auto] paragraph 9,
If the name of an entity with an undeduced placeholder type appears in an expression, the program is ill-formed.
This would thus appear to be an invalid expression resulting from substitution in the immediate context of the declaration and thus a substitution failure.
The other alternative would be to treat the presence of an undeduced placeholder type for a function template as satisfying the requirements of 13.9.2 [temp.inst] paragraph 4,
Unless a function template specialization has been explicitly instantiated or explicitly specialized, the function template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist or if the existence of the definition affects the semantics of the program.
and attempt to instantiate foo<int>. That instantiation fails because the definition is not provided, which would then be an error outside the immediate context of the declaration and thus a hard error instead of substitution failure.
CWG 2022-11-10
There is no implementation divergence on the handling of this example.
Possible resolution:
Change in 9.2.9.7.1 [dcl.spec.auto.general] paragraph 11 as follows:
If a variable or function with an undeduced placeholder type is named
by an expression (6.3 [basic.def.odr]), the program is
ill-formed. Once a non-discarded return statement has been seen in a
function, however, the return type deduced from that statement can be
used in the rest of the function, including in other return
statements.
[ Example:
...
template <typename T> auto f(T); // not defined
template <typename T> struct F {
template<class U>
static constexpr bool g(...) { return false; }
template<class U, class = decltype(f(U{})) >
static constexpr bool g(int) { return true; }
static constexpr bool value = g<T>(0);
};
static_assert(F<int>::value == false, "");
-- end example ]
According to 9.3.4.6 [dcl.fct] paragraph 5, top-level cv-qualifiers on parameter types are deleted when determining the function type. It is not clear how or whether this adjustment should be applied to parameters of function templates when the parameter has a dependent type, however. For example:
template<class T> struct A { typedef T arr[3]; }; template<class T> void f(const typename A<T>::arr) { } // #1 template void f<int>(const A<int>::arr); template <class T> struct B { void g(T); }; template <class T> void B<T>::g(const T) { } // #2
If the const in #1 is dropped, f<int> has a parameter type of A* rather than the const A* specified in the explicit instantiation. If the const in #2 is not dropped, we fail to match the definition of B::g to its declaration.
Rationale (November, 2010):
The CWG agreed that this behavior is intrinsic to the different ways cv-qualification applies to array types and non-array types.
Notes, January, 2012:
Additional discussion of this issue arose regarding the following example:
template<class T> struct A { typedef double Point[2]; virtual double calculate(const Point point) const = 0; }; template<class T> struct B : public A<T> { virtual double calculate(const typename A<T>::Point point) const { return point[0]; } }; int main() { B<int> b; return 0; }
The question is whether the member function in B<int> has the same type as that in A<int>: is the parameter-type-list instantiated directly (i.e., using the adjusted types) or regenerated from the individual parameter types?
(See also issue 1322.)
Additional notes (February, 2024)
There is implementation divergence for the following example (clang and EDG accept, gcc and MSVC reject):
template<typename U> int f(const U t); int v; auto test = f(v); template<typename U> int f(U t) { static_assert(std::is_const<decltype(t)>::value, "t should be non-const"); return 0; }
Possible resolution:
Change in 9.3.4.6 [dcl.fct] paragraph 6 as follows:
The type of a function is determined using the following rules. The type of each parameter (including function parameter packs) is determined from its own parameter-declaration (9.3 [dcl.decl]). After determining the type of each parameter, any parameter of type “array of T” or of function type T is adjusted to be “pointer to T”. After producing the list of parameter types, any top-level cv-qualifiers modifying a non-dependent parameter type are deleted when forming the function type. The resulting list of transformed parameter types and the presence or absence of the ellipsis or a function parameter pack is the function's parameter-type-list. ...
Subclause 9.3.4.6 [dcl.fct] paragraph 6 specifies
A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared static or virtual.
This does not address the situation when an explicit object member function becomes implicitly virtual by overriding an implicit object member function. That should be prevented.
This also does not address class-specific allocation and deallocation functions, which are implicitly static.
Proposed resolution (approved by CWG 2023-06-15) [SUPERSEDED]:
Change in 9.3.4.6 [dcl.fct] paragraph 6 as follows:
A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared staticor virtual.
Change in 9.3.4.6 [dcl.fct] paragraph 7 as follows:
... An implicit object member function is a non-static member function without an explicit object parameter. [ Note: An explicit object member function cannot be virtual (11.7.3 [class.virtual]). -- end note ]
Add a new paragraph before 11.7.3 [class.virtual] paragraph 7 as follows:
A virtual function shall not be an explicit object member function (9.3.4.6 [dcl.fct]).
[ Example:
struct B { virtual void g(); // #1 }; struct D : B { virtual void f(this D&); // error: explicit object member function cannot be virtual void g(this D&); // error: overrides #1; explicit object member function cannot be virtual };-- end example]
The ref-qualifier, or lack thereof, ...
Proposed resolution (approved by CWG 2023-07-14):
Change in 9.3.4.6 [dcl.fct] paragraph 6 as follows:
A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared staticor virtual.
Change in 9.3.4.6 [dcl.fct] paragraph 7 as follows:
... An implicit object member function is a non-static member function without an explicit object parameter. [ Note: An explicit object member function cannot be virtual (11.7.3 [class.virtual]). -- end note ]
Change in 11.4.11 [class.free] paragraph 1 as follows:
Any allocation function for a class T is a static member (even if not explicitly declared static); it shall not have an explicit object parameter.
Change in 11.4.11 [class.free] paragraph 3 as follows:
Any deallocation function for a class T is a static member (even if not explicitly declared static); it shall not have an explicit object parameter.
Change in 11.4.11 [class.free] paragraph 4 as follows:
[ Note: Since member allocation and deallocation functions are static they cannot be virtual. -- end note ]
Add a new paragraph before 11.7.3 [class.virtual] paragraph 7 as follows:
A virtual function shall not be an explicit object member function (9.3.4.6 [dcl.fct]).
[ Example:
struct B { virtual void g(); // #1 }; struct D : B { virtual void f(this D&); // error: explicit object member function cannot be virtual void g(this D&); // error: overrides #1; explicit object member function cannot be virtual };-- end example]
The ref-qualifier, or lack thereof, ...
CWG 2023-11-09
Progress of this issue is blocked on issue 2554.
(From submission #521.)
Consider:
template <class T> struct A { A() requires (false) = default; A() : t(42) {} T t; }; struct B : A<int> {};
According to the current wording, class B has a trivial default constructor, which is undesirable.
Proposed resolution (approved by CWG 2024-05-31):
Change in 11.4.5.2 [class.default.ctor] paragraph 3 as follows:
A default constructor for class X is trivial if it is not user-provided and if:Otherwise, the default constructor is non-trivial.
its classX has no virtual functions (11.7.3 [class.virtual]) and no virtual base classes (11.7.2 [class.mi]), and- no non-static data member of
its classX has a default member initializer (11.4 [class.mem]), and- all the direct base classes of its class have trivial default constructors, and
- the constructor selected to default-initialize each direct base class subobject of X is trivial,
- for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.
- for each direct non-static data member of X that is of class type (or array thereof), the constructor selected to default-initialize that member (or an array element of that member) is trivial.
Describing the handling of static data members with brace-or-equal-initializers, 11.4.9.3 [class.static.data] paragraph 4 says,
The member shall still be defined in a namespace scope if it is odr-used (6.3 [basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.
The word “shall” implies a required diagnostic, but this is describing an ODR violation (the static data member might be defined in a different translation unit) and thus should be “no diagnostic required.”
Additional notes (March, 2024)
This paragraph has more issues and is largely redundant with 6.2 [basic.def] paragraph 2.3 and paragraph 2.4.
Possible resolution:
Change 11.4.9.3 [class.static.data] paragraph 4 as follows:
If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression (7.7 [expr.const]). The member shall still be defined in a namespace scope if it is odr-used (6.3 [basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer . The declaration of an inline static data member (which is a definition) may specify a brace-or-equal-initializer . If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see D.5 [depr.static.constexpr]). Declarations of other static data members shall not specify a brace-or-equal-initializer.
A declaration of a static data member in a class definition shall not specify a brace-or-equal-initializer unless the member is either inline or of non-volatile const integral or enumeration type. In the latter case, the member shall be constant-initialized (7.7 [expr.const]).
Subclause 11.6 [class.local] paragraph 3 establishes restrictions on the definition of classes nested within local classes, but it is unclear which restrictions exist for other members of local classes.
Possible resolution [SUPERSEDED]:
Change in 11.6 [class.local] paragraph 3 as follows:
If class X is a local class, a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in the same scope as the definition of class X.A class nested within a local class is a local class. A member of a local class X shall be declared only in the definition of X or the nearest enclosing block scope of X.
CWG 2024-06-14
The implementation status quo is that no members of local classes other than nested classes can be defined at block scope.
Possible resolution:
Change in 11.6 [class.local] paragraph 3 as follows:
If class X is a local class, a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in the same scope as the definition of class X.A class nested within a local class is a local class. A member of a local class X shall be declared only in the definition of X or, if the member is a nested class, in the nearest enclosing block scope of X.
Consider:
struct B { virtual void f(); // #1 }; struct D : B { void f(); // #2 };
Subclause 11.7.3 [class.virtual] paragraph 2 says:
If a virtual member function F is declared in a class B, and, in a class D derived (directly or indirectly) from B, a declaration of a member function G corresponds (6.4.1 [basic.scope.scope]) to a declaration of F, ignoring trailing requires-clauses, then G overrides [ Footnote: ... ] F .
Subclause 6.4.1 [basic.scope.scope] paragraph 4 defines "corresponds" as follows:
Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless
- ...
- each declares a function or function template, except when
- both declare functions with the same non-object-parameter-type-list, equivalent (13.7.7.2 [temp.over.link]) trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]), and, if both are non-static members, they have corresponding object parameters, or
- both declare function templates with...
Subclause 6.4.1 [basic.scope.scope] paragraph 3 defines "corresponding object parameters" as follows:
Two non-static member functions have corresponding object parameters if:
- exactly one is an implicit object member function with no ref-qualifier and the types of their object parameters (9.3.4.6 [dcl.fct]), after removing top-level references, are the same, or
- their object parameters have the same type.
In the example, B::f has an object parameter of type B, but D::f has an object parameter of type D. Thus, the two functions do not correspond, and thus D::f does not override B::f. That is an unintended alteration of the status quo ante.
See also issue 2553.
Proposed resolution:
Change in 11.7.3 [class.virtual] paragraph 2 as follows:
If a virtual member function F is declared in a class B, and, in a class D derived (directly or indirectly) from B, a declaration of a member function G corresponds (6.4.1 [basic.scope.scope]) to a declaration of F as if declared in D (12.2.2.1 [over.match.funcs.general]), ignoring trailing requires-clauses, and, if G is an explicit object member function, ignoring object parameters, and, if G is an implicit object member function, F and G have the same ref-qualifier (or absence thereof), then G overrides [ Footnote: ... ] F .
Remove 11.7.3 [class.virtual] paragraph 7 as follows:
The ref-qualifier , or lack thereof, of an overriding function shall be the same as that of the overridden function.
Subclause 6.7.3 [basic.life] paragraph 1 specifies:
... The lifetime of an object of type T begins when:except that ...
- storage with the proper alignment and size for type T is obtained, and
- its initialization (if any) is complete (including vacuous initialization) (9.4 [dcl.init]),
It is unclear whether initialization is considered complete when the (ultimate) target constructor completes, or when the outermost delegating constructor completes. Subclause 14.3 [except.ctor] paragraph 4 suggests it is the former:
If the compound-statement of the function-body of a delegating constructor for an object exits via an exception, the object's destructor is invoked. ...
Proposed resolution (approved by CWG 2023-07-14):
Split and change 11.9.3 [class.base.init] paragraph 9 as follows:
[Note 3: An abstract class ... -- end note ]
An attempt to initialize more than one non-static data member of a union renders the program ill-formed.[Note 4: After the call to a constructor for class X ... -- end note ] [Example 6: ... -- end example ]An attempt to initialize more than one non-static data member of a union renders the program ill-formed.
An object's initialization is considered complete when a non-delegating constructor for that object returns. [Note: Therefore, an object's lifetime can begin (6.7.3 [basic.life]) before all delegating constructors have completed. -- end note]
Change in 6.7.3 [basic.life] bullet 1.2 as follows:
... The lifetime of an object of type T begins when:except that ...
- storage with the proper alignment and size for type T is obtained, and
- its initialization (if any) is complete (including vacuous initialization) (9.4 [dcl.init], 11.9.3 [class.base.init]),
CWG 2023-10-20
Utterances about "during construction or destruction" in 11.9.5 [class.cdtor] need to be adjusted.
Possible resolution:
Split and change 11.9.3 [class.base.init] paragraph 9 as follows:
[Note 3: An abstract class ... -- end note ]
An attempt to initialize more than one non-static data member of a union renders the program ill-formed.[Note 4: After the call to a constructor for class X ... -- end note ] [Example 6: ... -- end example ]An attempt to initialize more than one non-static data member of a union renders the program ill-formed.
An object's initialization is considered complete when a non-delegating constructor for that object returns. [Note: Therefore, an object's lifetime can begin (6.7.3 [basic.life]) before all delegating constructors have completed. -- end note]
Change in 6.7.3 [basic.life] bullet 1.2 as follows:
... The lifetime of an object of type T begins when:except that ...
- storage with the proper alignment and size for type T is obtained, and
- its initialization (if any) is complete (including vacuous initialization) (9.4 [dcl.init], 11.9.3 [class.base.init]),
Change in 11.9.5 [class.cdtor] paragraph 2 as follows:
During theconstructioninitialization of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified.
Change in 11.9.5 [class.cdtor] paragraph 4 as follows:
Member functions, including virtual functions (11.7.3 [class.virtual]), can be called during construction or destruction (11.9.3 [class.base.init]). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during theconstructioninitialization or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x)under construction or destructionbeing initialized or destroyed, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access (7.6.1.5 [expr.ref]) and the object expression refers to the complete object of x or one of that object's base class subobjects but not x or one of its base class subobjects, the behavior is undefined.
Change in 11.9.5 [class.cdtor] paragraph 5 as follows:
The typeid operator (7.6.1.8 [expr.typeid]) can be used during construction or destruction (11.9.3 [class.base.init]). When typeid is used in a constructor (including the mem-initializer or default member initializer (11.4 [class.mem]) for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of typeid refers to the objectunder construction or destructionbeing initialized or destroyed, typeid yields the std::type_info object representing the constructor or destructor's class. If the operand of typeid refers to the objectunder construction or destructionbeing initialized or destroyed and the static type of the operand is neither the constructor or destructor's class nor one of its bases, the behavior is undefined.
Change in 11.9.5 [class.cdtor] paragraph 6 as follows:
dynamic_casts (7.6.1.7 [expr.dynamic.cast]) can be used during construction or destruction (11.9.3 [class.base.init]). When a dynamic_cast is used in a constructor (including the mem-initializer or default member initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of the dynamic_cast refers to the objectunder construction or destructionbeing initialized or destroyed, this object is considered to be a most derived object that has the type of the constructor or destructor's class. If the operand of the dynamic_cast refers to the objectunder construction or destructionbeing initialized or destroyed and the static type of the operand is not a pointer to or object of the constructor or destructor's own class or one of its bases, the dynamic_cast results in undefined behavior.
Subclause 6.7.3 [basic.life] paragraph 6 specifies:
... For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise, ...
However, the referenced subclause does not discuss deleting the object or deallocating its storage.
See also issue 2258.
Proposed resolution (reviewed by CWG 2023-10-20):
Insert before 11.9.5 [class.cdtor] paragraph 2 as follows:
During the initialization or destruction of an object, invoking the destructor for the object or releasing or reusing (6.7.3 [basic.life]) the storage which the object occupies results in undefined behavior. [ Note: Creating an object nested within some object o does not reuse the storage for o. -- end note ] [ Example:
struct A() { A() { ::operator delete(this); } }; A *p1 = new A; // undefined behavior struct B() { B() { delete this; } }; B *p2 = new B; // undefined behavior struct S { constexpr S() { this->~S(); } constexpr S(int) {} }; constexpr int f() { S s(0); s.~S(); std::construct_at(&s); // #1 return 0; } constexpr int x = f(); // error: undefined behavior at #1-- end example ]
During the construction of an object, if the value of the object ...
Change in 11.9.5 [class.cdtor] paragraph 4 as follows:
Member functions, including virtual functions (11.7.3 [class.virtual]),other than the destructor can be called during construction or destruction (11.9.3 [class.base.init]). When a virtual function (11.7.3 [class.virtual]) is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and ...
The resolution accepted for issue 2539 does not actually address the example in the issue, because overload resolution is never performed for expressions involved only built-in types.
Suggested resolution:
Change in 11.10.3 [class.spaceship] paragraph 1 as follows:
The synthesized three-way comparison of type R (17.11.2 [cmp.categories]) of glvalues a and b of the same type is defined as follows:
- If a <=> b is usable (11.10.1 [class.compare.default]) and can be explicitly converted to R using static_cast, static_cast<R>(a <=> b).
- Otherwise, if a <=> b is usable or overload resolution for a <=> b is performed and finds at least one viable candidate, the synthesized three-way comparison is not defined.
- Otherwise, ...
Subclause 12.2.2.3 [over.match.oper] paragraph 4 specifies:
A non-template function or function template F named operator== is a rewrite target with first operand o unless a search for the name operator!= in the scope S from the instantiation context of the operator expression finds a function or function template that would correspond (6.4.1 [basic.scope.scope]) to F if its name were operator==, where S is the scope of the class type of o if F is a class member, and the namespace scope of which F is a member otherwise. A function template specialization named operator== is a rewrite target if its function template is a rewrite target.
The meaning of "corresponds" has changed with paper P0847 such that this example from the Working Paper is now ill-formed:
struct B { bool operator==(const B&); }; struct C : B { C(); C(B); bool operator!=(const B&); }; bool c1 = B() == C(); // was OK, now ill-formed
The reason is that the definition of "corresponds" now also considers the object parameters, which are different in the example above.
Possible resolution [SUPERSEDED]:
Change and split in 6.4.1 [basic.scope.scope] paragraph 3 as follows:
...
Two functions have corresponding non-object signatures if they have the same non-object-parameter-type-list. Two function templates have corresponding non-object signatures if their template-parameter-lists have the same length, their corresponding template-parameter s are equivalent, and they have equivalent non-object-parameter-type-lists and return types (if any)
, and, if both are non-static members, they have corresponding object parameters. Two functions or function templates have corresponding signatures if they have corresponding non-object signatures and, if both are non-static members, they have corresponding object parameters.
Change in 6.4.1 [basic.scope.scope] paragraph 4 as follows:
Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unlessTwo function or function template declarations declare corresponding overloads if:
- either is a using-declarator , or
- one declares a type (not a typedef-name) and the other declares a variable, non-static data member other than of an anonymous union (11.5.2 [class.union.anon]), enumerator, function, or function template, or
- each declares a function or function template and they do not declare corresponding overloads.
- both declare functions with
the same non-object-parameter-type-list, equivalent (13.7.7.2 [temp.over.link]) trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]), and, if both are non-static members, they have corresponding object parameters,corresponding signatures or both declare function templates with corresponding signatures andequivalent template-heads and trailing requires-clauses (if any).- both have equivalent (13.7.7.2 [temp.over.link]) template-heads and trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]).
Change in 12.2.2.3 [over.match.oper] paragraph 4 as follows:
A non-template function or function template F named operator== is a rewrite target with first operand o unless a search for the name operator!= in the scope S from the instantiation context of the operator expression finds a function or function template F2 such thatwould correspondF and F2 have corresponding non-object signatures (6.4.1 [basic.scope.scope])to F if its name were operator==, where S is the scope of the class type of o if F is a class member, and the namespace scope of which F is a member otherwise. A function template specialization named operator== is a rewrite target if its function template is a rewrite target.
CWG 2023-10-06
CWG raised the question to EWG whether the special rule that allows the example in this issue for backward compatibility should be extended to member functions with explicit object parameters. This is tracked via paper issue 1645.
EWG 2024-03-18
EWG has no consensus to extend the rules to explicit object parameters.
Possible resolution:
Change and split in 6.4.1 [basic.scope.scope] paragraph 3 as follows:
...
Two functions have corresponding non-object signatures if they have the same non-object-parameter-type-list. Two function templates have corresponding non-object signatures if their template-parameter-lists have the same length, their corresponding template-parameter s are equivalent, and they have equivalent non-object-parameter-type-lists and return types (if any)
, and, if both are non-static members, they have corresponding object parameters. Two functions or function templates have corresponding signatures if they have corresponding non-object signatures and, if both are non-static members, they have corresponding object parameters.
Change in 6.4.1 [basic.scope.scope] paragraph 4 as follows:
Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unlessTwo function or function template declarations declare corresponding overloads if:
- either is a using-declarator , or
- one declares a type (not a typedef-name) and the other declares a variable, non-static data member other than of an anonymous union (11.5.2 [class.union.anon]), enumerator, function, or function template, or
- each declares a function or function template and they do not declare corresponding overloads.
- both declare functions with
the same non-object-parameter-type-list, equivalent (13.7.7.2 [temp.over.link]) trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]), and, if both are non-static members, they have corresponding object parameters,corresponding signatures or both declare function templates with corresponding signatures andequivalent template-heads and trailing requires-clauses (if any).- both have equivalent (13.7.7.2 [temp.over.link]) template-heads and trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]).
Change in 12.2.2.3 [over.match.oper] paragraph 4 as follows:
A non-template function or function template F named operator== is a rewrite target with first operand o unless a search for the name operator!= in the scope S from the instantiation context of the operator expression finds a function or function template F2 such thatwould correspondF2 is an implicit object member function and F and F2 have corresponding non-object signatures (6.4.1 [basic.scope.scope])to F if its name were operator==, where S is the scope of the class type of o if F is a class member, and the namespace scope of which F is a member otherwise. A function template specialization named operator== is a rewrite target if its function template is a rewrite target.
Consider:
template <class T> T f(T); // #1 template <class T> T* f(T*); // #2 auto p = &f<int>;
Accoring to 12.3 [over.over] paragraph 3 and 12.3 [over.over] paragraph 5:
The specialization, if any, generated by template argument deduction (13.10.4 [temp.over], 13.10.3.3 [temp.deduct.funcaddr], 13.10.2 [temp.arg.explicit]) for each function template named is added to the set of selected functions considered.
[...]Any given function template specialization F1 is eliminated if the set contains a second function template specialization whose function template is more specialized than the function template of F1 according to the partial ordering rules of 13.7.7.3 [temp.func.order]. After such eliminations, if any, there shall remain exactly one selected function.
Major implementations reject the example as ambiguous, yet the wording specifies to unambiguously choose #2.
Suggested resolution [SUPERSEDED]:
Change in 12.3 [over.over] paragraph 5 as follows:
Any given function template specialization F1 is eliminated if the set contains a second function template specialization whose function template ismore specializedbetter than the function template of F1. If there is no target, a function template is better than another if it is more constrained than the other; otherwise a function template is better than another if it is more specialized than the other according to the partial ordering rules of 13.7.7.3 [temp.func.order]. After such eliminations, if any, there shall remain exactly one selected function.
Proposed resolution:
Change in 12.3 [over.over] paragraph 5 as follows:
AnyIf there is a target, any given function template specialization F1 is eliminated if the set contains a second function template specialization whose function template is more specialized than the function template of F1 according to the partial ordering rules of 13.7.7.3 [temp.func.order]. After such eliminations, if any, there shall remain exactly one selected function.
Consider:
struct S { template<typename> void f(); }; template<typename = int> void S::f() { } // ok?
There is implementation divergence in the treatment of this example. The relevant wording appears to be 13.2 [temp.param] paragraph 12:
A default template-argument shall not be specified in the template-parameter-lists of the definition of a member of a class template that appears outside of the member's class.
However, the example above deals with a member of an ordinary class, not a class template, but it is not clear why there should be a difference between a member template of a class template and a member template of a non-template class.
Alternatively, it is not clear why the example above should be treated differently from a non-member function template, e.g.,
template<typename> void f(); template<typename = int> void f() { }
which is explicitly permitted.
Proposed resolution:
Change in 13.2 [temp.param] paragraph 10 as follows:
... A default template-argument may be specified in a template declaration. A default template-argument shall not be specified in the template-parameter-lists of the definition of a member of aclass templatetemplated class C that appears outside of themember's classclass-specifier of C. A default template-argument shall not be specified in a friend class template declaration. If a friend function template declaration D specifies a default template-argument, that declaration shall be a definition and there shall be no other declaration of the function template which is reachable from D or from which D is reachable.
Consider:
template<class T> concept ctible = requires { T(); }; class A { template <class T> friend struct B; A(); }; template <class T> struct B; template <ctible T> struct B<T> { T t; }; B<A> b; // #1 template <class T> struct C { }; template <ctible T> struct C<T> { T t; }; C<A> c; // #2
Should the context of instantiation be considered for satisfaction checking? If satisfaction checking were always performed in an unrelated context, neither partial specialization is used, and #1 would be ill-formed (because B is incomplete), but #2 would be well-formed. If the satisfaction checking were performed in the context of the constrained declaration, #1 would be well-formed and #2 would be ill-formed, no diagnostic required, because the validity of A() is different in that context. That rule, however, could also consider the context, in which case #2 would also be well-formed.
The decision affects the amount of caching that an implementation can perform.
Subclause 13.5.2.3 [temp.constr.atomic] paragraph 3 should be clarified one way or another:
To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression, the constraint is not satisfied. Otherwise, the lvalue-to-rvalue conversion (7.3.2 [conv.lval]) is performed if necessary, and E shall be a constant expression of type bool. The constraint is satisfied if and only if evaluation of E results in true. If, at different points in the program, the satisfaction result is different for identical atomic constraints and template arguments, the program is ill-formed, no diagnostic required.
Proposed resolution:
Change in 13.5.2.3 [temp.constr.atomic] paragraph 3 as follows:
To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression, the constraint is not satisfied; access checking is performed in the context in which the constraint-expression or requires-expression appears. Otherwise, the lvalue-to-rvalue conversion (7.3.2 [conv.lval]) is performed if necessary, and E shall be a constant expression of type bool. ...
Consider:
template <typename T> struct A { T f(this auto, auto); // #1 int f(auto); // #2 }; int main() { A<int>().f(42); }
There is implementation divergence in the handling of this example: Clang and MSVC consider the example ambiguous, EDG chooses #1.
Proposed resolution (reviewed by CWG 2024-01-19):
Change in 13.7.7.3 [temp.func.order] paragraph 3 as follows:
Each function template M that is a member function with no explicit object parameter is considered to have a new first parameter of type X(M), described below, inserted in its function parameter list. If exactly one of the function templates was considered by overload resolution via a rewritten candidate (12.2.2.3 [over.match.oper]) with a reversed order of parameters, then the order of the function parameters in its transformed template is reversed. For a function template M with cv-qualifiers cv that is a non-static member of a class A:For a function template M that is a static member of a class A:
- The type X(M) is “rvalue reference to cv A” if the optional ref-qualifier of M is && or if M has no ref-qualifier and the positionally-corresponding parameter of the other transformed template has rvalue reference type; if this determination depends recursively upon whether X(M) is an rvalue reference type, it is not considered to have rvalue reference type.
- Otherwise, X(M) is “lvalue reference to cv A”.
[Note 2: This allows a non-static member to be ordered with respect to a static or non-member function and for the results to be equivalent to the ordering of two equivalent non-members. —end note]
- The type X(M) is the type of the object parameter of the other transformed template, if any.
- Otherwise, no new first parameter is inserted.
CWG 2024-01-19
It seems #2 should be chosen, because it is more specialized than #1.
The Standard does not appear to specify the linkage of a template specialization. 13.9.2 [temp.inst] paragraph 11 does say,
Implicitly instantiated class and function template specializations are placed in the namespace where the template is defined.
which could be read as implying that the specialization has the same linkage as the template itself. Implementation practice seems to be that the weakst linkage of the template and the arguments is used for the specialization.
Additional notes (February, 2023)
Template specializations do not have linkage.
Consider:
template<typename T> struct S {
S(const S&) = default;
S& operator=(const S&) = default;
int n;
};
// Are the explicit specializations OK?
template<> S<int>::S(const S&) noexcept { }
template<> S<int>& S<int>::operator=(const S& other) noexcept {
return *this;
}
If the explicit specialization were allowed, would the answer to std::is_trivially_copyable<S<int>> change? What is decltype(&S::operator=) (the defaulted definition is noexcept, yet no instantiation or implicit definition is triggered)?
Proposed resolution (2023-10-20) [SUPERSEDED]:
Change in 13.9.2 [temp.inst] paragraph 3 and add bullets as follows:
The implicit instantiation of a class template specialization causesThe implicit instantiation of a class template specialization does not cause the implicit instantiation of default arguments or noexcept-specifiers of the class member functions.
- the implicit instantiation of the declarations, but not of the definitions, of the
non-deleteduser-provided class member functions, member classes, scoped member enumerations, static data members, member templates, and friends; and- the implicit instantiation of the definitions of
[ Note: The implicit instantiation determines whether a defaulted function is deleted, but a non-deleted defaulted function is implicitly defined only when it is odr-used or needed for constant evaluation. -- end note ]
- defaulted and deleted member functions as defaulted (9.5.2 [dcl.fct.def.default]) or deleted (9.5.3 [dcl.fct.def.delete]),
- unscoped member enumerations, and
- member anonymous unions.
Additional notes (October, 2023)
It is desirable to require a diagnostic for such attempted explicit specializations.
Furthermore, there are situations where the "potentially-throwing" property of a non-deleted function is queried:
template<int...> struct C { // This class has no eligible copy assignment operator at all. void operator=(const C&) requires false; void operator=(int) noexcept; operator int() const; }; void f(C<> &c) { c = c; // Convert to int, then construct from int. } struct D { C<> c; }; bool g(D d) { return noexcept(d = d); // #1. If this is valid, what does it return? } struct E { C<> c; E &operator=(const E &o) { c = o.c; } };
All major implementations agree that E is valid. However, clang, gcc, and EDG delete the copy assignment operator of D, thus sidestepping the question at #1. (MSVC accepts, but #1 returns true despite the potentially-throwing conversion to int.) Apparently, user-defined conversions for the first argument of C's assignment operator are ignored in implementations other than MSVC. However, the specification is silent on that.
Possible resolution:
Change in 13.9.2 [temp.inst] paragraph 3 and add bullets as follows:
The implicit instantiation of a class template specialization causesThe implicit instantiation of a class template specialization does not cause the implicit instantiation of default arguments or noexcept-specifiers of the class member functions.
- the implicit instantiation of the declarations, but not of the definitions, of the
non-deleteduser-provided class member functions, member classes, scoped member enumerations, static data members, member templates, and friends; and- the implicit instantiation of the definitions of
[ Note: The implicit instantiation determines whether a defaulted function is deleted, but a non-deleted defaulted function is implicitly defined only when it is odr-used or needed for constant evaluation. -- end note ]
- defaulted and deleted member functions as defaulted (9.5.2 [dcl.fct.def.default]) or deleted (9.5.3 [dcl.fct.def.delete]),
- unscoped member enumerations, and
- member anonymous unions.
Change in 13.9.4 [temp.expl.spec] paragraph 7 as follows:
If a template, a member template or a member of a templated classtemplateis explicitly specialized, a declaration of that specialization shall be reachable from every use of that specialization that would cause an implicit instantiation to take place, in every translation unit in which such a use occurs; no diagnostic is required unless the specialization is for a member of a templated class whose definition is implicitly instantiated as a result of the implicit instantiation of the class (13.9.2 [temp.inst]). ...
(From submission #482.)
Consider:
template<typename T>
concept C = sizeof(T) > sizeof(char);
template<typename T>
concept D = sizeof(T) > sizeof(int) ;
template<typename T>
struct A
{
template<typename U>
constexpr int f(U) requires C<U> { return 0; }
template<>
constexpr int f(int) requires D<T> { return 1; }
};
static_assert(A<int>().f(0) == 0); // #1
There is substantial implementation variance: GCC does not allow explicit specializations of function templates at class scope (contrary to the rule change introduced by issue 727), clang rejects them if a trailing-requires-clause is present, and EDG accepts, but ignores the constraint, causing #1 to fail.
Proposed resolution (reviewed by CWG 2024-03-01) [SUPERSEDED]:
Add a new paragraph before 13.9.4 [temp.expl.spec] paragraph 8 as follows:
An explicit specialization of a function shall not have a trailing requires-clause (9.3.1 [dcl.decl.general]). [ Example:
template<typename T> concept C = sizeof(T) <= sizeof(int); template<typename T> struct A { template<typename U> void f(U) requires C<U>; template<> void f(char); // OK template<> void f(short) requires (sizeof(T) >= 1); // error: trailing requires-clause not allowed }; template<> template<typename U> void A<int>::f(U) requires C<U> {} // OK, explicit specialization is a template-- end example ]
The placement of explicit specialization declarations for function templates ...
Add another example at the end of 13.9.4 [temp.expl.spec] paragraph 15 as follows:
[ Example:
template<typename T> struct D { template<typename U> static constexpr int f(U); // #1 template<typename U> static constexpr int f(U) requires (sizeof(T) == 1); // #2 template<> constexpr int f(int) // #3 { return 1; } }; template<> template<typename U> constexpr int D<signed char>::f(U) requires (sizeof(signed char) == 1) // #4 { return 0; } static_assert(D<char>::f(0) == 1); // overload resolution selects #2; #3 is a specialization for #2 static_assert(D<char[2]>::f(0) == 1); // overload resolution selects #1; #3 is a specialization for #1 static_assert(D<signed char>::f(0) == 1); // overload resolution selects #2; #3 is a specialization for #2 static_assert(D<signed char>::f(0.0) == 0); // overload resolution selects #2; #4 is a specialization for #2-- end example ]
Additional notes (April, 2024)
The phrasing "an explicit specialization of a function" does not make sense.
Possible resolution:
Add a new paragraph before 13.9.4 [temp.expl.spec] paragraph 8 as follows:
An explicit specialization that declares a function shall not have a trailing requires-clause (9.3.1 [dcl.decl.general]). [ Example:
template<typename T> concept C = sizeof(T) <= sizeof(int); template<typename T> struct A { template<typename U> void f(U) requires C<U>; template<> void f(char); // OK template<> void f(short) requires (sizeof(T) >= 1); // error: trailing requires-clause not allowed }; template<> template<typename U> void A<int>::f(U) requires C<U> {} // OK, explicit specialization is a template template<> template<> void A<short>::f(int) requires C<int> {} // error: trailing requires-clause for a declaration of a non-templated function-- end example ]
The placement of explicit specialization declarations for function templates ...
Add another example at the end of 13.9.4 [temp.expl.spec] paragraph 15 as follows:
[ Example:
template<typename T> struct D { template<typename U> static constexpr int f(U); // #1 template<typename U> static constexpr int f(U) requires (sizeof(T) == 1); // #2 template<> constexpr int f(int) // #3 { return 1; } }; template<> template<typename U> constexpr int D<signed char>::f(U) requires (sizeof(signed char) == 1) // #4 { return 0; } static_assert(D<char>::f(0) == 1); // overload resolution selects #2; #3 is a specialization for #2 static_assert(D<char[2]>::f(0) == 1); // overload resolution selects #1; #3 is a specialization for #1 static_assert(D<signed char>::f(0) == 1); // overload resolution selects #2; #3 is a specialization for #2 static_assert(D<signed char>::f(0.0) == 0); // overload resolution selects #2; #4 is a specialization for #2-- end example ]
(From submission #537.)
Core issue 2355 added support for deducing the exception specification of a function template. However, implementations uniformly reject the following example:
template<bool B> void f() noexcept(B); // #1 template<> void f() noexcept; // explicit specialization of #1?
This is arguably the correct approach, because exception specifications are instantiated separately and do not cause substitution failure in the immediate context (13.10.3.1 [temp.deduct.general]). For class-scope explicit specializations, the noexcept-specifier has not even been parsed, yet. If we exclude such deduction from function declarations, we also need to avoid considering the exception specification for partial ordering, otherwise partial ordering would always fail.
A similar consideration applies to the address-of-function-template case:
struct A { static constexpr bool x = true; }; template<typename T, typename U> void f(T, U*) noexcept(T::x); // #1 template<typename T, typename U> void f(T, U) noexcept(T::y); // #2 void(&g)(A, int*) noexcept = f; // selects #1; not a hard error during deduction for #2
Suggested resolution [SUPERSEDED]:
Change in 13.10.3.7 [temp.deduct.decl] paragraph 1 as follows:
... In all these cases, P is the function type of the function template being considered as a potential match and A is either the function type from the declaration or the type of the deallocation function that would match the placement operator new as described in 7.6.2.8 [expr.new] , in all cases ignoring the noexcept-specifier (if any) of the function types. The deduction is done as described in 13.10.3.6 [temp.deduct.type].
Change in 13.10.3.5 [temp.deduct.partial] paragraph 3 as follows:
The types used to determine the ordering depend on the context in which the partial ordering is done:
- In the context of a function call, the types used are those function parameter types for which the function call has arguments. [ Footnote: ... ]
- In the context of a call to a conversion function, the return types of the conversion function templates are used.
- In other contexts (13.7.7.3 [temp.func.order]), the function template's function type outside of the noexcept-specifier is used.
Change in 13.10.3.3 [temp.deduct.funcaddr] paragraph 1 as follows:
Template arguments can be deduced from the type specified when taking the address of an overload set (12.3 [over.over]). If there is a target, the function template's function type and the target type are used as the types of P and A, ignoring the noexcept-specifier (if any) of both function types, and the deduction is done as described in 13.10.3.6 [temp.deduct.type]. Otherwise, deduction is performed with empty sets of types P and A.
CWG 2024-06-14
For the last case (address of overload set), the following example is currently well-formed, but is rendered ill-formed by the suggested resolution:
void f(T*); // #1 void f(T) noexcept; // #2 void (&g)(int*) noexcept = f; // status quo: selects #2; suggested resolution: selects #1 (more specialized) and fails the initialization
Possible resolution:
Change in 12.3 [over.over] paragraph 5 as follows:
All functions with associated constraints that are not satisfied (13.5.3 [temp.constr.decl]) are eliminated from the set of selected functions. Any function template specialization whose exception specification is originally non-dependent is eliminated from the set unless the function type of the specialization (after possibly applying the function pointer conversion (7.3.14 [conv.fctptr])) is identical to the function type of the target type. If more than one function in the set remains, all function template specializations in the set are eliminated if the set also contains a function that is not a function template specialization. Any given non-template function F0 is eliminated if the set contains a second non-template function that is more constrained than F0 according to the partial ordering rules of 13.5.5 [temp.constr.order]. Any given function template specialization F1 is eliminated if the set contains a second function template specialization whose function template is more specialized than the function template of F1 according to the partial ordering rules of 13.7.7.3 [temp.func.order]. After such eliminations, if any, there shall remain exactly one selected function.
Change in 13.10.3.7 [temp.deduct.decl] paragraph 1 as follows:
... In all these cases, P is the function type of the function template being considered as a potential match and A is either the function type from the declaration or the type of the deallocation function that would match the placement operator new as described in 7.6.2.8 [expr.new] , in all cases ignoring the noexcept-specifier (if any) of the function types. The deduction is done as described in 13.10.3.6 [temp.deduct.type].
Change in 13.10.3.5 [temp.deduct.partial] paragraph 3 as follows:
The types used to determine the ordering depend on the context in which the partial ordering is done:
- In the context of a function call, the types used are those function parameter types for which the function call has arguments. [ Footnote: ... ]
- In the context of a call to a conversion function, the return types of the conversion function templates are used.
- In other contexts (13.7.7.3 [temp.func.order]), the function template's function type outside of the noexcept-specifier is used.
Change in 13.10.3.3 [temp.deduct.funcaddr] paragraph 1 as follows:
Template arguments can be deduced from the type specified when taking the address of an overload set (12.3 [over.over]). If there is a target, the function template's function type and the target type are used as the types of P and A, ignoring the noexcept-specifier (if any) of both function types, and the deduction is done as described in 13.10.3.6 [temp.deduct.type]. Otherwise, deduction is performed with empty sets of types P and A.
C supports the following, C++ does not (see issues 232 and 2823):
void f() { char *p = 0; char *p2 = &*p; // OK in C, undefined behavior in C++ int a[5]; int *q = &a[5]; // OK in C, undefined behavior in C++ }
This incompatibility should be documented in Annex C.
Proposed resolution (approved by CWG 2024-06-26):
Add a new paragraph to C.7.4 [diff.expr] as follows:
Affected subclause: 7.6.2.2 [expr.unary.op]
Change: In certain contexts, taking the address of a dereferenced null or past-the-end pointer value is well-defined in C (and yields the original pointer value), but results in undefined behavior in C++. For example:void f() { char *p = 0; char *p2 = &*p; // well-defined in C, undefined behavior in C++ char *p3 = &p[0]; // well-defined in C, undefined behavior in C++ int a[5]; int *q = &a[5]; // well-defined in C, undefined behavior in C++ }Rationale: Consistent treatment of lvalues in C++.
Effect on original feature: Well-formed and well-defined C code exhibits undefined behavior in C++.
Difficulty of converting: Syntactic transformation to pointer arithmetic and possible addition of a check for null pointer values.
How widely used: Occasionally.
CWG 2024-06-26
Implementations are required to diagnose undefined behavior in constant expressions. The issue is kept in review status to allow time for submitting a paper to EWG to make the &a[5] case well-defined. See also C23 6.5.3.2p3.
The term "user-declared" is used 30 times throughout the standard, but it is not defined.
Proposed resolution:
Add a new entry after 3.64 [defns.unspecified] as follows:
user-declared [defns.user.declared]
not implicitly declared
CWG 2024-06-14
The proposed resolution does not work, because utterances such as "a default constructor is implicitly declared if no constructor is user-declared" become circular. The eventual definition should address the question whether constructors inherited from a base class are considered user-declared in the inheriting class.
5.4 [lex.pptoken] paragraph 2 specifies that there are 5 categories of tokens in phases 3 to 6. With 5.12 [lex.operators] paragraph 1, it is unclear whether new is an identifier or a preprocessing-op-or-punc; likewise for delete. This is relevant to answer the question whether
#define delete foo
is a well-formed control-line, since that requires an identifier after the define token.
(See also issue 189.)
According to 5.4 [lex.pptoken] paragraph 3,
If the input stream has been parsed into preprocessing tokens up to a given character:
If the next character begins a sequence of characters that could be the prefix and initial double quote of a raw string literal, such as R", the next preprocessing token shall be a raw string literal. Between the initial and final double quote characters of the raw string, any transformations performed in phases 1 and 2 (trigraphs, universal-character-names, and line splicing) are reverted; this reversion shall apply before any d-char, r-char, or delimiting parenthesis is identified.
However, phase 1 is defined as:
Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. The set of physical source file characters accepted is implementation-defined. Trigraph sequences (_N4140_.2.4 [lex.trigraph]) are replaced by corresponding single-character internal representations. Any source file character not in the basic source character set (5.3 [lex.charset]) is replaced by the universal-character-name that designates that character.
The reversion described in 5.4 [lex.pptoken] paragraph 3 specifically does not mention the replacement of physical end-of-line indicators with new-line characters. Is it intended that, for example, a CRLF in the source of a raw string literal is to be represented as a newline character or as the original characters?
The syntactic nonterminal punctuator appears in the grammar for token in 5.6 [lex.token], but it is nowhere defined. It should be merged with operator and given an appropriate list of tokens as a definition for the merged term.
Proposed resolution (October, 2017):
Change 5.5 [lex.digraph] paragraph 2 as follows
In all respects of the language except in an attribute-token (9.12.1 [dcl.attr.grammar]), each alternative token behaves the same, respectively, as its primary token, except for its spelling.18 The set of alternative tokens...
Change the grammar in 5.6 [lex.token] as follows:
{ } [ ] ( ) ; : ... ? :: . .* + - * / % ^ & | ~ ! = < > += -= *= /= %= ^= &= |= << >> <<= >>= == != <= >= && || ++ -- , ->* ->
Change 5.6 [lex.token] paragraph 1 as follows:
There are
fivefour kinds of tokens: identifiers, keywords, literals,19operators, and other separatorsand symbols.Blanks, horizontal and vertical tabs, newlines, formfeeds, and comments (collectively, “white space”), as described below, are ignored except as they serve to separate tokens. [Note: Some white space is required to separate otherwise adjacent identifiers, keywords, numeric literals, and alternative tokens containing alphabetic characters. —end note]Each preprocessing-token resulting from translation phase 6 is converted into the corresponding token as follows:
If the preprocessing-token is an identifier or is one of the preprocessing-op-or-punc tokens new or delete, the resulting token is a keyword if it is listed in Table 5, and otherwise is an identifier.
Otherwise, if the preprocessing-token is a pp-number with the lexical form of an integer-literal or floating-literal, or is a character-literal or string-literal, the resulting token is of the corresponding form.
Otherwise, if the preprocessing-token is a pp-number with the lexical form of a user-defined-integer-literal or user-defined-floating-literal or is a user-defined-character-literal or user-defined-string-literal, the resulting token is a user-defined-literal.
Otherwise, if the preprocessing-token is a preprocessing-op-or-punc, and there is a corresponding symbol (after converting alternative token representations to their primary tokens), the resulting token is that symbol.
Otherwise, the program is ill-formed.
[Note: Within an attribute-token (9.12.1 [dcl.attr.grammar]), a token formed from a preprocessing-token that satisfies the syntactic requirements of an identifier is considered to be an identifier with the spelling of the preprocessing-token. —end note]
Delete the final sentence of 5.12 [lex.operators] paragraph 1.
Each preprocessing-op-or-punc is converted to a single token in translation phase 7 (5.2 [lex.phases]).
The nonterminals operator and punctuator in 5.6 [lex.token] are not defined. There is a definition of the nonterminal operator in 12.4 [over.oper] paragraph 1, but it is apparent that the two nonterminals are not the same: the latter includes keywords and multi-token operators and does not include the nonoverloadable operators mentioned in paragraph 3.
There is a definition of preprocessing-op-or-punc in 5.12 [lex.operators] , with the notation that
Each preprocessing-op-or-punc is converted to a single token in translation phase 7 (2.1).However, this list doesn't distinguish between operators and punctuators, it includes digraphs and keywords (can a given token be both a keyword and an operator at the same time?), etc.
Suggested resolution:
Additional note (April, 2005):
The resolution for this problem should also address the fact that sizeof and typeid (and potentially others like decltype that may be added in the future) are described in some places as “operators” but are not listed in 12.4 [over.oper] paragraph 3 among the operators that cannot be overloaded.
(See also issue 369.)
According to 5.13.3 [lex.ccon] paragraph 1, a multicharacter literal like 'ab' is conditionally-supported and has type int.
According to 5.13.9 [lex.ext] paragraph 6,
If L is a user-defined-character-literal, let ch be the literal without its ud-suffix. S shall contain a literal operator (12.6 [over.literal]) whose only parameter has the type of ch and the literal L is treated as a call of the form
operator "" X(ch)
A user-defined-character-literal like 'ab'_foo would thus require a literal operator
However, that is not one of the signatures permitted by 12.6 [over.literal] paragraph 3.
Should multicharacter user-defined-character-literals be conditionally-supported? If so, 12.6 [over.literal] paragraph 3 should be adjusted accordingly. If not, a note in 5.13.9 [lex.ext] paragraph 6 saying explicitly that they are not supported would be helpful.
The description of the numeric literals occurring as part of user-defined-integer-literals and user-defined-floating-literals in 5.13.9 [lex.ext] says nothing about whether they are required to satisfy the same constraints as literals that are not part of a user-defined-literal. In particular, because it is the spelling, not the value, of the literal that is used for raw literal operators and literal operator templates, there is no particular reason that they should be restricted to the maximum values and precisions that apply to ordinary literals (and one could imagine that this would be a good notation for allowing literals of extended-precision types).
Is this relaxation of limits intended to be required, or is it a quality-of-implementation issue? Should something be said, either normatively or non-normatively, about this question?
According to 6.1 [basic.pre] paragraph 6,
A variable is introduced by the declaration of a reference other than a non-static data member or of an object.
In other words, non-static data members of reference type are not variables. This complicates the wording in a number of places, where the text refers to “variable or data member,” presumably to cover the reference case, but that phrasing could lead to the mistaken impression that all data members are not variables. It would be better if either there were a term for the current phrase “variable or data member” or if there were a less-unwieldy term for “non-static data member of reference type” that could be used in place of “data member” in the current phrasing.
According to 6.5.1 [basic.lookup.general] paragraphs 2-3,
...A declaration X precedes a program point P in a translation unit L if P follows X, X inhabits a class scope and is reachable from P, or else...
A single search in a scope S for a name N from a program point P finds all declarations that precede P to which any name that is the same as N (6.1 [basic.pre]) is bound in S.
These rules cause problems for finding enumerators when qualified by an exported name of its enumeration type, unlike a member of a class. For example:
export module A; enum class X { x }; enum Y { y }; export module B; import A; export using XB = X; export using YB = Y; // client code import B; int main() { XB x = XB::x; // should be OK because definition of X is reachable, even // though A is not imported YB y = YB::y; // similarly OK YB z = ::y; // error, because y from module A is not visible }
It would seem that this problem could be addressed by changing “inhabits a class scope” to “does not inhabit a namespace scope.”
According to 6.7.2 [intro.object] paragraph 7,
Unless it is a bit-field (11.4.10 [class.bit]), a most derived object shall have a nonzero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size.
Base class objects of zero size is a misleading term, as sizeof such an object is non-zero. Size should not be a property of an object, rather of a type.
The status of the following code should be explicitly indicated in the Standard to avoid surprise:
#include <new> int bar() { alignas(int) unsigned char space[sizeof(int)]; int *pi = new (static_cast<void *>(space)) int; *pi = 42; return [=]() mutable { return *std::launder(reinterpret_cast<int *>(space)); }(); }
In particular, it appears that the call to std::launder has undefined behaviour because the captured copy of space is not established to provide storage for an object of type int (sub 6.7.2 [intro.object] paragraph 1). Furthermore, the code has undefined behaviour also because it attempts to access the stored value of the int object through a glvalue of an array type other than one of the ones allowed by sub 7.2.1 [basic.lval] paragraph 8.
It is not intended that implicit object creation, as described in 6.7.2 [intro.object] paragraph 10, should occur during constant expression evaluation, but there is currently no wording prohibiting it.
Notes from the February, 2021 teleconference:
This issue was occasioned by issue 2464, which is also the subject of LWG issue 3495. CWG reviewed the proposed resolution and agrees with it. The intended approach for this issue is to wait for LWG to resolve that issue, then add a note in the core section pointing out the implications of that requirement for implicit object creation.
According to 6.7.3 [basic.life] paragraphs 5 and 6, a program has undefined behavior if a pointer or glvalue designating an out-of-lifetime object
is used to access a non-static data member or call a non-static member function of the object
It is not clear what the word “access” means in this context. A reasonable interpretation might be using the pointer or glvalue as the left operand of a class member access expression; alternatively, it might mean to read or write the value of that member, allowing a class member access expression that is used only to form an address or bind a reference.
This needs to be clarified. A relevant consideration is the recent adoption of the resolution of issue 597, which eased the former restriction on simple address manipulations involving out-of-lifetime objects: if base-class offset calculations are now allowed, why not non-static data member offset calculations?
(See also issue 1531 for other uses of the term “access.”)
Additional note (January, 2013):
A related question is the meaning of the phrase “before the constructor begins execution” in 11.9.5 [class.cdtor] paragraph 1 means:
For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior.
For example:
struct DerivedMember { ... }; struct Base { Base(DerivedMember const&); }; struct Derived : Base { DerivedMember x; Derived() : Base(x) {} }; Derived a;
Is the reference to Derived::x in the mem-initializer valid?
Additional note (March, 2013):
This clause is phrased in terms of the execution of the constructor. However, it is possible for an aggregate to have a non-trivial default constructor and be initialized without executing a constructor. The wording needs to be updated to allow for non-constructor initialization to avoid appearing to imply undefined behavior for an example like:
struct X { std::string s; } x = {}; std::string t = x.s; // No constructor called for x: undefined behavior?
(From submission #507.)
Subclause 6.7.3 [basic.life] paragraph 6 specifies:
Before the lifetime of an object has started but after the storage which the object will occupy has been allocated [ Footnote: ... ] or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, ...
"Before" and "after" are intended to refer to the "happens before" relation (which is not a total order), but the specific use here does not properly handle evaluations racing with the start of the lifetime of an object.
Possible resolution (option 1) [SUPERSEDED]:
Change in 6.7.3 [basic.life] paragraph 4 as follows:
The properties ascribed to objects and references throughout this document apply for a given object or reference only during its lifetime. [Note 2: In particular,before the lifetime of an object starts and after its lifetime endsthere are significant restrictions on the use ofthean object not within its lifetime, as described below, in 11.9.3 [class.base.init], and in 11.9.5 [class.cdtor]. Also, the behavior of an object under construction and destruction can differ from the behavior of an object whose lifetime has started and not ended. 11.9.3 [class.base.init] and 11.9.5 [class.cdtor] describe the behavior of an object during its periods of construction and destruction. —end note]
Change in 6.7.3 [basic.life] paragraph 6 as follows:
Before the lifetime of an object has started butIn an evaluation that occurs after the storage whichthean object will occupy has been allocated [ Footnote: ... ] but not after the lifetime of the object has started or, in an evaluation that occurs not beforeafterthe lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that represents the address of the storage location where the object will be or was located may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. ...
Change in 6.7.3 [basic.life] paragraph 7 as follows:
Similarly,before the lifetime of an object has started butin an evaluation that occurs after the storage whichthean object will occupy has been allocated but not after the lifetime of the object has started or, in an evaluation that occurs not beforeafterthe lifetime of an object has ended and before the storage which the object occupied is reused or released, any glvalue that refers to the original object may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. ...
Change in 6.7.3 [basic.life] paragraph 8 as follows:
If, in an evaluation that occurs not beforeafterthe lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if the original object is transparently replaceable (see below) by the new object. ...
Proposed resolution (approved by CWG 2024-04-19):
Change in 6.7.3 [basic.life] paragraph 4 as follows:
The properties ascribed to objects and references throughout this document apply for a given object or reference only during its lifetime. [Note 2: In particular,before the lifetime of an object starts and after its lifetime endsthere are significant restrictions on the use ofthean object not within its lifetime, as described below, in 11.9.3 [class.base.init], and in 11.9.5 [class.cdtor]. Also, the behavior of an object under construction and destruction can differ from the behavior of an object whose lifetime has started and not ended. 11.9.3 [class.base.init] and 11.9.5 [class.cdtor] describe the behavior of an object during its periods of construction and destruction. —end note]
Add a new paragraph before 6.7.3 [basic.life] paragraph 6 as follows:
An evaluation E is said to be in a storage-only phase of an object O if
- E happens after the storage which O will occupy is allocated and E does not happen after the lifetime of O starts; or
- E does not happen before the lifetime of O ends and E happens before the storage which O occupied is reused or released.
Change in 6.7.3 [basic.life] paragraph 6 as follows:
Before the lifetime of an object has started but after the storage which theobject will occupy has been allocated [ Footnote: ... ] but not after the lifetime of the object has started or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released,In an evaluation that is in a storage-only phase of an object, any pointer that represents the address of the storage location where the object will be or was located may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. ...
Change in 6.7.3 [basic.life] paragraph 7 as follows:
Similarly,before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released,in an evaluation that is in a storage-only phase of an object, any glvalue that refers to theoriginalobject may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. ...
Change in 6.7.3 [basic.life] paragraph 8 as follows:
If, in an evaluation that is in a storage-only phase of an object and where the evaluation does not happen beforeafterthe lifetime ofanthe object endshas ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if the original object is transparently replaceable (see below) by the new object. ...
Given the following example,
#include <new> int main() { unsigned char buf[sizeof(int)] = {}; int *ip = new (buf) int; return *ip; // 0 or undefined? }
Should the preceding initializsation of the buffer carry over to the value of *ip? According to 6.7.4 [basic.indet] paragraph 1,
When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced (7.6.19 [expr.ass]).
In this case, no new storage is being obtained for the int object created by the new-expression.
According to 6.7.5 [basic.stc] paragraph 2,
Static, thread, and automatic storage durations are associated with objects introduced by declarations (6.2 [basic.def]) and implicitly created by the implementation (6.7.7 [class.temporary]).
The apparent intent of the reference to 6.7.7 [class.temporary] is that a temporary whose lifetime is extended to be that of a reference with one of those storage durations is considered also to have that storage duration. This interpretation is buttressed by use of the phrase “an object with the same storage duration as the temporary” (twice) in 6.7.7 [class.temporary] paragraph 5.
There are two problems, however: first, the specification of lifetime extension of temporaries (also in 6.7.7 [class.temporary] paragraph 5) does not say anything about storage duration. Also, nothing is said in either of these locations about the storage duration of a temporary whose lifetime is not extended.
The latter point is important because 6.7.3 [basic.life] makes a distinction between the lifetime of an object and the acquisition and release of the storage the object occupies, at least for objects with non-trivial initialization and/or a non-trivial destructor. The assumption is made in 6.7.7 [class.temporary] and elsewhere that the storage in which a temporary is created is no longer available for reuse, as specified in 6.7.3 [basic.life], after the lifetime of the temporary has ended, but this assumption is not explicitly stated. One way to make that assumption explicit would be to define a storage duration for temporaries whose lifetime is not extended.
Do we need explicit language to forbid auto as the return type of allocation and deallocation functions?
(See also issue 1669.)
According to 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 4,
[Note: In particular, a global allocation function is not called to allocate storage for objects with static storage duration (6.7.5.2 [basic.stc.static]), for objects or references with thread storage duration (6.7.5.3 [basic.stc.thread]), for objects of type std::type_info (7.6.1.8 [expr.typeid]), or for an exception object (14.2 [except.throw]). —end note]
The restriction against allocating exception objects on the heap was intended to ensure that heap exhaustion could be reported by throwing an exception, i.e., that obtaining storage for std::bad_alloc could not fail because the heap was full. However, this implicitly relied on the assumption of a single thread and does not scale to large numbers of threads, so the restriction should be lifted and another mechanism found for guaranteeing the ability to throw std::bad_alloc.
Notes from the February, 2016 meeting:
The prohibition of using an allocation function appears only in a note, although there is a normative reference to the rule in 14.2 [except.throw] paragraph 4. CWG was in favor of retaining the prohibition of using a C++ allocation function for the memory of an exception object, with the implicit understanding that use of malloc would be permitted. The resolution for this issue should delete the note and move the prohibition to normative text in the relevant sections.
According to 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3,
If a deallocation function terminates by throwing an exception, the behavior is undefined.
This seems to be in conflict with the provisions of 14.5 [except.spec]: if a deallocation function throws an exception that is not allowed by its exception-specification, 14.5 [except.spec] paragraph 10 would appear to give the program defined behavior (calling std::unexpected() or std::terminate()). (Note that 14.5 [except.spec] paragraph 18 explicitly allows an explicit exception-specification for a deallocation function.)
6.7.6 [basic.align] speaks of “alignment requirements,” and 6.7.5.5.2 [basic.stc.dynamic.allocation] requires the result of an allocation function to point to “suitably aligned” storage, but there is no explicit statement of what happens when these requirements are violated (presumably undefined behavior).
The term "temporary expression" is used in 11.9.3 [class.base.init] paragraph 8 and 11.9.3 [class.base.init] paragraph 11, but is never defined.
Proposed resolution (approved by CWG 2023-11-10):
Change in 6.7.7 [class.temporary] paragraph 6 as follows:
The third context is when a reference binds to a temporary object. [ Footnote: ... ]The temporary object to which the reference is bound or the temporary object that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference if the glvalue to which the reference is bound wasA temporary expression is a glvalue obtained through one of the following:If a reference is bound to an object O that is the result of a temporary expression, the complete object of O persists for the lifetime of the reference.
- a temporary materialization conversion (7.3.5 [conv.rval]),
- ( expression ), where expression is
one of these expressionsa temporary expression,- subscripting (7.6.1.2 [expr.sub]) of an array operand, where that operand is
one of these expressionsa temporary expression,- a class member access (7.6.1.5 [expr.ref]) using the . operator where the left operand is
one of these expressionsa temporary expression and the right operand designates a non-static data member of non-reference type,- a pointer-to-member operation (7.6.4 [expr.mptr.oper]) using the .* operator where the left operand is
one of these expressionsa temporary expression and the right operand is a pointer to data member of non-reference type,- a
converting, without a user-defined conversion, a glvalue operand that is
- const_cast (7.6.1.11 [expr.const.cast]),
- static_cast (7.6.1.9 [expr.static.cast])
- dynamic_cast (7.6.1.7 [expr.dynamic.cast]), or
- reinterpret_cast (7.6.1.10 [expr.reinterpret.cast])
one of these expressionsa temporary expression to a glvalue that refers to the object designated by the operand, or to its complete object or a subobject thereof,- a conditional expression (7.6.16 [expr.cond]) that is a glvalue where the second or third operand is
one of these expressionsa temporary expression, or- a comma expression (7.6.20 [expr.comma]) that is a glvalue where the right operand is
one of these expressionsa temporary expression.
Change in 11.9.3 [class.base.init] paragraph 8 as follows:
A temporary expression bound toBinding a reference member to an object that is the result of a temporary expression in a mem-initializer is ill-formed.
Change in 11.9.3 [class.base.init] paragraph 11 as follows:
A temporary expression bound toBinding a reference member to an object that is the result of a temporary expression from a default member initializer is ill-formed.
CWG 2024-03-21
The phrasing "If a reference is bound to an object O that is the result of a temporary expression, ..." can be interpreted to refer to any temporary expression, even one entirely unrelated to the initializer of the reference.
According to 6.8 [basic.types] paragraph 4,
The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T).
However, it is not clear that a “sequence” can be indexed, as an array can and as is required for the implementation of memcpy and similar code.
Additional note, November, 2014:
An additional point of concern has been raised as to whether it is appropriate to refer to the constituent bytes of an object as being “objects” themselves, along with the interaction of this specification with copying or not copying parts of the object representation that do not participate in the value representation of the object (“padding” bytes).
The current wording of 6.9.3.2 [basic.start.static] allows deferral of static and thread_local initialization until a variable or function in the containing translation unit is odr-used. This requires implementations to avoid optimizing away the relevant odr-uses. We should consider relaxing the rule to allow for such optimizations.
Proposed resolution (November, 2014):
For a variable V with thread or static storage duration, let X be the set of all variables with the same storage duration as V that are defined in the same translation unit as V. If the observable behavior of the abstract machine (6.7.2 [intro.object]) depends on the value of V through an evaluation E, and E is not sequenced before the end of the initialization of any variable in X, then the end of the initialization of all variables in X is sequenced before E.
There is also a problem (submitted by David Majnemer) if the odr-use occurs in a constexpr context that does not require the variable to be constructed. For example,
struct A { A(); }; thread_local A a; constexpr bool f() { return &a != nullptr; }
It doesn't seem possible to construct a before its odr-use in f.
There is implementation divergence in the handling of this example.
Notes from the November, 2014 meeting:
CWG determined that the second part of the issue (involving constexpr) is not a defect because the address of an object with thread storage duration is not a constant expression.
Additional note, May, 2015:
CWG failed to indicate where and how to apply the wording in the proposed resolution. In addition, further review has raised concern that “sequenced before” may be the wrong relation to use for the static storage duration case because it implies “in the same thread.”
Notes from the October, 2015 meeting:
The suggested wording is intended to replace some existing wording in 6.9.3.2 [basic.start.static] paragraph 2. CWG affirmed that the correct relationship is “happens before” and not “sequenced before.”
The terms “ordered” and “unordered” initialization are only defined in 6.9.3.2 [basic.start.static] paragraph 2 for entities with static storage duration. They should presumably apply to entities with thread storage duration as well.
According to 6.9.3.3 [basic.start.dynamic] paragraph 3,
A non-initialization odr-use is an odr-use (6.3 [basic.def.odr]) not caused directly or indirectly by the initialization of a non-local static or thread storage duration variable.
Paragraphs 4-6 uses this term to exclude such odr-uses from consideration in determining the point by which a deferred initialization must be performed. A static_assert or a template argument expression can odr-use a variable, but it cannot be said to define any time during execution.
Suggestion: Add constant expression evaluation to the definition. Rename the term to “initializing odr-use” (based on effect rather than cause). Add a note saying that no such odr-use can occur before main begins.
Notes from the February, 2021 teleconference:
CWG agreed with the direction.
The phrases “name”, “qualified name” and “unqualified name” are used in various places. It is not clear that all names are either one or the other; there could, in fact, be a third kind of name that is neither.
See also editorial issue 4793.
According to 7.5.5.5 [expr.prim.id.dtor] paragraph 2,
If the id-expression names a pseudo-destructor, T shall be a scalar type and the id-expression shall appear as the right operand of a class member access (7.6.1.5 [expr.ref]) that forms the postfix-expression of a function call (7.6.1.3 [expr.call]).
This would appear to make the following example ill-formed, because it is the parenthesized expression and not the class member access that is the postfix-expression in the function call:
typedef int T;
void f(int* p) {
(p->~T)(); // Ill-formed?
}
Presumably this is an oversight.
Whether a reference is odr-used or not has less to do with the context where it is named and more to do with its initializer. In particular, 7.5.6 [expr.prim.lambda] bullet 12.2 leads to cases where references that can never be odr-used are implicitly captured:
A lambda-expression with an associated capture-default that does not explicitly capture this or a variable with automatic storage duration (this excludes any id-expression that has been found to refer to an init-capture's associated non-static data member), is said to implicitly capture the entity (i.e., this or a variable) if the compound-statement:
odr-uses (6.3 [basic.def.odr]) the entity, or
names the entity in a potentially-evaluated expression (6.3 [basic.def.odr]) where the enclosing full-expression depends on a generic lambda parameter declared within the reaching scope of the lambda-expression.
For example, ref should not be captured in the following:
struct A { A() = default; A(const A &) = delete; } globalA; constexpr bool bar(int &, const A &a) { return &a == &globalA; } int main() { A &ref = globalA; [=](auto q) { static_assert(bar(q, ref), ""); }(0); }
According to 7.6.1.4 [expr.type.conv] paragraph 4,
Similarly, a simple-type-specifier or typename-specifier followed by a braced-init-list creates a temporary object of the specified type direct-list-initialized (9.4.5 [dcl.init.list]) with the specified braced-init-list, and its value is that temporary object as a prvalue.
This wording does not handle the case where T is a reference type: it is not possible to create a temporary object of that type, and presumably the result would be an xvalue, not a prvalue.
P0135R1 (Wording for guaranteed copy elision through simplified value categories) removes complete type requirements from 7.6.1.3 [expr.call] (under the assumption that subclause 9.4 [dcl.init] has them; apparently it does not) and from 7.6.1.8 [expr.typeid] paragraph 3. These both appear to be bad changes and should presumably be reverted.
Consider:
struct A { static int x; }; struct B { using type = A; }; int y = B().type::x;
There seems to be no requirement that the member named in a class member access actually is a member of the class of the object expression. Subclause 7.5.5.1 [expr.prim.id.general] paragraph 3 does not cover static members:
An id-expression that denotes a non-static data member or non-static member function of a class can only be used:
- as part of a class member access in which the object expression refers to the member's class or a class derived from that class, or
- ...
Suggested resolution:
Change in 7.6.1.5 [expr.ref] paragraph 4 as follows:
Otherwise, the object expression shall be of class type. The class type shall be complete unless the class member access appears in the definition of that class.
[Note: The program is ill-formed if the result differs from that when the class is complete (6.5.2 [class.member.lookup]). —end note]
[Note: 6.5.5 [basic.lookup.qual] describes how names are looked up after the . and -> operators. —end note] If E2 is a qualified-id, the terminal name of its nested-name-specifier shall denote the type of E1 or a base class thereof.[Example:
struct A { static int x; }; struct B { static int x; }; struct D : B { using type = A; }; int y1 = D().B::x; // OK, B is a base class of D int y2 = D().type::x; // error: A is not a base class of D int y3 = D::type::x; // OK, evaluates A::x—end example ]
Change in 7.6.1.5 [expr.ref] bullet 6.5 as follows:
[Example:
enum E { e }; struct X { using E::e; }; int f(X x) { return x.e; }
—end example ]
Change in 7.5.5.1 [expr.prim.id.general] paragraph 3 as follows:
An id-expression that denotes a non-static data member or non-static member function of a class can only be used:
- as part of a class member access (7.6.1.5 [expr.ref])
in which the object expression refers to the member's class[ Footnote: ... ]or a class derived from that class, or- to form a pointer to member (7.6.2.2 [expr.unary.op]), or
- if that id-expression denotes a non-static data member and it appears in an unevaluated operand.
The specification of dynamic_cast in 7.6.1.7 [expr.dynamic.cast] paragraph 2 (and const_cast in 7.6.1.11 [expr.const.cast] is the same) says that the operand of a cast to an lvalue reference type must be an lvalue, so that
struct A { virtual ~A(); }; A &&make_a(); A &&a = dynamic_cast<A&&>(make_a()); // ok const A &b = dynamic_cast<const A&>(make_a()); // ill-formed
The behavior of static_cast is an odd hybrid:
struct B : A { }; B &&make_b(); A &&c = static_cast<A&&>(make_b()); // ok const A &d = static_cast<const A&>(make_b()); // ok const B &e = static_cast<const B&>(make_a()); // ill-formed
(Binding a const lvalue reference to an rvalue is permitted by 7.6.1.9 [expr.static.cast] paragraph 4 but not by paragraphs 2 and 3.)
There is implementation divergence on the treatment of these examples.
Also, const_cast permits binding an rvalue reference to a class prvalue but not to any other kind of prvalue, which seems like an unnecessary restriction.
Finally, 7.6.1.9 [expr.static.cast] paragraph 3 allows binding an rvalue reference to a class or array prvalue, but not to other kinds of prvalues; those are covered in paragraph 4. This would be less confusing if paragraph 3 only dealt with binding rvalue references to glvalues and left all discussion of prvalues to paragraph 4, which adequately handles the class and array cases as well.
Notes from the May, 2015 meeting:
CWG reaffirmed the status quo for dynamic_cast but felt that const_cast should be changed to permit binding an rvalue reference to types that have associated memory (class and array types).
The term “implicit conversion sequence” is now used in some non-call contexts (e.g., 7.6.1.9 [expr.static.cast] paragraph 4, 7.6.16 [expr.cond] paragraph 4, 7.6.10 [expr.eq] paragraph 4) ) and it is not clear that the current definition is suited for these additional uses. In particular, passing an argument in a function call is always copy-initialization, but some of these contexts require consideration of direct-initialization.
Notes from the December, 2016 teleconference:
The problem is that overload resolution relies on copy initalization and thus does not describe direct initialization. See also issue 1781.
(From submissions #526, #232, and #342.)
The resolution of issue 891 intended to make const_cast<int&&>(2) ill-formed. However, combined with the temporary materialization conversion turning prvalues into glvalues (7.1 [expr.pre] paragraph 7, this is now well-formed.
Also, the current rules regrettably allow const_cast<int>(0) and const_casts involving function pointers and pointers to member functions. The latter is non-normatively considered disallowed by the note in 7.6.1.11 [expr.const.cast] paragraph 9.
Major implementations except MSVC agree with the proposed direction of this issue.
Suggested resolution [SUPERSEDED]:
Change in 7.6.1.11 [expr.const.cast] paragraph 3 as follows:
For two similar types T1 and T2 (7.3.6 [conv.qual]), a prvalue of type T1 may be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each Pi1 is the same as Pi2 for all i. The result of a const_cast refers to the original entity.
If T is an object pointer type or pointer to data member type, the type of v shall be similar to T and the corresponding Pi components of the qualification decompositions of T and the type of v shall be the same (7.3.6 [conv.qual]). If v is a null pointer or null member pointer, the result is a null pointer or null member pointer, respectively. Otherwise, the result points to or past the end of the same object or member, respectively, as v.
[ Example: ... ]
Change in 7.6.1.11 [expr.const.cast] paragraph 4 as follows:
For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:
- an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
- a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
- if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>.
The result of a reference const_cast refers to the original object if the operand is a glvalue and to the result of applying the temporary materialization conversion (7.3.5 [conv.rval]) otherwise.
Otherwise, T shall be a reference type. Let T1 be the type of v and T2 be the type referred to by T. A const_cast from "pointer to T1" to "pointer to T2" shall be valid. If T is an lvalue reference type, v shall be an lvalue. Otherwise, if T2 is a class type and v is a prvalue, the temporary materialization conversion (7.3.5 [conv.rval]) is applied. Otherwise, the temporary materialization conversion is not applied and v shall be a glvalue. The result refers to the same object as the (possibly converted) operand.
Remove 7.6.1.11 [expr.const.cast] paragraph 5:
A null pointer value (6.8.4 [basic.compound]) is converted to the null pointer value of the destination type. The null member pointer value (7.3.13 [conv.mem]) is converted to the null member pointer value of the destination type.
Suggested resolution [SUPERSEDED]:
Change in 7.6.1.11 [expr.const.cast] paragraph 1 as follows:
The result of the expression const_cast<T>(v) is of type T. If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the expression v. The temporary materialization conversion (7.3.5 [conv.rval]) is not performed on v, other than as specified below. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast.
Change in 7.6.1.11 [expr.const.cast] paragraph 3 as follows:
For two similar object pointer or pointer to data member types T1 and T2 (7.3.6 [conv.qual]), a prvalue of type T1maycan be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each Pi1 is the same as Pi2 for all i.The result of a const_cast refers to the original entity.If v is a null pointer or null member pointer, the result is a null pointer or null member pointer, respectively. Otherwise, the result points to or past the end of the same object, or points to the same member, respectively, as v.
Change in 7.6.1.11 [expr.const.cast] paragraph 4 as follows:
For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:The result
- an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
- a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
- if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>. The temporary materialization conversion is performed on v.
of a reference const_castrefers to theoriginalsame objectifas the (possibly converted) operandis a glvalue and to the result of applying the temporary materialization conversion (7.3.5 [conv.rval]) otherwise.
Remove 7.6.1.11 [expr.const.cast] paragraph 5:
A null pointer value (6.8.4 [basic.compound]) is converted to the null pointer value of the destination type. The null member pointer value (7.3.13 [conv.mem]) is converted to the null member pointer value of the destination type.
CWG 2024-05-31
The existing example in 7.6.1.11 [expr.const.cast] paragraph 3 shows the temporary materialization conversion applied to an array type. The example would be made ill-formed by the suggested resolution above. More investigation is advised.
It is not clear from 7.6.2.8 [expr.new] whether a deleted operator delete is referenced by a new-expression in which there is no initialization or in which the initialization cannot throw an exception, rendering the program ill-formed. (The question also arises as to whether such a new-expression constitutes a “use” of the deallocation function in the sense of 6.3 [basic.def.odr].)
Notes from the July, 2009 meeting:
The rationale for defining a deallocation function as deleted would presumably be to prevent such objects from being freed. Treating the new-expression as a use of such a deallocation function would mean that such objects could not be created in the first place. There is already an exemption from freeing an object if “a suitable deallocation function [cannot] be found;” a deleted deallocation function should be treated similarly.
Additional notes (April, 2023):
An additional use-case for a deleted deallocation function would be to ensure that the initialization of the object is not potentially-throwing.
For cases where the deallocation function is never called from the constructor, access checking for it should not be done.
We should require that a program that replaces the aligned form of operator delete also replaces the sized+aligned form. We only allow a program to replace the non-sized form without replacing the sized form for backwards compatibility. This is not needed for the alignment feature, which is new.
Notes from the March, 2018 meeting:
CWG concurred with the recommendation.
Subclause 7.6.2.8 [expr.new] paragraph 28 specifies the lookup for the deallocation function that is invoked when the construction of the object in a new-expression exits via an exception. However, a destroying operator delete (6.7.5.5.3 [basic.stc.dynamic.deallocation]) should never be used, because the object in question has not yet been fully created.
Suggested resolution [SUPERSEDED]:
Change in 7.6.2.8 [expr.new] paragraph 28 as follows:
A declaration of a placement deallocation function matches the declaration of a placement allocation function if it has the same number of parameters and, after parameter transformations (9.3.4.6 [dcl.fct]), all parameter types except the first are identical. If the lookup finds a single matching deallocation function, that function will be called; otherwise, no deallocation function will be called. If the lookup finds a usual deallocation function and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed. For a non-placement allocation function, the normal deallocation function lookup is used to find the matching deallocation function (7.6.2.9 [expr.delete]) , except that any destroying operator delete (6.7.5.5.3 [basic.stc.dynamic.deallocation]) is ignored.
The common code sequence used by most implementations for pointer subtraction involves subtracting the pointer values to determine the number of bytes and then shifting to scale for the size of the array element. This produces incorrect results when the difference in bytes is larger than can be represented by a ptrdiff_t. For example, assuming a 32-bit ptrdiff_t:
int *a, *b; a = malloc(0x21000000 * sizeof(int)); b = a + 0x21000000; printf("%lx\n", (long)(b - a));
This will typically print e1000000 instead of 21000000.
Getting the right answer would require using a more expensive code sequence. It would be better to make this undefined behavior.
The current direction for issue 1776 (see paper P0137) calls into question the validity of doing pointer arithmetic to address separately-allocated but contiguous objects in a container like std::vector. A related question is whether there should be some allowance made for allowing pointer arithmetic using a pointer to a base class if the derived class is a standard-layout class with no non-static data members. It is possible that std::launder could play a part in the resolution of this issue.
Notes from the February, 2016 meeting:
This issue is expected to be resolved by the resolution of issue 1776. The major problem is when the elements of the vector contain constant or reference members; 6.7.3 [basic.life] paragraph 7 implies that pointer arithmetic leading to such an object produces undefined behavior, and CWG expects this to continue. Some changes to the interface of std::vector may be required, perhaps using std::launder as part of iterator processing.
The conditional operator converts pointer operands to their composite pointer type (7.6.16 [expr.cond] bullets 6.3 and 6.4). Similar treatment should be afforded to operands of reference type.
See also issue 2018.
According to 7.6.16 [expr.cond] paragraph 4,
Attempts are made to form an implicit conversion sequence from an operand expression E1 of type T1 to a target type related to the type T2 of the operand expression E2 as follows:
...
If E2 is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type:
if T1 and T2 are the same class type (ignoring cv-qualification) and T2 is at least as cv-qualified as T1, the target type is T2,
otherwise, if T2 is a base class of T1, the target type is cv1 T2, where cv1 denotes the cv-qualifiers of T1,
otherwise, the target type is the type that E2 would have after applying the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions.
It seems that to satisfy the conditions in the first two sub-bullets, T2 must be a class type, in which case T2 is the same as the type described in the third sub-bullet, since the lvalue-to-rvalue conversion does not change types and the other two conversions do not apply to a class type. Thus, this bullet and sub-bullets could be simplified to:
if E2 is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type, the target type is the type that E2 would have after applying the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions.
Notes from the August, 2020 teleconference:
This issue and suggested resolution predate the resolution of issue 2321, which added the second sub-bullet (the citation above reflects the wording after adoption of issue 2321), giving the result the cv-qualification of T1 instead of that of T2. The suggested resolution would revert that accepted resolution.
The specification of 7.6.19 [expr.ass] paragraph 9 is presumably intended to allow use of a braced-init-list as the operand of a compound assignment operator as well as a simple assignment operator, although the normative wording does not explicitly say so. (The example in that paragraph does include
complex<double> z;
z += { 1, 2 }; // meaning z.operator+=({1,2})
for instance, which could be read to imply compound assignment operators for scalar types as well.)
However, the details of how this is to be implemented are not clear. Paragraph 7 says,
The behavior of an expression of the form E1 op = E2 is equivalent to E1 = E1 op E2 except that E1 is evaluated only once.
Applying this pattern literally to a braced-init-list yields invalid code: x += {1} would become x = x + {1}, which is non-syntactic.
Another problem is how to apply the prohibition against narrowing conversions to a compound assignment. For example,
char c; c += {1};
would presumably always be a narrowing error, because after integral promotions, the type of c+1 is int. The similar issue 1078 was classified as "NAD" because the workaround was simply to add a cast to suppress the error; however, there is no place to put a similar cast in a compound assignment.
Notes from the October, 2012 meeting:
The incorrect description of the meaning of a compound assignment with a braced-init-list should be fixed by CWG. The question of whether it makes sense to apply narrowing rules to such assignments is better addressed by EWG.
See also issue 2399.
The current wording of the Standard is not sufficiently clear regarding the interaction of class scope (which treats the bodies of member functions as effectively appearing after the class definition is complete) and the use of constexpr member functions within the class definition in contexts requiring constant expressions. For example, an array bound cannot use a constexpr member function that relies on the completeness of the class or on members that have not yet been declared, but the current wording does not appear to state that.
Additional note (October, 2013):
This question also affects function return type deduction (the auto specifier) in member functions. For example, the following should presumably be prohibited, but the current wording is not clear:
struct S { static auto f() { return 42; } auto g() -> decltype(f()) { return f(); } };
CWG 2023-06-15
Definitions of member functions need an "as-needed" treatment. See issues 1890 and 2335.
According to 7.7 [expr.const] bullet 2.3, an expression is a constant expression unless (among other reasons) it would evaluate
an invocation of an undefined constexpr function or an undefined constexpr constructor;
This does not address the question of the point at which a constexpr function must be defined. The intent, in order to allow mutually-recursive constexpr functions, was that the function must be defined prior to the outermost evaluation that eventually results in the invocation, but this is not clearly stated.
Similar to the concern of issue 2166, the requirement of 7.7 [expr.const] bullet 2.7.1 for
a non-volatile glvalue of integral or enumeration type that refers to a complete non-volatile const object with a preceding initialization, initialized with a constant expression, or
does not specify the point at which the determination of “preceding initialization” is made: is it at the point at which the reference to the variable appears lexically, or is it the point at which the outermost constant evaluation occurs? There is implementation divergence on this point.
Converting a consteval lambda to a function pointer in a non-immediate context should be immediately-escalating.
Currently, this is well-formed:
auto x = &*[]() consteval { return 42; };
Suggested resolution:
Make the conversion function of a lambda whose call operator is an immediate function also an immediate function.
A simple example like
int main() { int k = 0; for (auto x : { 1, 2, 3 }) k += x; return k; }
requires that the <initializer_list> header be included, because the expansion of the range-based for involves a declaration of the form
auto &&__range = { 1, 2, 3 };
and a braced-init-list causes auto to be deduced as a specialization of std::initializer_list. This seems unnecessary and could be eliminated by specifying that __range has an array type for cases like this.
(It should be noted that EWG is considering a proposal to change auto deduction for cases involving braced-init-lists, so resolution of this issue should be coordinated with that effort.)
Notes from the September, 2013 meeting:
CWG felt that this issue should be resolved by using the array variant of the range-based for implementation.
The relative ordering between destruction of automatic variables on exit from a block and the release of the variables' storage is not specified by the Standard: are all the destructors executed first and then the storage released, or are they interleaved?
Notes from the February, 2016 meeting:
CWG agreed that the storage should persist until all destructions are complete, although the “as-if” rule would allow for unobservable optimizations of this ordering.
Because the restriction that a trailing-return-type can appear only in a declaration with “the single type-specifier auto” (9.3.4.6 [dcl.fct] paragraph 2) is a semantic, not a syntactic, restriction, it does not influence disambiguation, which is “purely syntactic” (8.9 [stmt.ambig] paragraph 3). Consequently, some previously unambiguous expressions are now ambiguous. For example:
struct A { A(int *); A *operator()(void); int B; }; int *p; typedef struct BB { int C[2]; } *B, C; void foo() { // The following line becomes invalid under C++0x: A (p)()->B; // ill-formed function declaration // In the following, // - B()->C is either type-id or class member access expression // - B()->C[1] is either type-id or subscripting expression // N3126 subclause 8.2 [dcl.ambig.res] does not mention an ambiguity // with these forms of expression A a(B ()->C); // function declaration or object declaration sizeof(B ()->C[1]); // sizeof(type-id) or sizeof on an expression }
Notes from the March, 2011 meeting:
CWG agreed that the presence of auto should be considered in disambiguation, even though it is formally handled semantically rather than syntactically.
CWG 2023-05-12
Both 8.9 [stmt.ambig] and 9.3.3 [dcl.ambig.res] need to be adjusted.
CWG 2023-06-13
Addressed by paper P2915R0.
According to 9.2.6 [dcl.constexpr] paragraph 6,
If no specialization of the template would satisfy the requirements for a constexpr function or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed; no diagnostic required.
This should say “instantiated template specialization” instead of just “specialization” to clarify that an explicit specialization is not in view here.
It is not clear whether the auto specifier can appear in a trailing-return-type.
The current wording allows something like
struct S { operator auto() { return 0; } } s;
If it is intended to be permitted, the details of its handling are not clear. Also, a similar syntax has been discussed as a possible future extension for dealing with proxy types in deduction which, if adopted, could cause confusion.
Additional note, November, 2013:
Doubt was expressed during the 2013-11-25 drafting review teleconference as to the usefulness of this provision. It is therefore being left open for further consideration after C++14 is finalized.
Notes from the February, 2014 meeting:
CWG continued to express doubt as to the usefulness of this construct but felt that if it is permitted, the rules need clarification.
Additional note (December, 2021):
See duplicate issue 2493 for additional details.
9.2.9 [dcl.type] paragraph 2 describes the auto specifier as “a placeholder for a type to be deduced.” Elsewhere, the Standard refers to the type represented by the auto specifier as a “placeholder type.” This usage has been deemed confusing by some, requiring either a definition of one or both terms or rewording to avoid them.
The grammar for type-id in 11.3 [class.name] paragraph 1 has two problems. First, the fact that we allow an abstract-pack-declarator makes some uses of type-id (template arguments, alignment specifiers, exception-specifications) ambiguous: T... could be parsed either as a type-id, including the ellipsis, or as the type-id T with a following ellipsis. There does not appear to be any rule to disambiguate these parses.
The other problem is that we do not allow parentheses in an abstract-pack-declarator, which makes
template<typename...Ts> void f(Ts (&...)[4]);
ill-formed because (&...)() is not an abstract-pack-declarator. There is implementation variance on this point.
According to 9.3.4.6 [dcl.fct] paragraph 5,
The type of a function is determined using the following rules. The type of each parameter (including function parameter packs) is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type “array of T” or “function returning T” is adjusted to be “pointer to T” or “pointer to function returning T,” respectively. After producing the list of parameter types, any top-level cv-qualifiers modifying a parameter type are deleted when forming the function type. The resulting list of transformed parameter types and the presence or absence of the ellipsis or a function parameter pack is the function's parameter-type-list. [Note: This transformation does not affect the types of the parameters. For example, int(*)(const int p, decltype(p)*) and int(*)(int, const int*) are identical types. —end note]
This is not sufficiently clear to specify the intended handling of an example like
void f(int a[10], decltype(a) *p );
Should the type of p be int(*)[10] or int**? The latter is the intended result, but the phrase “after determining the type of each parameter” makes it sound as if the adjustments are performed after all the parameter types have been determined from the decl-specifier-seq and declarator instead of for each parameter individually.
See also issue 1444.
9.3.4.6 [dcl.fct] paragraph 3 specifies the grammar for parameter-declaration:
parameter-declaration: attribute-specifier-seqopt thisopt decl-specifier-seq declarator attribute-specifier-seqopt thisopt decl-specifier-seq declarator = initializer-clause attribute-specifier-seqopt thisopt decl-specifier-seq abstract-declaratoropt attribute-specifier-seqopt thisopt decl-specifier-seq abstract-declaratoropt = initializer-clause
This is overly permissive; using a defining-type-specifier-seq instead of a decl-specifier-seq is sufficient.
Proposed resolution (November, 2022):
Change in 9.2.2 [dcl.stc] paragraph 4 as follows:
There can be no static function declarations within a block, nor any static function parameters.
Change in 9.2.2 [dcl.stc] paragraph 5 as follows:
The extern specifier shall not be used in the declaration of a class memberor function parameter.
Change in 9.2.4 [dcl.typedef] paragraph 1 as follows:
The typedef specifier shall not be combined in a decl-specifier-seq with any other kind of specifier except a defining-type-specifier, and it shall not be usedin the decl-specifier-seq of a parameter-declaration (9.3.4.6 [dcl.fct]) norin the decl-specifier-seq of a function-definition (9.5 [dcl.fct.def]).
Change in 9.2.8 [dcl.inline] paragraph 4 as follows:
The inline specifier shall not appear on a block scope declarationor on the declaration of a function parameter.
Change in 9.3.4.6 [dcl.fct] paragraph 3 as follows:
parameter-declaration: attribute-specifier-seqopt thisoptdecl-specifier-seqdefining-type-specifier-seq declarator attribute-specifier-seqopt thisoptdecl-specifier-seqdefining-type-specifier-seq declarator = initializer-clause attribute-specifier-seqopt thisoptdecl-specifier-seqdefining-type-specifier-seq abstract-declaratoropt attribute-specifier-seqopt thisoptdecl-specifier-seqdefining-type-specifier-seq abstract-declaratoropt = initializer-clause
CWG 2023-02-07
Additional drafting is needed to address references to decl-specifier-seq in other parts of the standard. A list is here. Furthermore, reducing the grammar to a type-specifier-seq appears to be sufficient.
The standard is not precise enough about when the default arguments of member functions are parsed. This leads to confusion over whether certain constructs are legal or not, and the validity of certain compiler implementation algorithms.
9.3.4.7 [dcl.fct.default] paragraph 5 says "names in the expression are bound, and the semantic constraints are checked, at the point where the default argument expression appears"
However, further on at paragraph 9 in the same section there is an example, where the salient parts are
int b; class X { int mem2 (int i = b); // OK use X::b static int b; };which appears to contradict the former constraint. At the point the default argument expression appears in the definition of X, X::b has not been declared, so one would expect ::b to be bound. This of course appears to violate 6.4.7 [basic.scope.class] paragraph 1(2) "A name N used in a class S shall refer to the same declaration in its context and when reevaluated in the complete scope of S. No diagnostic is required."
Furthermore 6.4.7 [basic.scope.class] paragraph 1(1) gives the scope of names declared in class to "consist not only of the declarative region following the name's declarator, but also of .. default arguments ...". Thus implying that X::b is in scope in the default argument of X::mem2 previously.
That previous paragraph hints at an implementation technique of saving the token stream of a default argument expression and parsing it at the end of the class definition (much like the bodies of functions defined in the class). This is a technique employed by GCC and, from its behaviour, in the EDG front end. The standard leaves two things unspecified. Firstly, is a default argument expression permitted to call a static member function declared later in the class in such a way as to require evaluation of that function's default arguments? I.e. is the following well formed?
class A { static int Foo (int i = Baz ()); static int Baz (int i = Bar ()); static int Bar (int i = 5); };If that is well formed, at what point does the non-sensicalness of
class B { static int Foo (int i = Baz ()); static int Baz (int i = Foo()); };become detected? Is it when B is complete? Is it when B::Foo or B::Baz is called in such a way to require default argument expansion? Or is no diagnostic required?
The other problem is with collecting the tokens that form the default argument expression. Default arguments which contain template-ids with more than one parameter present a difficulty in determining when the default argument finishes. Consider,
template <int A, typename B> struct T { static int i;}; class C { int Foo (int i = T<1, int>::i); };The default argument contains a non-parenthesized comma. Is it required that this comma is seen as part of the default argument expression and not the beginning of another of argument declaration? To accept this as part of the default argument would require name lookup of T (to determine that the '<' was part of a template argument list and not a less-than operator) before C is complete. Furthermore, the more pathological
class D { int Foo (int i = T<1, int>::i); template <int A, typename B> struct T {static int i;}; };would be very hard to accept. Even though T is declared after Foo, T is in scope within Foo's default argument expression.
Suggested resolution:
Append the following text to 9.3.4.7 [dcl.fct.default] paragraph 8.
The default argument expression of a member function declared in the class definition consists of the sequence of tokens up until the next non-parenthesized, non-bracketed comma or close parenthesis. Furthermore such default argument expressions shall not require evaluation of a default argument of a function declared later in the class.
This would make the above A, B, C and D ill formed and is in line with the existing compiler practice that I am aware of.
Notes from the October, 2005 meeting:
The CWG agreed that the first example (A) is currently well-formed and that it is not unreasonable to expect implementations to handle it by processing default arguments recursively.
Additional notes, May, 2009:
Presumably the following is ill-formed:
int f(int = f());
However, it is not clear what in the Standard makes it so. Perhaps there needs to be a statement to the effect that a default argument only becomes usable after the complete declarator of which it is a part.
Notes from the August, 2011 meeting:
In addition to default arguments, commas in template argument lists also cause problems in initializers for nonstatic data members:
struct S { int n = T<a,b>(c); // ill-formed declarator for member b // or template argument? };
(This is from #16 of the IssuesFoundImplementingC0x.pdf document on the Bloomington wiki.
Additional notes (August, 2011):
Notes from the February, 2012 meeting:
It was decided to handle the question of parsing an initializer like T<a,b>(c) (a template-id or two declarators) in this issue and the remaining questions in issue 361. For this issue, a template-id will only be recognized if there is a preceding declaration of a template.
Additional note (November, 2020):
Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.
It is not clear, either from 9.3.4.7 [dcl.fct.default] or 13.9.3 [temp.explicit], whether it is permitted to add a default argument in an explicit instantiation of a function template:
template<typename T> void f(T, int) { }
template void f<int>(int, int=0); // Permitted?
Notes from the April, 2013 meeting:
The intent is to prohibit default arguments in explicit instantiations.
Consider an example like:
struct Cat {}; struct Dog { operator Cat(); }; Dog d; Cat c(d);
This goes to 9.4 [dcl.init] bullet 17.6.2:
Otherwise, if the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified version of the source type is the same class as, or a derived class of, the class of the destination, constructors are considered. The applicable constructors are enumerated (12.2.2.4 [over.match.ctor]), and the best one is chosen through overload resolution (12.2 [over.match]). The constructor so selected is called to initialize the object, with the initializer expression or expression-list as its argument(s). If no constructor applies, or the overload resolution is ambiguous, the initialization is ill-formed.
Overload resolution selects the move constructor of Cat. Initializing the Cat&& parameter of the constructor results in a temporary, per 9.4.4 [dcl.init.ref] bullet 5.2.1.2. This precludes the possitiblity of copy elision for this case.
This seems to be an oversight in the wording change for guaranteed copy elision. We should presumably be simultaneously considering both constructors and conversion functions in this case, as we would for copy-initialization, but we'll need to make sure that doesn't introduce any novel problems or ambiguities.
See also issue 2311.
This issue is addressed by paper P2828.
According to 11.9.3 [class.base.init] paragraph 11,
A temporary expression bound to a reference member from a brace-or-equal-initializer is ill-formed. [Example:
struct A { A() = default; // OK A(int v) : v(v) { } // OK const int& v = 42; // OK }; A a1; // error: ill-formed binding of temporary to reference A a2(1); // OK, unfortunately—end example]
The rule is intended to apply only if an actual initialization results in such a binding, but it could be read as applying to the declaration of A::v itself. It would be clearer if the restriction were moved into bullet 9.1, e.g.,
if the entity is a non-static data member that has a brace-or-equal-initializer and either
the constructor's class is a union (11.5 [class.union]), and no other variant member of that union is designated by a mem-initializer-id or
the constructor's class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id,
the entity is initialized as specified in 9.4 [dcl.init], and the program is ill-formed if the entity is a reference member and this initialization binds it to a temporary;
The example in 9.4.3 [dcl.init.string] paragraph 1 says,
char msg[] = "Syntax error on line %s\n";shows a character array whose members are initialized with a string-literal. Note that because '\n' is a single character and because a trailing '\0' is appended, sizeof(msg) is 25.
However, there appears to be no normative specification of how the size of the array is to be calculated.
Currently an attempt to bind an rvalue reference to a reference-unrelated lvalue succeeds, binding the reference to a temporary initialized from the lvalue by copy-initialization. This appears to be intentional, as the accompanying example contains the lines
int i3 = 2; double&& rrd3 = i3; // rrd3 refers to temporary with value 2.0
This violates the expectations of some who expect that rvalue references can be initialized only with rvalues. On the other hand, it is parallel with the handling of an lvalue reference-to-const (and is handled by the same wording). It also can add efficiency without requiring existing code to be rewritten: the implicitly-created temporary can be moved from, just as if the call had been rewritten to create a prvalue temporary from the lvalue explicitly.
On a related note, assuming the binding is permitted, the intent of the overload tiebreaker found in 12.2.4.3 [over.ics.rank] paragraph 3 is not clear:
Standard conversion sequence S1 is a better conversion sequence than standard conversion sequence S2 if
...
S1 and S2 are reference bindings (9.4.4 [dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and S1 binds an rvalue reference to an rvalue and S2 binds an lvalue reference.
At question is what “to an rvalue” means here. If it is referring to the value category of the initializer itself, before conversions, then the supposed performance advantage of the binding under discussion does not occur because the competing rvalue and lvalue reference overloads will be ambiguous:
void f(int&&); // #1 void f(const int&); void g(double d) { f(d); // ambiguous: #1 does not bind to an rvalue }
On the other hand, if “to an rvalue” refers to the actual object to which the reference is bound, i.e., to the temporary in the case under discussion, the phrase would seem to be vacuous because an rvalue reference can never bind directly to an lvalue.
Notes from the February, 2012 meeting:
CWG agreed that the binding rules are correct, allowing creation of a temporary when binding an rvalue reference to a non-reference-related lvalue. The phrase “to an rvalue” in 12.2.4.3 [over.ics.rank] paragraph 3 is a leftover from before binding an rvalue reference to an lvalue was prohibited and should be removed. A change is also needed to handle the following case:
void f(const char (&)[1]); // #1 template<typename T> void f(T&&); // #2 void g() { f(""); //calls #2, should call #1 }
Additional note (October, 2012):
Removing “to an rvalue,” as suggested, would have the effect of negating the preference for binding a function lvalue to an lvalue reference instead of an rvalue reference because the case would now fall under the preceding bullet of 12.2.4.3 [over.ics.rank] bullet 3.1, sub-bullets 4 and 5:
Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:
Standard conversion sequence S1 is a better conversion sequence than standard conversion sequence S2 if
...
S1 and S2 are reference bindings (9.4.4 [dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and S1 binds an rvalue reference to an rvalue and S2 binds an lvalue reference... or, if not that,
S1 and S2 are reference bindings (9.4.4 [dcl.init.ref]) and S1 binds an lvalue reference to a function lvalue and S2 binds an rvalue reference to a function lvalue.
Presumably if the suggested resolution is adopted, the order of these two bullets should be inverted.
In the following case,
struct A { operator int &&() const; operator int &&() volatile; operator long(); }; int main() { int &&x = A(); }
the conversion for direct binding cannot be used because of the ambiguity, so indirect binding is used, which allows the use of the conversion to long in creating the temporary.
Is this intended? There is implementation variation.
Notes from the February, 2014 meeting:
CWG agreed that an ambiguity like this should make the initialization ill-formed instead of falling through to do indirect binding.
The specification for list-initialization of a reference does not consider the existence of conversion functions. Consequently, the following example is ill-formed:
struct S { operator struct D &(); } s; D &d{s};
Consider:
struct A { A(); A(A const&) = delete; }; int main(){ A a = A(A(A())); // #1 A b = A{A{A{}}}; // #2 }
#1 is well-formed per 9.4.1 [dcl.init.general] bullet 16.6.1. However, even though #2 is intended to have a similar effect, the relevant rule excludes non-aggregates, making the example ill-formed (because the relevant constructor is deleted); see 9.4.5 [dcl.init.list] bullet 3.2:
If T is an aggregate class and the initializer list has a single element of type cv U, where U is T or a class derived from T, the object is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization).
There is implementation divergence: gcc and clang accept, MSVC rejects.
See also issue 2311.
Suggested resolution:
Change in 9.4.5 [dcl.init.list] bullet 3.2 as follows:
If T isan aggregate classa class type and the initializer list has a single element of type cv U, where U is T or a class derived from T, the object is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization).
CWG 2024-03-01
Recent MSVC no longer rejects the example. The suggested resolution would revert issue 2137, which is not desirable. The consensus is to try an initializer-list constructor first and then fall back to copy-initialization (with guaranteed copy elision). A change to 12.2.4.2.6 [over.ics.list] may be necessary.
The resolution of issue 1778 means that whether an explicitly-defaulted function is deleted or not cannot be known until the end of the class definition. As a result, new rules are required to disallow references (in, e.g., decltype) to explicitly-defaulted functions that might later become deleted.
Notes from the June, 2014 meeting:
The approach favored by CWG was to make any reference to an explicitly-defaulted function ill-formed if it occurs prior to the end of the class definition.
Subclause 9.5.4 [dcl.fct.def.coroutine] paragraph 7 specifies:
The expression promise.get_return_object() is used to initialize the returned reference or prvalue result object of a call to a coroutine. The call to get_return_object is sequenced before the call to initial-suspend and is invoked at most once.
It is unclear:
There is implementation divergence.
Note that a user-defined conversion may be involved in the initialization of the coroutine's prvalue result object from get_return_object(). Note also that the return type of get_return_object might be non-copyable and non-movable. However, there are certain programming patterns that would benefit from a late-initialized return value.
See also compiler explorer.
Suggested resolution [SUPERSEDED]:
Change in 9.5.4 [dcl.fct.def.coroutine] paragraph 7 as follows:
The expression promise.get_return_object() is used to initialize theThe returned reference or prvalue result object of a call to a coroutine is copy-initialized with promise.get_return_object(). Thecall to get_return_objectinitialization is sequenced before the call to initial-suspendand is invoked at most once.
Additional notes (January, 2023)
See also clang bug report #56532.
Forwarded to EWG with paper issue 1414, by decision of the CWG chair.
EWG 2023-02-06
EWG agrees that get_return_object is invoked outside of the try-block and that, if a conversion is needed, the return value of get_return_object is considered an xvalue that is later converted to the result object.
The scope in which the names of enumerators are entered for a member unscoped opaque enumeration is not clear. According to 9.7.1 [dcl.enum] paragraph 10,
Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier.
In the case of a member opaque enumeration defined outside its containing class, however, it is not clear whether the enumerator names are declared in the class scope or in the lexical scope containing the definition. Declaring them in the class scope would be a violation of 11.4 [class.mem] paragraph 1:
The member-specification in a class definition declares the full set of members of the class; no member can be added elsewhere.
Declaring the names in the lexical scope containing the definition would be contrary to the example in 13.7.2.6 [temp.mem.enum] paragraph 1:
template<class T> struct A { enum E : T; }; A<int> a; template<class T> enum A<T>::E : T { e1, e2 }; A<int>::E e = A<int>::e1;
There also appear to be problems with the rules for dependent types and members of the current instantiation.
Notes from the October, 2012 meeting:
CWG agreed that an unscoped opaque enumeration in class scope should be forbidden.
The declaration
enum E;
is ambiguous: it could be either a simple-declaration comprising the elaborated-type-specifier enum E and no init-declarator-list, or it could be an opaque-enum-declaration with an omitted enum-base (both of which are ill-formed, for different reasons).
(See also issue 2363.)
According to 9.8.2.2 [namespace.unnamed] paragraph 1,
An unnamed-namespace-definition behaves as if it were replaced by
inlineopt namespace unique { /* empty body */ }
using namespace unique ;
namespace unique { namespace-body }where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the translation unit.
The use of a single identifier for all occurrences of unique within a translation unit leads to problems when an inline unnamed namespace contains a nested unnamed namespace, e.g.,
inline namespace { namespace { } }
In this case, the unnamed namespace cannot be reopened because the lookup for unique finds both the outer and inner namespaces and is thus ambiguous.
Suggested resolution:
Change 9.8.2.2 [namespace.unnamed] paragraph 1 as follows:
...where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in each scope in a translation unit are replaced by the same scope-specific identifier, and this identifier differs from all other identifiers in the translation unit.
Notes from the December, 2021 teleconference:
The suggested resolution deals specifically with unnamed namespaces, but there are related examples that do not involve unnamed namespaces. The problem needs to be solved more generally in the specification of lookup.
Consider:
template<int I> struct C { }; struct B { C<1> foo(); C<1> bar(); }; struct D : B { using B::foo; C<2> foo(this B &); using B::bar; C<2> bar(this D &); }; struct DD : D { using D::foo; using D::bar; }; void bar(D d, DD dd) { d.foo(); dd.foo(); d.bar(); dd.bar(); }
Which functions are called?
Subclause 9.9 [namespace.udecl] paragraph 11 specifies:
The set of declarations named by a using-declarator that inhabits a class C does not include member functions and member function templates of a base class that correspond to (and thus would conflict with) a declaration of a function or function template in C.
The definition of "corresponds" considers the type of the implicit object parameter, which is a deviation from the status quo ante for a simple example like this one:
struct B {
void f(); // #1
};
struct D : B {
void f();
using B::f; // should not name #1
};
Suggested resolution:
Change in 9.9 [namespace.udecl] paragraph 11 as follows:
The set of declarations named by a using-declarator that inhabits a class C does not include member functions and member function templates of a base class that, when considered as members of C, correspond to (and thus would conflict with) a declaration of a function or function template in C.[ Example:
struct B { virtual void f(int); virtual void f(char); void g(int); void h(int); void i(); void j(); }; struct D : B { using B::f; void f(int); // OK, D::f(int) overrides B::f(int) using B::g; void g(char); // OK using B::h; void h(int); // OK, D::h(int) hides B::h(int) using B::i; void i(this B &); // OK using B::j; void j(this D &); // OK, D::j() hides B::j() }; void k(D* p) { p->f(1); // calls D::f(int) p->f('a'); // calls B::f(char) p->g(1); // calls B::g(int) p->g('a'); // calls D::g(char) p->i(); // calls B::i, because B::i as a member of D is a better match than D::i p->j(); // calls D::j } ...
According to 9.1 [dcl.pre] paragraph 2,
Unless otherwise stated, utterances in Clause 9 [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof refer only to those components of the declaration that are not nested within scopes nested within the declaration.
This contradicts the intent of 9.11 [dcl.link] paragraph 4, which says,
In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification.
Also, one of the comments in the example in paragraph 4 is inconsistent with the intent:
extern "C" { static void f4(); // the name of the function f4 has // internal linkage (not C language // linkage) and the function's type // has C language linkage. } extern "C" void f5() { extern void f4(); // OK: Name linkage (internal) // and function type linkage (C // language linkage) gotten from // previous declaration. }
The language linkage for the block-scope declaration of f4 is presumably determined by the fact that it appears in a C-linkage function, not by the previous declaration.
Proposed resolution (February, 2014):
Change 9.11 [dcl.link] paragraph 4 as follows:
Linkage specifications nest. When linkage specifications nest, the innermost one determines the language linkage. A linkage specification does not establish a scope. A linkage-specification shall occur only in namespace scope (6.4 [basic.scope]). In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification, including those appearing in scopes nested inside the linkage specification and not inside a nested linkage-specification. [Example:
...
extern "C" { static void f4(); // the name of the function f4 has // internal linkage (not C language // linkage) and the function's type // has C language linkage. } extern "C" void f5() { extern void f4(); // OK: Name linkage (internal) //and function type linkage (C//language linkage)gotten from // previous declaration.; function type // linkage (C language // linkage) gotten // from linkage specification }
Additional note, November, 2014:
The issue has been returned to "drafting" status to clarify the circumstances under which a preceding declaration supplies the language linkage for a declaration (for example, not when the declaration uses a typedef, which carries the language linkage, but only when the declaration uses a function declarator).
The grammar for alignment-specifier in 9.12.1 [dcl.attr.grammar] paragraph 1 is:
where the ellipsis indicates pack expansion. Naively, one would expect that the expansion would result in forms like
alignas() alignas(1, 2) alignas(int, double)
but none of those forms is given any meaning by the current wording. Instead, 13.7.4 [temp.variadic] paragraph 4 says,
In an alignment-specifier (9.12.2 [dcl.align]); the pattern is the alignment-specifier without the ellipsis.
Presumably this means that something like alignas(T...) would expand to something like
alignas(int) alignas(double)
This is counterintuitive and should be reexamined.
See also messages 24016 through 24021.
Notes from the February, 2014 meeting:
CWG decided to change the pack expansion of alignas so that the type-id or assignment-expression is repeated inside the parentheses and to change the definition of alignas to accept multiple arguments with the same meaning as multiple alignas specifiers.
According to 9.12.2 [dcl.align] paragraph 4,
The alignment requirement of an entity is the strictest non-zero alignment specified by its alignment-specifiers, if any; otherwise, the alignment-specifiers have no effect.
It is not clear whether this applies to specifiers within a single declaration, or if it is intended to apply to the union of all declarations.
Similarly, paragraph 6 says,
If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment. No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.
This only talks about agreement between definitions and non-defining declarations. What about an example where an entity is not defined but is declared with different alignment-specifiers?
struct alignas(16) A; struct alignas(32) A;
If A is not defined, is this, or should it be, ill-formed?
Notes from the February, 2017 meeting:
CWG agreed that the intent of the wording is that the “strictest” requirement is intended to apply to a single declaration, and the requirement for compatibility should apply to all declarations, whether the entity is defined or not.
Consider:
// module interface unit export module M; export enum E : int; enum E : int { e }; // other translation unit import M; auto a = E::e; // #1: OK? auto b = e; // #2: OK?
It is unclear whether the enumerator name e is or ought to be visible in the other translation unit.
See also issues 2588 (friend declarations) and 2480.
CWG 2022-11-10
See 10.2 [module.interface] paragraph 7.
Consider an example like:
struct A { struct B { auto foo() { return 0; } }; decltype(B().foo()) x; };
There does not appear to be a prohibition of cases like this, where the type of a member depends on the definition of a member function.
(See also issues 1360, 1397, and 2335.)
Additional notes (January, 2023):
The following example might be related:
#include <type_traits> struct Bar { struct Baz { int a = 0; }; static_assert(std::is_default_constructible_v<Baz>); };
Additional notes (November, 2023):
The following example is also rejected by implementations, but it is allowed by the rules:
struct A { static bool f2() { return f(); } static consteval bool f() { return true; } };
According to 11.4.5 [class.ctor] paragraph 5,
A defaulted default constructor for class X is defined as deleted if:
X is a union-like class that has a variant member with a non-trivial default constructor,
...
X is a union and all of its variant members are of const-qualified type (or array thereof),
X is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof),
...
Because the presence of a non-static data member initializer is the moral equivalent of a mem-initializer, these rules should probably be modified not to define the generated constructor as deleted when a union member has a non-static data member initializer. (Note the non-normative references in 11.5 [class.union] paragraphs 2-3 and 9.2.9.2 [dcl.type.cv] paragraph 2 that would also need to be updated if this restriction is changed.)
It would also be helpful to add a requirement to 11.5 [class.union] requiring either a non-static data member initializer or a user-provided constructor if all the members of the union have const-qualified types.
On a more general note, why is the default constructor defined as deleted just because a member has a non-trivial default constructor? The union itself doesn't know which member is the active one, and default construction won't initialize any members (assuming no brace-or-equal-initializer). It is up to the “owner” of the union to control the lifetime of the active member (if any), and requiring a user-provided constructor is forcing a design pattern that doesn't make sense. Along the same lines, why is the default destructor defined as deleted just because a member has a non-trivial destructor? I would agree with this restriction if it only applied when the union also has a user-provided constructor.
See also issues 1460, 1562, 1587, and 1621.
It is not clear when, if ever, a constructor template can be considered to provide a default constructor. For example:
struct A { template<typename ...T> A(T...); // #1 A(std::initializer_list<long>); // #2 }; A a{};
According to 9.4.5 [dcl.init.list] paragraph 3, A will be value-initialized if it has a default constructor, and there is implementation divergence whether this example calls #1 or #2.
Similarly, for an example like
struct B { template<typename T=int> B(T = 0); };
it is not completely clear whether a default constructor should be implicitly declared or not.
More generally, do utterances in the Standard concerning “constructors” also apply to constructor templates?
Notes from the February, 2014 meeting:
One possibility discussed was that we may need to change places that explicitly refer to a default constructor to use overload resolution, similar to the change that was made a few years ago with regard to copy construction vs “copy constructor.” One additional use of “default constructor” is in determining the triviality of a class, but it might be a good idea to remove the concept of a trivial class altogether. This possibility will be explored.
Notes from the February, 2016 meeting:
CWG reaffirmed the direction from the preceding note and also determined that the presence of a constructor template should suppress implicit declaration of a default constructor.
Additional notes (April, 2024)
The standard does not seem to contain a use of "trivial class" outside of examples. See paper P3247R0 Deprecate the notion of trivial types.
Additional examples to consider for the definition of "trivial class":
struct A { A() = default; //eligible, second constructor unsatisfied template<class... Args> A(Args&&... args) requires (sizeof...(Args) > 0) {} }; struct B { B() = default; //ineligible, second constructor more constrained template<class... Args> B(Args&&... args) requires (sizeof...(Args) == 0) {} }; struct C { C() = default; //eligible, but template<class... Args> //also eligible and non-trivial C(Args&&... args) {} };
See also issue 1363.
(See also submission #545.)
Consider:
struct A { int n; }; struct B : A { using A::A; B(int); };
Does B have a default constructor?
Suggested resolution [SUPERSEDED]:
Change in 9.9 [namespace.udecl] paragraph 4 as follows:
If a constructor or assignment operator brought from a base class into a derived class has the signature of a default constructor or copy/move constructor or assignment operator for the derived class (11.4.5.2 [class.default.ctor], 11.4.5.3 [class.copy.ctor], 11.4.6 [class.copy.assign]), the using-declaration does not by itself suppress the implicit declaration of the derived class member; the member from the base class is hidden or overridden by the implicitly-declaredcopy/move constructor or assignment operatorspecial member function of the derived class, as described below.
Change in 11.4.5.2 [class.default.ctor] paragraph 1 as follows:
A default constructor for a class X is a constructor of class X for which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters). If there is no user-declared constructor for class X, or if X inherits (9.9 [namespace.udecl]) one or more default constructors and there is no user-declared default constructor for X, a non-explicit constructor having no parameters is implicitly declared as defaulted (9.5 [dcl.fct.def]). An implicitly-declared default constructor is an inline public member of its class.
Proposed resolution [SUPERSEDED]:
(This also resolves issue 2632.)
Change in 9.9 [namespace.udecl] paragraph 4 as follows:
If a constructor or assignment operator brought from a base class into a derived class has the signature of a copy/move constructor or assignment operator for the derived class (11.4.5.3 [class.copy.ctor], 11.4.6 [class.copy.assign]), the using-declaration does not by itself suppress the implicit declaration of the derived class member; the member from the base class is hidden or overridden by the implicitly-declared copy/move constructor or assignment operator of the derived class, as described below.[ Note: A using-declarator that names a member function of a base class does not suppress the implicit declaration of a special member function in the derived class, even if their signatures are the same (11.4.5.2 [class.default.ctor], 11.4.5.3 [class.copy.ctor], 11.4.6 [class.copy.assign]). -- end note ]
Add a new paragraph before 11.4.1 [class.mem.general] paragraph 2 as follows:
... For any other member-declaration, each declared entity that is not an unnamed bit-field (11.4.10 [class.bit]) is a member of the class, termed a user-declared member, and each such member-declaration shall either declare at least one member name of the class or declare at least one unnamed bit-field.
Change in 11.4.5.2 [class.default.ctor] paragraph 1 as follows:
A default constructor for a class X is a constructor of class X for which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters). Ifthere is noa class X does not have a user-declared constructorfor class X, or if X inherits (9.9 [namespace.udecl]) one or more default constructors and X does not have a user-declared default constructor, a non-explicit constructor having no parameters is implicitly declared as defaulted (9.5 [dcl.fct.def]). An implicitly-declared default constructor is an inline public member of its class. [ Example:struct A {}; struct B {}; struct C : A, B { using A::A, B::B; C(int); }; C c; // OK struct X { X(int = 0, int = 0); }; // #1 struct Y { Y(int = 0); }; // #2 struct Z : X, Y { using X::X, Y::Y; }; Z z2(1, 1); // OK, invokes X(1, 1) and Y() Z z1(1); // error: ambiguous between #1 and #2 Z z0; // OK, invokes X() and Y()-- end example ]
Change in 11.4.5.3 [class.copy.ctor] paragraph 6 as follows:
If the classdefinitiondoes notexplicitly declarehave a user-declared copy constructor, a non-explicit one is declared implicitly. ...
Change in 11.4.5.3 [class.copy.ctor] paragraph 8 as follows:
Ifthe definition ofa class X does notexplicitly declarehave a user-declared move constructor, a non-explicit one will be implicitly declared as defaulted if and only if ...
Add a new paragraph before 11.4.5.3 [class.copy.ctor] paragraph 11 as follows:
[ Note: A using-declaration in a derived class C that names a constructor from a base class never suppresses the implicit declaration of a copy/move constructor of C, even if the base class constructor would be a copy or move constructor if declared as a member of C. -- end note]
A copy/move constructor for class X is trivial if it is not user-provided and if: ...
Change in 11.4.6 [class.copy.assign] paragraph 2 as follows:
If the classdefinitiondoes notexplicitly declarehave a user-declared copy assignment operator, one is declared implicitly. If the classdefinition declareshas a user-declared move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defaulted (9.5 [dcl.fct.def]). The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor (D.7 [depr.impldec]). ...
Additional notes (November, 2023)
How does access checking interact with the proposed resolution above?
struct B { protected: B(int = 0); }; struct A : B { using B::B; A(void *); }; A a; // okay? A aa(42); // not okay
CWG 2023-11-06
CWG resolved not to declare a default constructor in the derived class, but instead apply the usual rules for inherited constructors for this case. The wording should be changed so that the presence of a default constructor is never checked, in particular for "trivial class" (11.2 [class.prop], fixed by P3247R1 (Deprecate the notion of trivial types)), vacuous initialization (6.7.3 [basic.life] paragraph 1, fixed by issue 2859), and value initialization (9.4.1 [dcl.init.general] paragraph 9, also fixed by issue 2859).
Moving to always doing overload resolution for determining exception specifications and implicit deletion creates some unfortunate cycles:
template<typename T> struct A { T t; }; template <typename T> struct B { typename T::U u; }; template <typename T> struct C { C(const T&); }; template <typename T> struct D { C<B<T> > v; }; struct E { typedef A<D<E> > U; }; extern A<D<E> > a; A<D<E> > a2(a);
If declaring the copy constructor for A<D<E>> is part of instantiating the class, then we need to do overload resolution on D<E>, and thus C<B<E>>. We consider C(const B<E>&), and therefore look to see if there's a conversion from C<B<E>> to B<E>, which instantiates B<E>, which fails because it has a field of type A<D<E>> which is already being instantiated.
Even if we wait until A<D<E>> is considered complete before finalizing the copy constructor declaration, declaring the copy constructor for B<E> will want to look at the copy constructor for A<D<E>>, so we still have the cycle.
I think that to avoid this cycle we need to short-circuit consideration of C(const T&) somehow. But I don't see how we can do that without breaking
struct F { F(F&); }; struct G; struct G2 { G2(const G&); }; struct G { G(G&&); G(const G2&); }; struct H: F, G { }; extern H h; H h2(h);
Here, since G's move constructor suppresses the implicit copy constructor, the defaulted H copy constructor calls G(const G2&) instead. If the move constructor did not suppress the implicit copy constructor, I believe the implicit copy constructor would always be viable, and therefore a better match than a constructor taking a reference to another type.
So perhaps the answer is to reconsider that suppression and then disqualify any constructor taking (a reference to) a type other than the constructor's class from consideration when looking up a subobject constructor in an implicitly defined constructor. (Or assignment operator, presumably.)
Another possibility would be that when we're looking for a conversion from C<B<E>> to B<E> we could somehow avoid considering, or even declaring, the B<E> copy constructor. But that seems a bit dodgy.
Additional note (October, 2010):
An explicitly declared move constructor/op= should not suppress the implicitly declared copy constructor/op=; it should cause it to be deleted instead. This should prevent a member function taking a (reference to) an un-reference-related type from being chosen by overload resolution in a defaulted member function.
And we should clarify that member functions taking un-reference-related types are not even considered during overload resolution in a defaulted member function, to avoid requiring their parameter types to be complete.
The current wording of 11.4.5.3 [class.copy.ctor] paragraph 31 refers only to constructors and destructors:
When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects.
However, in some cases (e.g., auto_ptr) a conversion function is also involved in the copying, and it could presumably also have visible side effects that would be eliminated by copy elision. (Some additional contexts that may also require changes in this regard are mentioned in the resolution of issue 535.)
Additional note (September, 2012):
The default arguments of an elided constructor can also have side effects and should be mentioned, as well; however, the elision should not change the odr-use status of functions and variables appearing in those default arguments.
The implicit declaration of a special member function sometimes requires overload resolution, in order to select a special member to use for base classes and non-static data members. This can be required to determine whether the member is or would be deleted, and whether the member is trivial, for instance. The standard appears to require such overload resolution be performed at the end of the definition of the class, but in practice, implementations perform it lazily. This optimization appears to be non-conforming, in the case where overload resolution would hit an error. In order to enable this optimization, such errors should be “no diagnostic required.”
Additional note (March, 2013):
See also issue 1360.
Notes from the September, 2013 meeting:
The problem with this approach is that hard errors (not in the immediate context) can occur, affecting portability. There are some cases, such as a virtual assignment operator in the base class, where lazy evaluation cannot be done, so it cannot be mandated.
Consider:
struct A
{
A();
A(A&);
explicit A(int);
operator int() const;
};
struct B
{
B(B&& other);
A a;
};
B::B(B&& other) : a(static_cast<B&&>(other).a) {}
// B::B(B&& other) = default; // ill-formed
void f(B& b1)
{
B b2 = static_cast<B&&>(b1);
}
The user-defined move constructor is well-formed because B::a can be initialized via A::operator int() and A::A(int); however, Clang and GCC believe a defaulted one would be ill-formed.
What about the following, which is considered well-formed by compilers and calls A::A(C&&)?
struct C {}; struct A : C { A(); A(A&); A(C&&); }; struct B { B(B&& other); A a; }; B::B(B&& other) = default;
It appears that the following example may have unwanted undefined behavior in C++, although not in C:
struct A { int x, y; }; A passthrough(A a) { return a; } int main(void) { A a; a.x = 0; return passthrough(a).x; }
The default memberwise copying operation is not specified to be done in a way that is insensitive to indeterminate values.
Bullet 4 of 11.4.5.3 [class.copy.ctor] paragraph 23 says that a defaulted copy/move assignment operator is defined as deleted if the class has
a non-static data member of class type M (or array thereof) that cannot be copied/moved because overload resolution (12.2 [over.match]), as applied to M's corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator
The intent of this is that if overload resolution fails to find a corresponding copy/move assignment operator that can validly be called to copy/move a member, the class's assignment operator will be defined as deleted. However, this wording does not cover an example like the following:
struct A { A(); }; struct B { B(); const A a; }; typedef B& (B::*pmf)(B&&); pmf p =&B::operator=;
Here, the problem is simply that overload resolution failed to find a callable function, which is not one of the cases listed in the current wording. A similar problem exists for base classes in the fifth bullet.
Additional note (January, 2013):
A similar omission exists in paragraph 11 for copy constructors.
An example like the following,
class A { private: A& operator=(const A&); }; class B : virtual public A { public: B& operator = (const B& src); }; class C: public B { public: void f(const C* psrc) { *this = *psrc; } };
is presumably well-formed, even though the copy assignment operator of A is inaccessible in C, because 11.4.6 [class.copy.assign] paragraph 12 says that only direct, not virtual, base class object assignment operators are invoked by the generated assignment operator (although there is implementation divergence on this question).
Should the example also be well-formed if A were a direct virtual base of C? That is, if a direct virtual base also has an indirect derivation path, its direct derivation can be ignored for generated assignment operators.
Possibly relevant to this question is the permission for an implementation to assign virtual base class objects more than once:
It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy/move assignment operator.
According to 11.4.7 [class.dtor] paragraph 12,
At the point of definition of a virtual destructor (including an implicit definition (11.4.5.3 [class.copy.ctor])), the non-array deallocation function is looked up in the scope of the destructor's class (6.5.2 [class.member.lookup]), and, if no declaration is found, the function is looked up in the global scope. If the result of this lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function or a function with a deleted definition (9.5 [dcl.fct.def]), the program is ill-formed. [Note: This assures that a deallocation function corresponding to the dynamic type of an object is available for the delete-expression (11.4.11 [class.free]). —end note]
However, bullet 5.3 of that section says that such a lookup failure causes the destructor to be defined as deleted, rather than making the program ill-formed. It appears that paragraph 12 was overlooked when deleted functions were added to the language. See also 11.4.11 [class.free] paragraph 7.
Consider the following example:
#include <stdio.h> struct Base { Base *p; virtual void f() { puts("base"); } ~Base() { p->f(); } }; struct Derived : Base { Derived() { p = this; } void f() { puts("derived"); } void g() { p->f(); delete this; } }; void h() { Derived *p = new Derived; p->g(); }
Should this have defined behavior? On the one hand, the Derived object is in its period of destruction, so the behavior of the p->f() call in the Base destructor should be to call Base::f(). On the other hand, p is a pointer to a Derived object whose lifetime has ended, and the rules in 6.7.3 [basic.life] don't appear to allow the call. (Calling this->f() from the Base destructor would be OK — the question is whether you can do that for a pointer that used to point to the derived object, or if you can only do it for a pointer that was “created” after the dynamic type of the object changed to be Base.)
If the above is valid, it has severe implications for devirtualization. The purpose of 6.7.3 [basic.life] paragraph 7 appears to be to allow an implementation to assume that if it will perform two loads of a constant field (for instance, a const member, the implicit pointer for a reference member, or a vptr), and the two loads are performed on the “same pointer value”, then they load the same value.
Should there be a rule for destructors similar to that of 11.4.5 [class.ctor] paragraph 12?
During the construction of a const object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified.
According to 11.4.9.3 [class.static.data] paragraph 4,
Unnamed classes and classes contained directly or indirectly within unnamed classes shall not contain static data members.
There is no such restriction on member functions, and there is no rationale for this difference, given that both static data members and member functions can be defined outside a unnamed class with a typedef name for linkage purposes. (Issue 406 acknowledged the lack of rationale by removing the specious note in 11.4.9.3 [class.static.data] that attempted to explain the restriction but left the normative prohibition in place.)
It would be more consistent to remove the restriction for classes with a typedef name for linkage purposes.
Additional note (August, 2012):
It was observed that, since no definition of a const static data member is required if it is not odr-used, there is no reason to prohibit such members in an unnamed class even without a typedef name for linkage purposes.
It is not clear how an example like the following should be treated:
template <class ...> struct partition_indices { static auto compute_right () {} static constexpr auto right = compute_right; }; auto foo () -> partition_indices<>; void f() { foo(); };
The initialization of right is in a context that must be done during the initial parse of the class, but the function body of compute_right is not supposed to be evaluated until the class is complete. Current implementations appear to accept the template case but not the equivalent non-template case. It's not clear why those cases should be treated differently.
If you change the example to include a forward dependency in the body of compute_right, e.g.,
template <int> struct X {}; template <class T> struct partition_indices { static auto compute_right () { return X<I>(); } static constexpr auto right = compute_right; static constexpr int I = sizeof(T); }; auto foo () -> partition_indices<int>; void f() { foo(); };
current implementations reject the code, but it's not clear that there is a rationale for the different behavior.
Notes from the March, 2018 meeting:
It was proposed that one direction might be to disallow instantiating member functions while the containing class template is being instantiated. However, overnight implementation experience indicated that this approach breaks seemingly-innocuous and currently-accepted code like:
template <class T> struct A { static constexpr int num() { return 42; } int ar[num()]; }; A<int> a;
There was divergence of opinion regarding whether the current rules describe the current behavior for the two original examples or whether additional explicit rules are needed to clarify the difference in behavior between template and non-template examples, as well as whether there should be a difference at all..
Notes from the June, 2018 meeting:
The consensus of CWG was to treat templates and classes the same by "instantiating" delayed-parse regions when they are needed instead of at the end of the class.
See also issues 1626 and 1890.
According to 11.5 [class.union] paragraph 4,
[Note: In general, one must use explicit destructor calls and placement new operators to change the active member of a union. —end note] [Example: Consider an object u of a union type U having non-static data members m of type M and n of type N. If M has a non-trivial destructor and N has a non-trivial constructor (for instance, if they declare or inherit virtual functions), the active member of u can be safely switched from m to n using the destructor and placement new operator as follows:
u.m.~M(); new (&u.n) N;—end example]
This pattern is only “safe” if the original object that is being destroyed does not involve any const-qualified or reference types, i.e., satisfies the requirements of 6.7.3 [basic.life] paragraph 7, bullet 3:
the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type
Although paragraph 4 of 11.5 [class.union] is a note and an example, it should at least refer to the lifetime issues described in 6.7.3 [basic.life].
Additional note (October, 2013):
See also issue 1776, which suggests possibly changing the restriction in 6.7.3 [basic.life]. If such a change is made, this issue may become moot.
11.5 [class.union] paragraph 5 defines an anonymous union as follows:
A union of the form
union { member-specification } ;
is called an anonymous union; it defines an unnamed object of unnamed type.
It is obviously intended that a declaration like
static union { int i; float f; };
is a declaration of that form (cf paragraph 6, which requires the static keyword for anonymous unions declared in namespace scope). However, it would be clearer if the definition were recast in more descriptive terms, e.g.,
An anonymous union is an unnamed class that is defined with the class-key union in a simple-declaration in which the init-declarator-list is omitted. Such a simple-declaration is treated as if it contained a single declarator declaring an unnamed variable of the union's type.
(Note that this definition would require some additional tweaking to apply to class member anonymous union declarations, since simple-declarations are not included as member-declarations.)
As a related point, it is not clear how the following examples are to be treated, and there is implementation variance on some:
void f() { thread_local union { int a; }; } void g() { extern union { int b; }; } thread_local union { int c; }; // static is implied by thread_local static thread_local union { int d; }; static const union { int e = 0; }; // is e const? Clang says yes, gcc says no static constexpr union { int f = 0; };
Additional notes (July, 2023)
This issue is addressed by issue 2767.
Consider this example from issue 7:
class Foo { }; class A : virtual private Foo { }; class Bar : public A { };
This example should cause Bar's defaulted default constructor to be deleted, because it does not have access to the injected-class-name Foo.
Notes from the December, 2016 teleconference:
The injected-class-name is irrelevant to the example, which is ill-formed. The access should be permitted only if conversion of the this pointer to a pointer to the base class would succeed.
Does the restriction in 11.8.5 [class.protected] apply to upcasts across protected inheritance, too? For instance,
struct B { int i; }; struct I: protected B { }; struct D: I { void f(I* ip) { B* bp = ip; // well-formed? bp->i = 5; // aka "ip->i = 5;" } };
I think the rationale for the 11.8.5 [class.protected] restriction applies equally well here — you don't know whether ip points to a D object or not, so D::f can't be trusted to treat the protected B subobject consistently with the policies of its actual complete object type.
The current treatment of “accessible base class” in 11.8.3 [class.access.base] paragraph 4 clearly makes the conversion from I* to B* well-formed. I think that's wrong and needs to be fixed. The rationale for the accessibility of a base class is whether “an invented public member” of the base would be accessible at the point of reference, although we obscured that a bit in the reformulation; it seems to me that the invented member ought to be considered a non-static member for this purpose and thus subject to 11.8.5 [class.protected].
(See also issues 385 and 471.).Notes from October 2004 meeting:
The CWG tentatively agreed that casting across protective inheritance should be subject to the additional restriction in 11.8.5 [class.protected].
Proposed resolution (April, 2011)
Change 11.8.3 [class.access.base] paragraph 4 as follows:
A base class B of N is accessible at R, if
an invented public member of B would be a public member of N, or
R occurs in a member or friend of class N, and an invented public member of B would be a private or protected member of N, or
R occurs in a member or friend of a class P derived from N, and an invented public member of B would be a private
or(but not a protected [Footnote: A protected invented member is disallowed here for the same reason the additional check of 11.8.5 [class.protected] is applied to member access: it would allow casting a pointer to a derived class to a protected base class that might be a subobject of an object of a class that is different from the class context in which the reference occurs. —end footnote]) member of P, orthere exists a class S such that B is a base class of S accessible at R and S is a base class of N accessible at R.
[Example:
class B { public: int m; }; class S: private B { friend class N; }; class N: private S { void f() { B* p = this; // OK because class S satisfies the fourth condition // above: B is a base class of N accessible in f() because // B is an accessible base class of S and S is an accessible // base class of N. } }; class N2: protected B { }; class P2: public N2 { void f2(N2* n2p) { B* bp = n2p; // error: invented member would be protected and naming // class N2 not the same as or derived from the referencing // class P2 n2p->m = 0; // error (cf 11.8.5 [class.protected]) for the same reason } };—end example]
According to 11.8.5 [class.protected] paragraph 1, except when forming a pointer to member,
All other accesses involve a (possibly implicit) object expression (7.6.1.5 [expr.ref]).
It is not clear that this is strictly true for the invocation of a base class constructor from a mem-initializer. A wording tweak may be advisable.
The following line in the example in 11.8.5 [class.protected] paragraph 1 is no longer allowed following the change from issue 1873:
class B { protected: int i; static int j; }; // ... class D2 : public B { friend void fr(B*, D1*, D2*); void mem(B*, D1*); }; void fr(B* pb, D1* p1, D2* p2) { // ... p2->B::i = 4; // OK (access through a D2, even though naming class is B) // ... }
The example line ought to work, but none of the bullets in 11.8.3 [class.access.base] paragraph 5 apply:
A member m is accessible at the point R when named in class N if
- m as a member of N is public, or
- m as a member of N is private, and R occurs in a direct member or friend of class N, or
- m as a member of N is protected, and R occurs in a direct member or friend of class N, or in a member of a class P derived from N, where m as a member of P is public, private, or protected, or
- there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B.
One aproach might be that 11.8.3 [class.access.base] bullet 5.3 should also consider friends of a class P derived from N where P is the type of the object expression (if any) or a base class thereof, and m as a member of P is public, protected, or private.
According to 11.9.3 [class.base.init] paragraph 16,
Member functions (including virtual member functions, 11.7.3 [class.virtual]) can be called for an object under construction. Similarly, an object under construction can be the operand of the typeid operator (7.6.1.8 [expr.typeid]) or of a dynamic_cast (7.6.1.7 [expr.dynamic.cast]). However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the result of the operation is undefined.
The example in that paragraph reads, in significant part,
class B { public: int f(); }; class C { public: C(int); }; class D : public B, C { public: D() : C(f()) // undefined: calls member function // but base \tcode{C} not yet initialized {} };
However, the construction of B, the object for which the member function is being called) has completed its construction, so it is not clear why this should be undefined behavior.
(See also issue 1517.)
Given the following example,
struct Noncopyable { Noncopyable(); Noncopyable(const Noncopyable &) = delete; }; Noncopyable make(int kind = 0); struct AsBase : Noncopyable { AsBase() : Noncopyable(make()) {} // #1 }; struct AsMember { Noncopyable nc; AsMember() : nc(make()) { } // #2? };
All implementations treat #1 as an error, invoking the deleted copy constructor, while #2 is accepted. It's not clear from the current wording why they should be treated differently.
Additional note (August, 2022):
If there are concerns about reuse of tail padding in #1, requiring a copy for some implementation reason, similar concerns should apply to #2 if the data member is declared with [[no_unique_address]].
Furthermore, the following example using a delegating constructor shows implementation divergence:
struct Noncopyable { Noncopyable(); Noncopyable(const Noncopyable &) = delete; Noncopyable(int) : Noncopyable(Noncopyable()) {} // #3? };
The current wording of 11.9.5 [class.cdtor] paragraph 4 does not describe the behavior of calling a virtual function in a mem-initializer for a base class, only for a non-static data member. Also, the changes for issue 1202 should have been, but were not, applied to the description of the behavior of typeid and dynamic_cast in paragraphs 5 and 6.
In addition, the resolution of issue 597 allowing the out-of-lifetime conversion of pointers/lvalues to non-virtual base classes, should have been, but were not, applied to paragraph 3.
(See also issue 2056.)
Proposed resolution (August, 2013):
Change 11.9.5 [class.cdtor] paragraph 1 as follows:
For an object with a non-trivial constructor, referring to any non-static member or virtual base class of the object before the constructor begins execution results in undefined behavior. For an object with a non-trivial destructor, referring to any non-static member or virtual base class of the object after the destructor finishes execution results in undefined behavior. [Example:struct X { int i; }; struct Y : X { Y(); }; // non-trivial struct A { int a; }; struct B :publicvirtual A { int j; Y y; }; // non-trivial extern B bobj; B* pb = &bobj; // OK int* p1 = &bobj.a; // undefined, refers to base class member int* p2 = &bobj.y.i; // undefined, refers to member's member A* pa = &bobj; // undefined, upcast to a virtual base class type B bobj; // definition of bobj extern X xobj; int* p3 = &xobj.i; //OK, X is a trivial class X xobj;
Change 11.9.5 [class.cdtor] paragraphs 3-6 as follows:
To explicitly or implicitly convert a pointer (a glvalue) referring to an object of class X to a pointer (reference) to a direct or indirect virtual base class B of X, the construction of X and the construction of all of its direct or indirect bases
that directly or indirectly derive fromfor which B is a direct or indirect virtual base shall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior. To form a pointer to (or access the value of) a direct non-static member...
Member functions, including virtual functions (11.7.3 [class.virtual]), can be called during construction or destruction (11.9.3 [class.base.init]). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access (7.6.1.5 [expr.ref]) and the object expression refers to the complete object of x or one of that object's base class subobjects but not to x or one of its base class subobjects, the behavior is undefined.The period of construction of an object or subobject whose type is a class type C begins immediately after the construction of all its base class subobjects is complete and concludes when the last constructor of class C exits. The period of destruction of an object or subobject whose type is a class type C begins when the destructor for C begins execution and concludes immediately before beginning the destruction of its base class subobjects. A polymorphic operation is a virtual function call (7.6.1.3 [expr.call]), the typeid operator (7.6.1.8 [expr.typeid]) when applied to a glvalue of polymorphic type, or the dynamic_cast operator (7.6.1.7 [expr.dynamic.cast]) when applied to a pointer to or glvalue of a polymorphic type. A polymorphic operand is the object expression in a virtual function call or the operand of a polymorphic typeid or dynamic_cast.During the period of construction or period of destruction of an object or subobject whose type is a class type C (call it x), the effect of performing a polymorphic operation in which the polymorphic operand designates x or a base class subobject thereof is as if the dynamic type of the object were class C. [Footnote: This is true even if C is an abstract class, which cannot be the type of a most-derived object. —end footnote] If a polymorphic operand refers to an object or subobject having class type C before its period of construction begins or after its period of destruction is complete, the behavior is undefined. [Note: This includes the evaluation of an expression appearing in a mem-initializer of C in which the mem-initializer-id designates C or one of its base classes. —end note] [Example:
struct V { V(); V(int); virtual void f(); virtual void g(); }; struct A : virtual V { virtual void f(); virtual int h(); A() : V(h()) { } // undefined behavior: virtual function h called // before A's period of construction begins }; struct B : virtual V { virtual void g(); B(V*, A*); }; struct D : A, B { virtual void f(); virtual void g(); D() : B((A*)this, this) { } }; B::B(V* v, A* a) { f(); // calls V::f, not A::f g(); // calls B::g, not D::g v->g(); // v is base of B, the call is well-defined, calls B::g a->f(); // undefined behavior, a's type not a base of B typeid(*this); // type_info for B typeid(*v); // well-defined: *v has type V, a base of B, // so its period of construction is complete; // yields type_info for B typeid(*a); // undefined behavior: A is not a base of B, // so its period of construction has not begun dynamic_cast<B*>(v); // well-defined: v has type V*, V is a base of B, // so its period of construction is complete; // results in this dynamic_cast<B*>(a); // undefined behavior: A is not a base of B, // so its period of construction has not begun }—end example]
The typeid operator (7.6.1.8 [expr.typeid]) can be used during construction or destruction (11.9.3 [class.base.init]). When typeid is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of typeid refers to the object under construction or destruction, typeid yields the std::type_info object representing the constructor or destructor's class. If the operand of typeid refers to the object under construction or destruction and the static type of the operand is neither the constructor or destructor's class nor one of its bases, the result of typeid is undefined.
dynamic_casts (7.6.1.7 [expr.dynamic.cast]) can be used during construction or destruction (11.9.3 [class.base.init]). When a dynamic_cast is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of the dynamic_cast refers to the object under construction or destruction, this object is considered to be a most derived object that has the type of the constructor or destructor's class. If the operand of the dynamic_cast refers to the object under construction or destruction and the static type of the operand is not a pointer to or object of the constructor or destructor's own class or one of its bases, the dynamic_cast results in undefined behavior. [Example:struct V { virtual void f(); }; struct A : virtual V { }; struct B : virtual V { B(V*, A*); }; struct D : A, B { D() : B((A*)this, this) { } }; B::B(V* v, A* a) { typeid(*this); // type_info for B typeid(*v); // well-defined: *v has type V, a base of B // yields type_info for B typeid(*a); // undefined behavior: type A not a base of B dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B // results in B* dynamic_cast<B*>(a); // undefined behavior, // a has type A*, A not a base of B
—end example]
Footnote 127 of 12.2.2.2.2 [over.call.func] paragraph 3 reads,
An implied object argument must be contrived to correspond to the implicit object parameter attributed to member functions during overload resolution. It is not used in the call to the selected function. Since the member functions all have the same implicit object parameter, the contrived object will not be the cause to select or reject a function.
It is not true that “the member functions all have the same implicit object parameter.” This statement does not take into account member functions brought into the class by using-declarations or cv-qualifiers and ref-qualifiers on the non-static member functions:
struct B { char f(); // B & }; struct D : B { using B::f; long f(); // D & char g() const; // D const & long g(); // D & char h() &; // D & long h() &&; // D && }; int main() { // D::f() has better match than B::f() decltype(D().f()) *p1 = (long *)0; // D::g() has better match than D::g() const decltype(D().g()) *p2 = (long *)0; // D::h() & is not viable function // D::h() && is viable function decltype(D().h()) *p3 = (long *)0; }
The value category of a contrived object expression is not specified by the rules and, probably, cannot be properly specified in presence of ref-qualifiers, so the statement “the contrived object will not be the cause to select or reject a function” should be normative rather than informative:
struct X
{
static void f(double) {}
void f(int) & {}
void f(int) && {}
};
int main()
{
X::f(0); // ???
}
Subclause 12.2.2.2.3 [over.call.object] paragraph 2 considers only
those conversion funtions matching a particular grammar pattern. This
unintendedly excludes conversion functions with an explicit object
parameter (and, as a pre-existing defect, noexcept
conversion functions):
In addition, for each non-explicit conversion function declared in T of the formoperator conversion-type-id ( ) cv-qualifier-seqopt ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt ;where the optional cv-qualifier-seq is the same cv-qualification as, or a greater cv-qualification than, cv, and where conversion-type-id denotes the type “pointer to function of (P1 , . . . , Pn ) returning R”, or the type “reference to pointer to function of (P1 , . . . , Pn ) returning R”, or the type “reference to function of (P1 , . . . , Pn ) returning R”, a surrogate call function with the unique name call-function and having the formR call-function ( conversion-type-id F, P1 a1 , ... , Pn an ) { return F (a1 , . . . , an ); }is also considered as a candidate function. Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration. [ Footnote: ...]
For example, there is implementation divergence in handling this example:
using fn_t = void(); struct C { operator fn_t * (this C const &); }; void foo(C c) { c(); }
The candidates selected by 12.2.2.3 [over.match.oper] include built-in candidates that will result in an error if chosen; this was affirmed by issue 1687. As a result, t+u is ill-formed because it is resolved to the built-in operator+(int*,std::ptrdiff_t), although most implementations do not (yet) agree:
struct Adaptor { Adaptor(int); }; struct List { }; void operator +(List &, Adaptor); struct DataType { operator int *() const = delete; operator List &() const; }; struct Yea; struct Nay { int theNaysHaveIt; }; template <typename T, typename U> Yea addCheck(int, T &&t, U &&u, char (*)[sizeof(t + u, 0)] = 0); template <typename T, typename U> Nay addCheck(void *, T &&t, U &&u); void test(DataType &data) { (void)sizeof(addCheck(0, data, 0.).theNaysHaveIt); }
It might be better to adjust the candidate list in 12.2.2.4 [over.match.ctor] bullet 3.3.3 to allow conversion only on class types and exclude the second standard conversion sequence.
Consider the following example:
struct T {
T() {}
T(struct S&) {}
};
struct S {
operator T() { return T(); }
};
int main()
{
S s;
T&& t(s); // #1
}
Because there are two possible conversions from S to T, one by conversion function and the other by converting constructor, one might expect that the initialization at #1 would be ambiguous. However, 12.2.2.7 [over.match.ref] (used in the relevant bullet of 9.4.4 [dcl.init.ref], paragraph 5.2.1.2) only deals with conversion functions and ignores converting constructors.
Notes from the November, 2014 meeting:
CWG agreed that 9.4.4 [dcl.init.ref] should be changed to consider converting constructors in this case.
In 12.2.2.7 [over.match.ref], candidates that produce non-class prvalues are considered, although that seems to contradict what 9.4.4 [dcl.init.ref] says. See also issue 2077.
According to 12.2.2.8 [over.match.list] paragraph 1 says,
If the initializer list has no elements and T has a default constructor, the first phase is omitted.
However, this case cannot occur. If T is a non-aggregate class type with a default constructor and the initializer is an empty initializer list, the object will be value-constructed, per 9.4.5 [dcl.init.list] bullet 3.4. Overload resolution is only necessary if default-initialization (or a check of its semantic constraints) is implied, with the relevant section concerning candidates for overload resolution being 12.2.2.4 [over.match.ctor].
See also issue 1518.
Proposed resolution (January, 2017):
Change 12.2.2.8 [over.match.list] paragraph 1 as follows:
When objects of non-aggregate class type T are list-initialized such that 9.4.5 [dcl.init.list] specifies that overload resolution is performed according to the rules in this section, overload resolution selects the constructor in two phases:
Initially, the candidate functions are the initializer-list constructors (9.4.5 [dcl.init.list]) of the class T and the argument list consists of the initializer list as a single argument.
If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list.
If the initializer list has no elements and T has a default constructor, the first phase is omitted.In copy-list-initialization, if an explicit constructor is chosen...
Additional notes, February, 2017:
The statement of the issue is incorrect. In an example like
struct A { A(); A(initializer_list<int>); }; void f(A a); int main() { f({}); }
the rule in question is not used for the initialization of the parameter. However, it is used to determine whether a valid implicit conversion sequence exists for a. It is unclear whether an additional change to resolve this discrepancy is needed or not.
Given the declarations
template<typename T = int> using X = vector<int>; X x = {1, 2, 3}; template<typename...> using Y = vector<int>; Y y = {1, 2, 3};
CTAD deduces vector<int>. Then we are asked to perform a check that the arguments of X and Y are deducible from vector<int>.
I think this check should succeed, deducing T = int in the first case and <pack> = <empty> in the second case, so both declarations should be valid. That seems consistent with what would happen for a non-alias with template parameters that CTAD can't deduce, where there is either a default template argument or the parameter is a pack. But what actually happens is that we're asked to form
template<typename T> struct AA; template<typename T = int> struct AA<X<T>>;
and
template<typename T> struct AA; template<typename ...Ts> struct AA<Y<Ts...>>;
However, both of those partial specializations are ill-formed: a partial specialization can't have default template arguments, and neither of these is more specialized than the primary template, because T / Ts are not used in deducible contexts.
I think we have the wrong model here, and should instead be considering (effectively) whether function template argument deduction would succeed for
template<typename T> struct AA {}; template<typename T = int> void f(AA<X<T>>);
and
template<typename T> struct AA {}; template<typename ...Ts> void f(AA<Y<Ts...>>);
respectively, when given an argument of type AA<deduced return type>. That is, get rid of the weird class template partial specialization restrictions, and instead add in the rules from function templates to use default template arguments and to default non-deduced packs to empty packs.
Consider the following example:
template<class T> struct S { template<class U> struct N { N(T) {} N(T, U) {} template<class V> N(V, U) {} }; }; S<int>::N x{2.0, 1};
The description of CTAD in 12.2.2.9 [over.match.class.deduct] doesn't really specify how nested classes work. If you are supposed to deduce all the enclosing class template arguments, the example is ill-formed because there is no way to deduce T. If you are supposed to consider S<int>::N as having a new constructor template, then it should probably be well-formed.
Notes from the March, 2021 teleconference:
CWG agreed that the intent is to use the partially-instantiated inner template with the explicitly-specified template argument int.
Consider:
struct A { A(); } a; A a1 = {a}, a2 = {{a}}, a3 = {{{a}}};
a1 and a2 are valid, a3 is ill-formed, because 12.2.4.2 [over.best.ics] bullet 4.5 allows one pair of braces and 12.2.4.2.6 [over.ics.list] paragraph 2 allows a second pair of braces. The implicit conversion sequence from {{a}} to A is a user-defined conversion.
Prior to the list-initialization-from-same-type changes via issues 1467 and 2076, a2 was ill-formed like a3.
Is this intended, or did DR2076 not go far enough in reintroducing the restriction? Perhaps a more extreme rule, such as saying that a copy/move constructor is simply not a candidate for list-initialization from a list that contains one element that is itself a list, would work better?
Notes from the July, 2017 meeting:
CWG agreed that the a2 example should be ill-formed but that the a1 example must remain for C compatibility.
According to 12.2.4.2.1 [over.best.ics.general] paragraphs 1 and 9,
An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called. The sequence of conversions is an implicit conversion as defined in 7.3 [conv], which means it is governed by the rules for initialization of an object or reference by a single expression (9.4 [dcl.init], 9.4.4 [dcl.init.ref]).
If no sequence of conversions can be found to convert an argument to a parameter type, an implicit conversion sequence cannot be formed.
However, 7.3.1 [conv.general] paragraph 3 says,
An expression E can be implicitly converted to a type T if and only if the declaration T t=E; is well-formed, for some invented temporary variable t (9.4 [dcl.init]).
This definition is too restrictive in the context of overload resolution's implicit conversion sequences. The intent, as stated in 12.2.1 [over.match.general] note 1, is that overload resolution ignores some factors that would make such an initialization ill-formed, and these are applied only after the best match is determined:
[Note 1: The function selected by overload resolution is not guaranteed to be appropriate for the context. Other restrictions, such as the accessibility of the function, can make its use in the calling context ill-formed. —end note]
For example,
struct A{ A(int) = delete; }; struct B{ B(int) {} }; void fun(A); // #1 void fun(B); // #2 int main() { fun(0); // #3 }
The intent is that overload #1 be viable with a valid implicit conversion sequence, making the call at #3 ambiguous, even though the hypothetical declaration
A t = 1;
would be ill-formed.
Proposed resolution (approved by CWG 2022-12-02):
Change 12.2.4.2.1 [over.best.ics.general] paragraph 1, merging it with paragraph 2,as follows:
An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called. The sequence of conversions is an implicit conversion as defined in 7.3 [conv], which means; it isgoverned bythus based on the rules for initialization of an object or reference by a single expression (9.4 [dcl.init], 9.4.4 [dcl.init.ref]). Implicit, except that implicit conversion sequences are concerned only with the type, cv-qualification, and value category of the argument and how these are converted to match the corresponding properties of the parameter. [Note: ... ]
CWG 2023-02-06
Additional drafting is needed to cover e.g. conversions from literal 0 to null pointer constants.
The resolution of issue 1604 broke the following example:
struct A {}; struct B { operator const A() const; }; void f(A const&); void f(A&&); int main() { B a; f(a); }
Overload resolution selects the A&& overload, but then initialization fails. This seems like a major regression; we're now required to reject
std::vector<A> va; B b; va.push_back(b);
Should we update 12.2.4.2.5 [over.ics.ref] to match the changes made to 9.4.4 [dcl.init.ref]?
See also issue 2108.
In determining the implicit conversion sequence for an initializer list argument passed to a reference parameter, the intent is that a temporary of the appropriate type will be created and bound to the reference, as reflected in 12.2.4.2.6 [over.ics.list] paragraph 5:
Otherwise, if the parameter is a reference, see 12.2.4.2.5 [over.ics.ref]. [Note: The rules in this section will apply for initializing the underlying temporary for the reference. —end note]
However, 12.2.4.2.5 [over.ics.ref] deals only with expression arguments, not initializer lists:
When a parameter of reference type binds directly (9.4.4 [dcl.init.ref]) to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base Conversion (12.2.4.2 [over.best.ics])... If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence (12.2.4.2.3 [over.ics.user]), with the second standard conversion sequence either an identity conversion or, if the conversion function returns an entity of a type that is a derived class of the parameter type, a derived-to-base Conversion.
When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert the argument expression to the underlying type of the reference according to 12.2.4.2 [over.best.ics]. Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the underlying type with the argument expression. Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.
(Note in particular that the reference binding refers to 9.4.4 [dcl.init.ref], which also does not handle initializer lists, and not to 9.4.5 [dcl.init.list].)
Either 12.2.4.2.5 [over.ics.ref] needs to be revised to handle binding references to initializer list arguments or 12.2.4.2.6 [over.ics.list] paragraph 5 needs to be clearer on how the expression specification is intended to be applied to initializer lists.
Consider the following example:
#include <initializer_list> struct A{ operator short(){ return 0; } }; struct B{ operator bool(){ return 0; } }; void fun(std::initializer_list<int>){} void fun(std::initializer_list<bool>){} int main(){ fun({A{},B{}}); }
According to 12.2.4.2.6 [over.ics.list] paragraph 6,
Otherwise, if the parameter type is std::initializer_list<X> and all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the worst conversion necessary to convert an element of the list to X, or if the initializer list has no elements, the identity conversion. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor.
In this example, all of the conversions from list elements to the initializer_list template argument type are user-defined conversions. According to 12.2.4.3 [over.ics.rank] bullet 3.3,
User-defined conversion sequence U1 is a better conversion sequence than another user-defined conversion sequence U2 if they contain the same user-defined conversion function or constructor or they initialize the same class in an aggregate initialization and in either case the second standard conversion sequence of U1 is better than the second standard conversion sequence of U2.
Since in both cases the two elements of the initializer-list argument involve different user-defined conversion functions, the two user-defined conversion sequences for the elements cannot be distinguished, so the determination of the “worst conversion” for the two candidates does not consider the second standard conversion sequence. This presumably makes it impossible to distinguish the conversion sequences for the two candidates in the function call, making the call ambiguous.
However, there is implementation divergence on the handling of this example, with g++ reporting an ambiguity and clang, MSVC, and EDG calling the int overload, presumably on the basis that short->int is a promotion while short->bool is a conversion.
Notes from the August, 2021 teleconference:
CWG agreed with the reasoning expressed in the analysis, that conversions involving different user-defined conversion functions cannot be compared, and thus the call is ambiguous. The use of the phrase “worst conversion” is insufficiently clear, however, and requires definition.
Proposed resolution, August, 2021:
Change 12.2.4.2.6 [over.ics.list] paragraphs 5 and 6 as follows:
Otherwise, if the parameter type is std::initializer_list<X> and either the initializer list is empty or all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the
worst conversionworst conversion necessary to convert an element of the list to X,or ifdefined as follows. If the initializer list has no elements, the worst conversion is the identity conversion. Otherwise, the worst conversion is an implicit conversion sequence for a list element that is not better than any other implicit conversion sequence required by list elements, compared as described in 12.2.4.3 [over.ics.rank]. If more than one implicit conversion sequence satisfies this criterion, then if they are user-defined conversion sequences that do not all contain the same user-defined conversion function or constructor, the worst conversion sequence is the ambiguous conversion sequence (12.2.4.2.1 [over.best.ics.general]); otherwise, it is unspecified which of those conversion sequences is chosen as worst. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor. [Example 2:void f(std::initializer_list<int>); f( {} ); // OK: f(initializer_list<int>) identity conversion f( {1,2,3} ); // OK: f(initializer_list<int>) identity conversion f( {'a','b'} ); // OK: f(initializer_list<int>) integral promotion f( {1.0} ); // error: narrowing struct A { A(std::initializer_list<double>); // #1 A(std::initializer_list<complex<double>>); // #2 A(std::initializer_list<std::string>); // #3 }; A a{ 1.0,2.0 }; // OK, uses #1 void g(A); g({ "foo", "bar" }); // OK, uses #3 typedef int IA[3]; void h(const IA&); h({ 1, 2, 3 }); // OK: identity conversion void x(std::initializer_list<int>); void x(std::initializer_list<bool>); struct S1 { operator short(); }; struct S2 { operator bool(); }; void y() { x({S1{}, S2{}}); // error: ambiguous. The ICSes for each list element are indistinguishable because // they do not contain the same conversion function, so the worst conversion is // the ambiguous conversion sequence. }—end example]
Otherwise, if the parameter type is “array of N X ” or “array of unknown bound of X”, if there exists an implicit conversion sequence from each element of the initializer list (and from {} in the former case if N exceeds the number of elements in the initializer list) to X, the implicit conversion sequence is the worst
such implicit conversion sequenceconversion necessary to convert an element of the list (including, if there are too few list elements, {}) to X, determined as described above for a std::initializer_list<X> with a non-empty initializer list.
CWG 2023-06-13
An ambiguous conversion for the function selected by overload resolution is ill-formed per 12.2.4.2.1 [over.best.ics.general]. Instead of attempting to define "worst conversion" (possibly with focusing on the second standard conversion sequence in user-defined conversions), it might be more consistent overall to use the rules for a succession of function arguments/parameters when comparing list-initializations.
There are overload tiebreakers that order reference/nonreference and base/derived conversions, but how they relate is not specified. For example:
struct A { A(); }; struct B : A {}; struct C : B {}; void f1(B&); void f1(A); void f2(B); void f2(A&); int main() { C v; f1(v); // all compilers choose f1(B&) f2(v); // all compilers choose f2(B) }
The Standard does not appear to specify what happens in this case.
Consider:
void f() noexcept {}
void g(void (*)() noexcept) {}
void g(void (&)()) {}
int main() {
g(f); // error: ambiguous
}
In contrast:
void f() noexcept {} void g(void (*)()) {} void g(void (&)()) {} // #1 int main() { g(f); // OK, calls #1 }
In both cases, binding void(&)() to void() noexcept is considered an identity conversion, without further disambiguation by 12.2.4.3 [over.ics.rank].
CWG 2024-06-26
Binding a reference to a function should not be considered an identity conversion if it strips a non-throwing exception specification. This amendment removes the ambiguity for the first example and makes the second example ambiguous, which is desirable.
According to 12.4.7 [over.inc] paragraph 1,
The user-defined function called operator++ implements the prefix and postfix ++ operator. If this function is a non-static member function with no parameters, or a non-member function with one parameter, it defines the prefix increment operator ++ for objects of that type. If the function is a non-static member function with one parameter (which shall be of type int) or a non-member function with two parameters (the second of which shall be of type int), it defines the postfix increment operator ++ for objects of that type.
According to 12.4 [over.oper] paragraph 8,
Operator functions cannot have more or fewer parameters than the number required for the corresponding operator, as described in the rest of this subclause.
This does not rule out an operator++ with more than two parameters, however, since there is no corresponding operator.
One possibility might be to add a sentence like,
A function named operator++ shall declare either a prefix or postfix increment operator.
Static data members of template classes and of nested classes of template classes are not themselves templates but receive much the same treatment as template. For instance, Clause 13 [temp] paragraph 1 says that templates are only "classes or functions" but implies that "a static data member of a class template or of a class nested within a class template" is defined using the template-declaration syntax.
There are many places in the clause, however, where static data members of one sort or another are overlooked. For instance, Clause 13 [temp] paragraph 6 allows static data members of class templates to be declared with the export keyword. I would expect that static data members of (non-template) classes nested within class templates could also be exported, but they are not mentioned here.
Paragraph 8, however, overlooks static data members altogether and deals only with "templates" in defining the effect of the export keyword; there is no description of the semantics of defining a static data member of a template to be exported.
These are just two instances of a systematic problem. The entire clause needs to be examined to determine which statements about "templates" apply to static data members, and which statements about "static data members of class templates" also apply to static data members of non-template classes nested within class templates.
(The question also applies to member functions of template classes; see issue 217, where the phrase "non-template function" in 9.3.4.7 [dcl.fct.default] paragraph 4 is apparently intended not to include non-template member functions of template classes. See also issue 108, which would benefit from understanding nested classes of class templates as templates. Also, see issue 249, in which the usage of the phrase "member function template" is questioned.)
Notes from the 4/02 meeting:
Daveed Vandevoorde will propose appropriate terminology.
Currently 13.1 [temp.pre] paragraph 6 forbids any template from having C linkage. Should alias templates be exempt from this prohibition, since they do not have any linkage?
Additional note, April, 2013:
It was suggested that relaxing this restriction for alias templates could provide a way of addressing the long-standing lack of a way of specifying a language linkage for a dependent function type (see issue 13).
Rationale (April, 2013):
CWG felt that this suggested use of alias templates should be considered in a broader context and thus was more appropriate for EWG.
EWG 2022-11-11
extern "C" on a template should be allowed, and should affect only calling convention, but not mangling. This is tracked in github issue cplusplus/papers#1373.
(From submission #506.)
Consider:
template <int> struct S {} v;
Is this a declaration of a class template or a declaration of a variable template, or neither? Implementations uniformly reject.
Another example:
template<class T> typedef struct C F;
Proposed resolution (approved by CWG 2024-04-05):
Change in 13.1 [temp.pre] paragraph 2 through 5 as follows:
The declaration in a template-declaration (if any) shall
The declaration shall not be an export-declaration or a simple-declaration whose decl-specifier-seq contains typedef.
- declare or define a function, a class, or a variable, or
- define a member function, a member class, a member enumeration, or a static data member of a class template or of a class nested within a class template, or
- define a member template of a class or class template, or
- be a deduction-guide, or
- be an alias-declaration.
A template-declaration is a declaration.Ifthe template-declaration declares a member of C, and the nested-name-specifier is treated as a non-dependent reference to C for the purpose of further interpreting the declaration. Otherwise:
- the declaration in a template-declaration introduces an entity using a declarative nested-name-specifier (7.5.5.3 [expr.prim.id.qual]) that is dependent and equivalent to the injected-class-name of a class template or class template partial specialization C, and
- the template-head of the template-declaration is equivalent to that of C,
[ Example:
- A class template is introduced by a template-declaration whose declaration is a simple-declaration that either contains a class-specifier in its decl-specifier-seq or consists solely of an elaborated-type-specifier.
- A function template is introduced by a template-declaration whose declaration declares a function.
- An alias template is introduced by a template-declaration whose declaration is an alias-declaration.
- A variable template is introduced by a template-declaration whose declaration declares a variable.
template<typename T> struct A { template<typename U> struct B; template<typename U> struct B<U*> { template<typename V> void f(); }; }; template<typename T> // #1 template<typename U> // #2 template<typename V> // #3 void A<T>::B<U*>::f() {}The template-declaration #1 declares a member of the class template A, because A<T> is equivalent to the injected-class-name of A. The template-declaration #2 declares a member of the class template partial specialization A<T>::B<U*>, because A<T>::B<U*> is equivalent to the injected-class-name of the partial specialization when A<T> is treated as a non-dependent reference to the primary template A. The template-declaration #3 declares a function template that is a member of the class template partial specialization. -- end example ]A declaration introduced by a template declaration of a variable is a variable template.A variable template at class scope is a static data member template. [ Example: ... ][Note 2: A template-declaration can appear only as a namespace scope or class scope declaration. —end note]
Its declaration shall not be an export-declaration.A template-declaration shall declare exactly one template or member of a template. [ Example:template <int> struct S {} v; // error: declares both a class template and a variable template template <int> struct V *v; // OK: declares a variable template-- end example ] In a function template declaration, the unqualified-id of the declarator-id shall be a name. [Note 3: A class or variable template declaration of a simple-template-id declares a partial specialization (13.7.6 [temp.spec.partial]). —end note]In a template-declaration, explicit specialization, or explicit instantiation, the init-declarator-list in the declaration shall contain at most one declarator. [ Note: When such a declaration is used to declare a class template, no declarator is permitted, because the declarator would be considered to declare a variable or function template in addition to the class template. -- end note ]
CWG 2024-06-26
This drafting lacks the required positional association of template-heads with the respective component of the nested-name-specifier.
The type adjustment of template non-type parameters described in 13.2 [temp.param] paragraph 8 appears to be underspecified. For example, implementations vary in their treatment of
template<typename T, T[T::size]> struct A {}; int dummy; A<int, &dummy> a;
and
template<typename T, T[1]> struct A; template<typename T, T*> struct A {}; int dummy; A<int, &dummy> a;
See also issues 1322 and 1668.
Additional note, February, 2021:
See the discussion regarding top-level cv-qualifiers on template parameters when determining the type in this compiler bug report.
Default function arguments are instantiated only when needed. Is the same true of default template arguments? For example, is the following well-formed?
#include <type_traits> template<class T> struct X { template<class U = typename T::type> static void foo(int){} static void foo(...){} }; int main(){ X<std::enable_if<false>>::foo(0); }
Also, is the effect on lookup the same? E.g.,
struct S { template<typename T = U> void f(); struct U {}; };
Additional note (November, 2020):
Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.
The Standard is not clear, and there is implementation divergence, for an example like the following:
template<class ...Types> struct Tuple_ { // _VARIADIC_TEMPLATE
template<Types ...T, int I> int f() {
return sizeof...(Types);
}
};
int main() {
Tuple_<char,int> a;
int b = a.f<1, 2, 3>();
}
The question is whether the 3 is accepted as the argument for I or an error, exceeding the number of arguments for T, which is set as 2 by the template arguments for Tuple_. See also issue 2383 for a related example.
According to 13.4.3 [temp.arg.nontype] paragraph 1 (newly revised by the adoption of paper N4268),
For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of):
a subobject (6.7.2 [intro.object]),
...
This change breaks an example like
template<int *p> struct X {}; int arr[32]; X<arr> x;
because the array-to-pointer decay produces a pointer to the first element, which is a subobject.
Suggested resolution:
Change the referenced bullet to read:
a subobject (6.7.2 [intro.object]) that is not the first element of a complete object of array type,
Note that this resolution also allows an example like
template<char &p> struct S { }; char arr[2]; S<arr[0]> s_arr;
which may not be exactly what we want.
See also issue 2401.
Consider an example like:
template <const char *N> struct A { static const int val; }; template <const char *N> const int A<N>::val = 0; static const char c[2] = ""; int main() { A<c> a; return A<c>::val; }
Formally, this appears to violate the prohibition of using the address of a subobject as a non-type template argument, since the array reference c in the argument decays to a pointer to the first element of the array. However, at least some implementations accept this example, and at least conceptually the template argument designates the complete object. Should an exception be made for the result of array decay?
See also issue 2043.
Notes from the July, 2019 meeting
CWG felt that the example should be allowed if the parameter type is a pointer to object type (thus prohibiting void*).
It is not clear how to handle an example like:
template<typename T1, typename T2 = char> class A { }; template<template<typename... T> class X> class S { X<int> x; }; S<A> a;
Issue 184 dealt with a similar question but did so in the era before variadic templates. This usage should be permitted in modern C++.
Notes from the February, 2016 meeting:
CWG felt that this usage should be permitted, but only for template template parameters with a parameter pack.. Furthermore, if the template template parameter has a default argument followed by a parameter pack, the parameter's default argument would be used, followed by any remaining default arguments from the template template argument.
Do the changes from P0522R0 regarding template template parameter matching apply to deduction? For example:
template<class T, class U = T> class B { /* ... */ }; template<template<class> class P, class T> void f(P<T>); int main() { f(B<int>()); // OK? f(B<int,float>()); // ill-formed, T deduced to int and float }
In deduction we can determine that P is more specialized than B, then substitute B into P<T>, and then compare B<T,T> to B<int,int>. This will allow deduction to succeed, whereas comparing <T> to <int,int> without this substitution would fail. I suppose this is similar to deducing a type parameter, substituting it into the type of a non-type parameter, then deducing the value of the non-type parameter
Does this make sense? Do we need more wording?
Consider also this example;
template<typename> struct match; template<template<typename> class t,typename T> struct match<t<T> > { typedef int type; }; // #1 template<template<typename,typename> class t,typename T0,typename T1> struct match<t<T0,T1> > { typedef int type; }; // #2 template<typename,typename = void> struct other { }; typedef match<other<void,void> >::type type;
Before this change, partial specialization #1 was not a candidate; now it is, and neither partial specialization is at least as specialized as the other, so we get an ambiguity. It seems that the consistent way to address this would be to use other during partial ordering, so we'd be comparing
template<typename T>
void fn (match<other<T>>); // i.e. other<T,void>
template<typename T0, typename T1>
void fn (match<other<T0,T1>>);
So #1 is more specialized, whereas before this change we chose #2.
Additional notes (May, 2024)
Paper P3310R0 (Solving partial ordering issues introduced by P0522R0) by Matheus Izvekov strives to solve this issue, currently with focus on partial ordering (only).
For the following example,
template<int N> struct A {}; template<short N> using B = A<N>; template<int N> void f(B<N>) {} // #1 template<int N> void f(A<N>) {} // #2
There is implementation variance as to whether there is one f or two. As with previously-discussed cases, these have different SFINAE effects, perhaps equivalent but not functionally equivalent. Should the argument to #1 be treated as something like A<(int)(short)N> and not just A<N>.
See also issues 1668 and 1979.
Is it permitted for a variable template to have an unnamed type?
The Standard appears to be silent on whether the types of non-type template arguments in a partial specialization must be the same as those of the primary template or whether conversions are permitted. For example,
template<char...> struct char_values {}; template<int C1, char C3> struct char_values<C1, 12, C3> { static const unsigned value = 1; }; int check0[char_values<1, 12, 3>::value == 1? 1 : -1];
The closest the current wording comes to dealing with this question is 13.7.6.1 [temp.spec.partial.general] bullet 9.1:
A partially specialized non-type argument expression shall not involve a template parameter of the partial specialization except when the argument expression is a simple identifier.
In this example, one might think of the first template argument in the partial specialization as (char)C1, which would violate the requirement, but that reasoning is tenuous.
It would be reasonable to require the types to match in cases like this. If this kind of usage is allowed it could get messy if the primary template were int... and the partial specialization had a parameter that was char because not all of the possible values from the primary template could be represented in the parameter of the partial specialization. A similar issue exists if the primary template takes signed char and the partial specialization takes unsigned int.
There is implementation variance in the treatment of this example.
(See also issues 1315, 2033, and 2127.)
An example like the following would seem to be plausible:
template<class T, T*> struct X { }; // We want to partially specialize for all nullptrs... template<class T> struct X<T, nullptr> { ... }; // NOT OK
This is disallowed by the rule in bullet 9.2 of 13.7.6.1 [temp.spec.partial.general]:
The type of a template parameter corresponding to a specialized non-type argument shall not be dependent on a parameter of the specialization.
(See also issues 1315, 1647, and 2033.)
According to 13.7.6.1 [temp.spec.partial.general] paragraph 1,
A partial specialization shall be declared before the first use of a class template specialization that would make use of the partial specialization as the result of an implicit or explicit instantiation in every translation unit in which such a use occurs; no diagnostic is required.
There are two problems with this wording. First, the “no diagnostic required” provision is presumably to avoid mandating cross-translation-unit analysis, but there is no reason not to require the diagnostic if the rule is violated within a single translation unit. Also, “would make use” is imprecise; it could be interpreted as applying only when the partial specialization would have been selected by a previous specialization, but it should also apply to cases where the partial specialization would have made a previous specialization ambiguous.
Making these two changes would guarantee that a diagnostic is issued for the following example:
template <class T1, class T2> class A; template <class T> struct A<T, void> { void f(); }; template <class T> void g(T) { A<char, void>().f(); } // #1 template<typename T> struct A<char, T> {}; A<char, void> f; // #2
It is unspecified whether the reference to A<char, void> at #1 is the “first use” or not. If so, A<char, void> is bound to the first partial specialization and, under the current wording, an implementation is not required to diagnose the ambiguity resulting from the second partial specialization. If #2 is the “first use,” it is clearly ambiguous and must result in a diagnostic. There is implementation divergence on the handling of this example that would be addressed by the suggested changes.
In the following example, the template parameter in the partial specialization is non-deducible:
template <class T> struct A { typedef T U; }; template <class T> struct C { }; template <class T> struct C<typename A<T>::U> { };
Several compilers issue errors for this case, but there appears to be nothing in the Standard that would make this ill-formed; it simply seems that the partial specialization will never be matched, so the primary template will be used for all specializations. Should it be ill-formed?
(See also issue 1246.)
Notes from the April, 2006 meeting:
It was noted that there are similar issues for constructors and conversion operators with non-deducible parameters, and that they should probably be dealt with similarly.
Additional note, December, 2021:
The original issue, but not the *#8220;similar issues *#8221; pointed out in the 2006-04 note, was resolved by the changes for issue 1315 and paper P0127R2.
According to 13.7.6.4 [temp.spec.partial.member] paragraph 2,
If a member template of a class template is partially specialized, the member template partial specializations are member templates of the enclosing class template; if the enclosing class template is instantiated (13.9.2 [temp.inst], 13.9.3 [temp.explicit]), a declaration for every member template partial specialization is also instantiated as part of creating the members of the class template specialization.
Does this imply that only partial specializations of member templates that are declared before the enclosing class is instantiated are considered? For example, in
template<typename A> struct X { template<typename B> struct Y; }; template struct X<int>; template<typename A> template<typename B> struct X<A>::Y<B*> { int n; }; int k = X<int>::Y<int*>().n;
is the last line valid? There is implementation variance on this point. Similarly, for an example like
template<typename A> struct Outer { template<typename B, typename C> struct Inner; }; Outer<int> outer; template<typename A> template<typename B> struct Outer<A>::Inner<typename A::error, B> {};
at what point, if at all, is the declaration of the partial specialization instantiated? Again, there is implementation variance in the treatment of this example.
Notes from the February, 2014 meeting:
CWG decided that partial specialization declarations should be instantiated only when needed to determine whether the partial specialization matches or not.
Additional note, November, 2014:
See also paper N4090.
Issue 1244 was resolved by changing the example in 13.6 [temp.type] paragraph 1 from
template<template<class> class TT> struct X { }; template<class> struct Y { }; template<class T> using Z = Y<T>; X<Y> y; X<Z> z;
to
template<class T> struct X { }; template<class> struct Y { }; template<class T> using Z = Y<T>; X<Y<int> > y; X<Z<int> > z;
In fact, the original intent was that the example should have been correct as written; however, the normative wording to make it so was missing. The current wording of 13.7.8 [temp.alias] deals only with the equivalence of a specialization of an alias template with the type-id after substitution. Wording needs to be added specifying under what circumstances an alias template itself is equivalent to a class template.
Proposed resolution (September, 2012):
Add the following as a new paragraph following 13.7.8 [temp.alias] paragraph 2:
When the type-id in the declaration of alias template (call it A) consists of a simple-template-id in which the template-argument-list consists of a list of identifiers naming each template-parameter of A exactly once in the same order in which they appear in A's template-parameter-list, the alias template is equivalent to the template named in the simple-template-id (call it T) if A and T have the same number of template-parameters. [Footnote: This rule is transitive: if an alias template A is equivalent to another alias template B that is equivalent to a class template C, then A is also equivalent to C, and A and B are also equivalent to each other. —end footnote] [Example:
template<typename T, U = T> struct A; template<typename V, typename W> using B = A<V, W>; // equivalent to A template<typename V, typename W> using C = A<V>; // not equivalent to A: // not all parameters used template<typename V> using D = A<V>; // not equivalent to A: // different number of parameters template<typename V, typename W> using E = A<W, V>; // not equivalent to A: // template-arguments in wrong order template<typename V, typename W = int> using F = A<V, W>; // equivalent to A: // default arguments not considered template<typename V, typename W> using G = A<V, W>; // equivalent to A and B template<typename V, typename W> using H = E<V, W>; // equivalent to E template<typename V, typename W> using I = A<V, typename W::type>; // not equivalent to A: // argument not identifier—end example]
Change 13.6 [temp.type] paragraph 1 as follows:
Two template-ids refer to the same class or function if
...
their corresponding template template-arguments refer to the same or equivalent (13.7.8 [temp.alias]) templates.
[Example:
...declares x2 and x3 to be of the same type. Their type differs from the types of x1 and x4.
template<class Ttemplate<class> class TT> struct X { }; template<class> struct Y { }; template<class T> using Z = Y<T>; X<Y<int>Y> y; X<Z<int>Z> z;declares y and z to be of the same type. —end example]
Additional note, November, 2014:
Concern has been expressed over the proposed resolution with regard to its handling of default template arguments that differ between the template and its alias, e.g.,
template<typename T, typename U = int> struct A {}; template<typename T, typename U = char> using B = A<T, U>; template<template<typename...> typename C> struct X { C<int> c; };
Notes from the May, 2015 meeting:
See also issue 1979, which CWG is suggesting to be resolved by defining a “simple” alias, one in which the SFINAE conditions are the same as the referenced template and that uses all template parameters.
The interaction of alias templates and access control is not clear from the current wording of 13.7.8 [temp.alias]. For example:
template <class T> using foo = typename T::foo;
class B {
typedef int foo;
friend struct C;
};
struct C {
foo<B> f; // Well-formed?
};
Is the substitution of B::foo for foo<B> done in the context of the befriended class C, making the reference well-formed, or is the access determined independently of the context in which the alias template specialization appears?
If the answer to this question is that the access is determined independently from the context, care must be taken to ensure that an access failure is still considered to be “in the immediate context of the function type” (13.10.3 [temp.deduct] paragraph 8) so that it results in a deduction failure rather than a hard error.
Notes from the October, 2012 meeting:
The consensus of CWG was that instantiation (lookup and access) for alias templates should be as for other templates, in the definition context rather than in the context where they are used. They should still be expanded immediately, however.
Additional note (February, 2014):
A related problem is raised by the definition of std::enable_if_t (21.3.3 [meta.type.synop]):
template <bool b, class T = void> using enable_if_t = typename enable_if<b,T>::type;
If b is false, there will be no type member. The intent is that such a substitution failure is to be considered as being “in the immediate context” where the alias template specialization is used, but the existing wording does not seem to accomplish that goal.
Additional note, November, 2014:
Concern has been expressed that the intent to analyze access in the context of the alias template definition is at odds with the fact that friendship cannot be granted to alias templates; if it could, the access violation in the original example could be avoided by making foo a friend of class B, but that is not possible.
Additional node, February, 2016:
The issue has been returned to "open" status to facilitate further discussion by CWG as to whether the direction in the October, 2012 note is still desirable.
Notes from the February, 2016 meeting:
CWG reaffirmed the direction described in the October, 2012 note above. With regard to the November, 2014 note regarding granting of friendship, it was observed that the same problem occurs with enumerators, which might refer to inaccessible names in the enumerator volue. The solution in both cases is to embed the declaration in a class and grant the class friendship. See issue 1844, dealing with the definition of “immediate context.”
CWG 2024-06-28
See issue 2296 for the definition of "immediate context".
In an example like
template<typename T> struct A {
struct B {
void f();
};
};
template<typename T> using X = typename A<T>::B;
template<typename T> void X<T>::f() { } // #1
should #1 be considered a definition of A<T>::B::f()?
Analogy with alias-declarations would suggest that it should, but alias template specializations involve issues like SFINAE on unused template parameters (see issue 1558) and possibly other complications.
(See also issues 1980, 2021, 2025, and 2037.)
Notes from the May, 2015 meeting:
CWG felt that this kind of usage should be permitted only via a “simple” alias, in which the SFINAE is the same as the template to which it refers and all the template parameters are used. See also issue 1286.
In an example like
template<typename T, typename U> using X = T; template<typename T> X<void, typename T::type> f(); template<typename T> X<void, typename T::other> f();
it appears that the second declaration of f is a redeclaration of the first but distinguishable by SFINAE, i.e., equivalent but not functionally equivalent.
Notes from the November, 2014 meeting:
CWG felt that these two declarations should not be equivalent.
There is implementation divergence for this example:
struct A { typedef int type; }; template <typename T> using ALIAS = A; template <typename T> void foo() { ALIAS<T>::type t; // Is typename required here? } int main() { foo<A>(); }
See also issues 1558, 1979, and 2037.
According to 13.8.2 [temp.local] paragraph 5,
A qualified-id is assumed to name a type if
it is a qualified name in a type-id-only context (see below), or
it is a decl-specifier of the decl-specifier-seq of a
simple-declaration or a function-definition in namespace scope,
member-declaration,
parameter-declaration in a member-declaration140, unless that parameter-declaration appears in a default argument,
parameter-declaration in a declarator of a function or function template declaration whose declarator-id is qualified, unless that parameter-declaration appears in a default argument,
...
There are two possible problems with this specification. First, consider an example like
template<typename T> struct S { static void (*pfunc)(T::name); // Omitted typename okay because it is a // member-declaration }; template<typename T> void (*S<T>::pfunc)(T::name) = nullptr; // Omitted typename ill-formed because not a function // or function template declaration
Should bullet 5.2.4 be extended to include function pointer and member function pointer declarations, as well as function and function template declarations?
Second, given an example like
template<typename T> struct Y {}; template<typename T> struct S { Y<int(T::type)> m; // Omitted typename okay because it is in a member-declaration? };
Should bullet 5.2.3 be restricted to parameter-declarations of the member being declared, rather than simply “in” such a member-declaration?
Notes from the December, 2020 teleconference:
The second issue was split off into issue 2468 to allow the resolutions to proceed independently.
According to 13.8.2 [temp.local] paragraph 5,
A qualified-id is assumed to name a type if
it is a qualified name in a type-id-only context (see below), or
it is a decl-specifier of the decl-specifier-seq of a
simple-declaration or a function-definition in namespace scope,
member-declaration,
parameter-declaration in a member-declaration140, unless that parameter-declaration appears in a default argument,
...
This specification would appear to allow an example like:
template<typename T> struct Y {}; template<typename T> struct S { Y<int(T::type)> m; // Omitted typename okay because it is in a member-declaration? };
The affected parameter-declarations should be only those of the member declarator, not in a member template's template parameter list.
(Note: this issue was spun off from issue 2462 to allow the resolutions to proceed independently.)
According to 13.8.3.2 [temp.dep.type] paragraph 8, a type is dependent (among other things) if it is
a simple-template-id in which either the template name is a template parameter or any of the template arguments is a dependent type or an expression that is type-dependent or value-dependent
This applies to alias template specializations, even if the resulting type does not depend on the template argument:
struct B { typedef int type; }; template<typename> using foo = B; template<typename T> void f() { foo<T>::type * x; //error: typename required }
Is a change to the rules for cases like this warranted?
Notes from the October, 2012 meeting:
CWG agreed that no typename should be required in this case. In some ways, an alias template specialization is like the current instantiation and can be known at template definition time.
The correct handling of an example like the following is unclear:
template<typename T> struct A { struct B: A { }; };
A type used as a base must be complete (11.7 [class.derived] paragraph 2) . The fact that the base class in this example is the current instantiation could be interpreted as indicating that it should be available for lookup, and thus the normal rule should apply, as members declared after the nested class would not be visible.
On the other hand, 13.8.3 [temp.dep] paragraph 3 says,
In the definition of a class or class template, if a base class depends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.
This wording refers not to a dependent type, which would permit lookup in the current instantiation, but simply to a type that “depends on a template-parameter,” and the current instantiation is such a type.
Implementations vary on the handling of this example.
(See also issue 1526 for another case related to the distinction between a “dependent type” and a “type that depends on a template-parameter.”)
Notes from the October, 2012 meeting:
CWG determined that the example should be ill-formed.
According to 13.8.3.2 [temp.dep.type] paragraph 9, a local class in a function template is dependent if and only if it contains a subobject of a dependent type. However, given an example like
template<typename T> void f() {
struct X {
typedef int type;
#ifdef DEPENDENT
T x;
#endif
};
X::type y; // #1
}
void g() { f<int>(); }
there is implementation variance in the treatment of #1, but whether or not DEPENDENT is defined appears to make no difference.
In a related question, should a value-dependent alignas specifier cause a type to be dependent? Given
template<int N> struct Y { typedef int type; };
template<int N> void h() {
struct alignas(N) X {};
Y<alignof(X)>::type z; // #2
}
void i() { h<4>(); }
Most/all implementations issue an error for a missing typename in #2.
Perhaps the right answer is that the types should be dependent but a member of the current instantiation, permitting name lookup without typename.
Additional notes (September, 2022):
At present, the term "current instantiation" is defined for class templates only, and thus does not apply to function templates.
Moreover, the resolution for this issue should also handle local enums, with particular attention to 9.7.2 [enum.udecl] paragraph 1:
The elaborated-enum-specifier shall not name a dependent type and...
This rule, without amendment, would disallow the following reasonable example if local enums were made dependent types:
template <class T> void f() { enum class E { e1, e2 }; using enum E; }
Consider:
struct B { template <class T> void h(); }; template <class T> struct A { template <class U> static U f(U); void g() { f(B()).h<int>(); // OK, f(B()) is non-type-dependent with type B. } };
A member template ought to be dependent only if it depends on template parameters of the current scope, but 13.8.3.3 [temp.dep.expr] paragraph 3 is silent on the matter.
Consider:
using T = int[]; using U = int[2]; template<auto M, int ...N> void f() { auto &&arr1 = T(N...); auto &&arr2 = T{N...}; auto &&arr3 = U(M, M); auto &&arr4 = U{M, M}; };
I think here T(N...) is not type-dependent, per 13.8.3.3 [temp.dep.expr] paragraph 3, but should be. (I think T{N...} is type-dependent.) Conversely, I think U{M, M} is type-dependent, per 13.8.3.3 [temp.dep.expr] paragraph 6, but should not be. (U(M, M) is not type-dependent.)
I think we should say that
are type-dependent if the type specifier names a dependent type, or if it names an array of unknown bound and the braced-init-list or expression-list is type-dependent.
(I think we could be a little more precise than that in the case where there is no top-level pack expansion: T{M, M} needs to be type-dependent for a general array of unknown bound T due to brace elision, but not in the case where the array element type is a scalar type. And T(M, M) does not need to be type-dependent because direct aggregate initialization can't perform brace elision. But I think the simpler rule is probably good enough.)
Notes from the August, 2021 teleconference:
CWG agreed with the suggested change. There was some support for the “more precise” approach mentioned in the description.
According to 13.8.3.5 [temp.dep.temp] paragraph 3,
a non-type template-argument is dependent if the corresponding non-type template-parameter is of reference or pointer type and the template-argument designates or points to a member of the current instantiation or a member of a dependent type.
Members of non-dependent base classes are members of the current instantiation, but using one as a non-type template argument should not be considered dependent.
template <class T> class Foo { public: typedef int Bar; Bar f(); }; template <class T> typename Foo<T>::Bar Foo<T>::f() { return 1;} --------------------In the class template definition, the declaration of the member function is interpreted as:
int Foo<T>::f();In the definition of the member function that appears outside of the class template, the return type is not known until the member function is instantiated. Must the return type of the member function be known when this out-of-line definition is seen (in which case the definition above is ill-formed)? Or is it OK to wait until the member function is instantiated to see if the type of the return type matches the return type in the class template definition (in which case the definition above is well-formed)?
Suggested resolution: (John Spicer)
My opinion (which I think matches several posted on the reflector recently) is that the out-of-class definition must match the declaration in the template. In your example they do match, so it is well formed.
I've added some additional cases that illustrate cases that I think either are allowed or should be allowed, and some cases that I don't think are allowed.
template <class T> class A { typedef int X; }; template <class T> class Foo { public: typedef int Bar; typedef typename A<T>::X X; Bar f(); Bar g1(); int g2(); X h(); X i(); int j(); }; // Declarations that are okay template <class T> typename Foo<T>::Bar Foo<T>::f() { return 1;} template <class T> typename Foo<T>::Bar Foo<T>::g1() { return 1;} template <class T> int Foo<T>::g2() { return 1;} template <class T> typename Foo<T>::X Foo<T>::h() { return 1;} // Declarations that are not okay template <class T> int Foo<T>::i() { return 1;} template <class T> typename Foo<T>::X Foo<T>::j() { return 1;}In general, if you can match the declarations up using only information from the template, then the declaration is valid.
Declarations like Foo::i and Foo::j are invalid because for a given instance of A<T>, A<T>::X may not actually be int if the class is specialized.
This is not a problem for Foo::g1 and Foo::g2 because for any instance of Foo<T> that is generated from the template you know that Bar will always be int. If an instance of Foo is specialized, the template member definitions are not used so it doesn't matter whether a specialization defines Bar as int or not.
Implementations differ in their treatment of the following code:
template <class T> struct A { typename T::X x; }; template <class T> struct B { typedef T* X; A<B> a; }; int main () { B<int> b; }
Some implementations accept it. At least one rejects it because the instantiation of A<B<int> > requires that B<int> be complete, and it is not at the point at which A<B<int> > is being instantiated.
Erwin Unruh:
In my view the programm is ill-formed. My reasoning:
So each class needs the other to be complete.
The problem can be seen much easier if you replace the typedef with
typedef T (*X) [sizeof(B::a)];
Now you have a true recursion. The compiler cannot easily distinguish between a true recursion and a potential recursion.
John Spicer:
Using a class to form a qualified name does not require the class to be complete, it only requires that the named member already have been declared. In other words, this kind of usage is permitted:
class A { typedef int B; A::B ab; };
In the same way, once B has been declared in A, it is also visible to any template that uses A through a template parameter.
The standard could be more clear in this regard, but there are two notes that make this point. Both 6.5.5.2 [class.qual] and _N4567_.5.1.1 [expr.prim.general] paragraph 7 contain a note that says "a class member can be referred to using a qualified-id at any point in its potential scope (6.4.7 [basic.scope.class])." A member's potential scope begins at its point of declaration.
In other words, a class has three states: incomplete, being completed, and complete. The standard permits a qualified name to be used once a name has been declared. The quotation of the notes about the potential scope was intended to support that.
So, in the original example, class A does not require the type of T to be complete, only that it have already declared a member X.
Bill Gibbons:
The template and non-template cases are different. In the non-template case the order in which the members become declared is clear. In the template case the members of the instantiation are conceptually all created at the same time. The standard does not say anything about trying to mimic the non-template case during the instantiation of a class template.
Mike Miller:
I think the relevant specification is 13.8.4.1 [temp.point] paragraph 3, dealing with the point of instantiation:
For a class template specialization... if the specialization is implicitly instantiated because it is referenced from within another template specialization, if the context from which the specialization is referenced depends on a template parameter, and if the specialization is not instantiated previous to the instantiation of the enclosing template, the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.
That means that the point of instantiation of A<B<int> > is before that of B<int>, not in the middle of B<int> after the declaration of B::X, and consequently a reference to B<int>::X from A<B<int> > is ill-formed.
To put it another way, I believe John's approach requires that there be an instantiation stack, with the results of partially-instantiated templates on the stack being available to instantiations above them. I don't think the Standard mandates that approach; as far as I can see, simply determining the implicit instantiations that need to be done, rewriting the definitions at their respective points of instantiation with parameters substituted (with appropriate "forward declarations" to allow for non-instantiating references), and compiling the result normally should be an acceptable implementation technique as well. That is, the implicit instantiation of the example (using, e.g., B_int to represent the generated name of the B<int> specialization) could be something like
struct B_int; struct A_B_int { B_int::X x; // error, incomplete type }; struct B_int { typedef int* X; A_B_int a; };
Notes from 10/01 meeting:
This was discussed at length. The consensus was that the template case should be treated the same as the non-template class case it terms of the order in which members get declared/defined and classes get completed.
Proposed resolution:
In 13.8.4.1 [temp.point] paragraph 3 change:
the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.
To:
the point of instantiation is the same as the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the nearest enclosing declaration. [Note: The point of instantiation is still at namespace scope but any declarations preceding the point of instantiation, even if not at namespace scope, are considered to have been seen.]
Add following paragraph 3:
If an implicitly instantiated class template specialization, class member specialization, or specialization of a class template references a class, class template specialization, class member specialization, or specialization of a class template containing a specialization reference that directly or indirectly caused the instantiation, the requirements of completeness and ordering of the class reference are applied in the context of the specialization reference.
and the following example
template <class T> struct A { typename T::X x; }; struct B { typedef int X; A<B> a; }; template <class T> struct C { typedef T* X; A<C> a; }; int main () { C<int> c; }
Notes from the October 2002 meeting:
This needs work. Moved back to drafting status.
The current wording of 13.8.4.1 [temp.point] does not define the point of instantiation of a variable template specialization. Presumably replacing the references to “static data member of a class template” with “variable template” in paragraphs 1 and 8 would be sufficient.
Additional note, July, 2017:
It has also been observed that there is no definition of the point of instantiation for an alias template. It is not clear that there is a need for normative wording for the point of instantiation of an alias template, but if not, a note explaining its absence would be helpful.
Consider:
template<typename T> struct X; extern X<int> *p; void *q = +p; // #1, complete type affects semantics via ADL template<typename T> struct X {}; X<int> x; // #2, ill-formed, X<int> is incomplete
According to the wording of issue 212, this program is ill-formed, because the single point of instantiation for X<int> is at #1, thus X<int> is an incomplete type even at #2 after the primary template has been completed.
Notes from the December, 2016 teleconference:
The consensus was that references to specializations before the template definition is seen are not points of instantiation.
Consider:
template<typename T> constexpr T f(); constexpr int g() { return f<int>(); } // #1 template<typename T> constexpr T f() { return 123; } int k[g()]; // #2
There are two points of instantiation for f<int>. At #1, the template isn't defined, so it cannot be instantiated there. At #2, it's too late, as the definition was needed when parsing the type of k.
Should we also treat the point of definition of (at least) a constexpr function template as a point of instantiation for all specializations that have a point of instantiation before that point? Note the possible interaction of such a resolution with 13.8.4.1 [temp.point] paragraph 7:
If two different points of instantiation give a template specialization different meanings according to the one-definition rule (6.3 [basic.def.odr]), the program is ill-formed, no diagnostic required.
Notes from the November, 2021 teleconference:
Another possibility for a point of instantiation, other than the definition of the template, would be the point at which the function is called. Similar questions have been raised regarding the points at which variables are initialized (issue 2186) and constexpr functions are defined (issue 2166).
According to 13.9.2 [temp.inst] paragraph 11,
If a function template f is called in a way that requires a default argument to be used, the dependent names are looked up, the semantics constraints are checked, and the instantiation of any template used in the default argument is done as if the default argument had been an initializer used in a function template specialization with the same scope, the same template parameters and the same access as that of the function template f used at that point, except that the scope in which a closure type is declared (7.5.6.2 [expr.prim.lambda.closure]) — and therefore its associated namespaces — remain as determined from the context of the definition for the default argument. This analysis is called default argument instantiation. The instantiated default argument is then used as the argument of f.
Some details are not clear from this description. For example, given
#include <type_traits> template<class T> struct Foo { Foo(T = nullptr) {} }; bool b = std::is_constructible<Foo<int>>::value; int main() {}
does “used” mean odr-used or used in any way? Is a failure of default argument instantiation in the immediate context of the call or is a failure a hard error? And does it apply only to function templates, as it says, or should it apply to member functions of class templates? There is implementation divergence on these questions.
Notes from the March, 2018 meeting:
CWG felt that such errors should be substitution failures, not hard errors.
Additional notes (March, 2024)
This issue is related to issue 2296.
According to 13.9.2 [temp.inst] paragraph 6,
If the function selected by overload resolution (12.2 [over.match]) can be determined without instantiating a class template definition, it is unspecified whether that instantiation actually takes place.
There are other contexts in which a smart implementation could presumably avoid instantiations, such as when doing argument-dependent lookup involving a class template specialization when the template definition contains no friend declarations or checking base/derived relationships involving incomplete class template definitions. It would be helpful to enumerate such contexts.
The instantiation of default arguments for friends defined in a templated entity is not covered by 13.7.1 [temp.decls.general] paragraph 3 or 13.9.2 [temp.inst] paragraph 2. Consider:
template <typename T> struct A { friend void foo(A &&, int = T::happy) { } }; int main(void) { foo(A<int>(), 0); }
There is implementation divergence in the treatment of this example.
Notes from the December, 2016 teleconference:
This issue should be resolved by the resolution of issue 2174.
It is not clear how to handle parameter packs that are expanded during instantiation in parallel with those that are not yet concrete. In particular, does the following example require a diagnostic?
template<typename ...T> struct Tuple; template<class T, class U> struct Outer; template<class ...T, class ...U> struct Outer<Tuple<T ...>, Tuple<U ...> > { template<class X, class Y> struct Inner; template<class ...Y> struct Inner<Tuple<T, Y> ...> { }; template<class ...Y> struct Inner<Tuple<U, Y> ...> { }; }; Outer<Tuple<int, void>, Tuple<int, void> > outer;
Notes from the March, 2018 meeting:
CWG felt that ill-formed, no diagnostic required was the correct approach.
Consider:
struct Base {}; template<int N> struct S : public Base { friend int foo(Base&) requires (N == 1) { return 1; } friend int foo(Base&) requires (N == 2) { return 3; } }; int main() { S<1> s1{}; S<2> s2{}; // #1 }
The current wording does not seem to cover what happens for this case. In particular, 13.9.2 [temp.inst] paragraph 17 does not cover constrained non-template friends.
See also the Itanium ABI issue 24.
Suggested resolution:
Change in 13.7.5 [temp.friend] paragraph 9 as follows:
A non-template friend declaration with a requires-clause shall be a definition. A friend function template with a constraint that depends on a template parameter from an enclosing template shall be a definition. Such a constrained friend function or function template declaration does not declare the same function or function template as a declarationininhabiting any other scope.
Change in 13.9.2 [temp.inst] paragraph 17 as follows:
The type-constraints and requires-clause of a template specialization ormembertemplated function are not instantiated along with the specialization or function itself, even for a member function of a local class; substitution into the atomic constraints formed from them is instead performed as specified in 13.5.3 [temp.constr.decl] and 13.5.2.3 [temp.constr.atomic] when determining whether the constraints are satisfied or as specified in 13.5.3 [temp.constr.decl] when comparing declarations.[ Note 7: ... ]
[ Example 10: ... ]
[ Example:
struct Base {}; template<int N> struct S : Base { friend int foo(Base&) requires (N == 1) { return 1; } // #1 friend int foo(Base&) requires (N == 2) { return 3; } // #2 }; S<1> s1; S<2> s2; // OK, no conflict between #1 and #2 int x = foo(s1); // OK, selects #1 int y = foo(s2); // OK, selects #2-- end example ]
[ Example 11: ... ]
CWG 2022-11-10
The friend definitions should conflict with friend definitions from other instantiations of the same class template, consistent with how non-constrained friends would work. Note that the enclosing dependent class type does not appear in the friend function's signature, which is unusual.
Consider a case like
struct X { template<typename T> void f(T); void f(int); }; template void X::f(int);
or
template<typename T> void f(T) {} void f(int); template void f(int);
Presumably in both these cases the explicit instantiation should refer to the template and not to the non-template; however, 13.7.3 [temp.mem] paragraph 2 says,
A normal (non-template) member function with a given name and type and a member function template of the same name, which could be used to generate a specialization of the same type, can both be declared in a class. When both exist, a use of that name and type refers to the non-template member unless an explicit template argument list is supplied.
This would appear to give the wrong answer for the first example. It's not clearly stated, but consistency would suggest a similar wrong answer for the second. Presumably a statement is needed somewhere that an explicit instantiation directive applies to a template and not a non-template function if both are visible.
Additional note, January, 2014:
A related example has been raised:
template<typename T> class Matrix { public: Matrix(){} Matrix(const Matrix&){} template<typename U> Matrix(const Matrix<U>&); }; template Matrix<int>::Matrix(const Matrix&); Matrix<int> m; Matrix<int> mm(m);
If the explicit instantiation directive applies to the constructor template, there is no way to explicitly instantiate the copy constructor.
An explicit instantiation of a class template specialization also explicitly instantiates member functions of that class template specialization whose constraints are satisfied, even those that are not callable because a more-constrained overload exists which would always be selected by overload resolution. Ideally, we would not explicitly instantiate definitions of such uncallable functions.
Notes from the August, 2020 teleconference:
CWG felt that the concept of “eligible” might form a basis for the resolution of this issue.
CWG determined that issue 2488 was not a defect. However, the discussion uncovered an issue regarding the handling of an explicit instantiation of a class template containing such members. According to 13.9.3 [temp.explicit] paragraph 10,
An explicit instantiation that names a class template specialization is also an explicit instantiation of the same kind (declaration or definition) of each of its direct non-template members that has not been previously explicitly specialized in the translation unit containing the explicit instantiation, provided that the associated constraints, if any, of that member are satisfied by the template arguments of the explicit instantiation (13.5.3 [temp.constr.decl], 13.5.2 [temp.constr.constr]), except as described below.
Paragraph 12 says,
An explicit instantiation of a prospective destructor (11.4.7 [class.dtor]) shall correspond to the selected destructor of the class.
Perhaps the virtual and constrained members could be handled in an analogous fashion.
Notes from the November, 2021 teleconference:
Issue 2488 is being reopened due to subsequent comments.
CWG 2022-11-10
For each explicit instantiation, there shall be exactly one member whose constraints are more specialized than any other member with the same signature. Use the "address of function" model to determine this member.
Paragraph 17 of 13.9.4 [temp.expl.spec] says,
A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized.
This is curious, because paragraph 3 only allows explicit specialization of members of implicitly-instantiated class specializations, not explicit specializations. Furthermore, paragraph 4 says,
Definitions of members of an explicitly specialized class are defined in the same manner as members of normal classes, and not using the explicit specialization syntax.
Paragraph 18 provides a clue for resolving the apparent contradiction:
In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member.
It appears from this and the following example that the phrase “explicitly specialized” in paragraphs 17 and 18, when referring to enclosing class templates, does not mean that explicit specializations have been declared for them but that their names in the qualified-id are followed by template argument lists. This terminology is confusing and should be changed.
Proposed resolution (October, 2005):
Change 13.9.4 [temp.expl.spec] paragraph 17 as indicated:
A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class templatethat is explicitly specializedspecialization. [Example:...
Change 13.9.4 [temp.expl.spec] paragraph 18 as indicated:
In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized,except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as wellthat is, the template-id naming the template may be composed of template parameter names rather than template-arguments.InFor each unspecialized template in such an explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding theexplicit specializationdeclaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. In such declarations, an unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...
Notes from the April, 2006 meeting:
The revised wording describing “unspecialized” templates needs more work to ensure that the parameter names in the template-id are in the correct order; the distinction between template arguments and parameters is also probably not clear enough. It might be better to replace this paragraph completely and avoid the “unspecialized” wording altogether.
Proposed resolution (February, 2010):
Change 13.9.4 [temp.expl.spec] paragraph 17 as follows:
A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class templatethat is explicitly specializedspecialization. [Example:...
Change 13.9.4 [temp.expl.spec] paragraph 18 as follows:
In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized,except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition.that is, the corresponding template prefix may specify a template-parameter-list instead of template<> and the template-id naming the template be written using those template-parameters as template-arguments. In such a declaration, the number, kinds, and types of the template-parameters shall be the same as those specified in the primary template definition, and the template-parameters shall be named in the template-id in the same order that they appear in the template-parameter-list. An unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...
The resolution of issue 941 permits a non-deleted explicit specialization of a deleted function template. For example:
template<typename T> void f() = delete; decltype(f<int>()) *p; template<> void f<int>();
However, the existing normative wording is not adequate to handle this usage. For one thing, =delete is formally, at least, a function definition, and an implementation is not permitted to instantiate a function definition unless it is used; presumably, then, an implementation could not reject the decltype above as a reference to a deleted specialization. Furthermore, there should be a requirement that a non-deleted explicit specialization of a deleted function template must precede any reference to that specialization. (I.e., the example should be ill-formed as written but well-formed if the last two lines were interchanged.)
Issue 531 surveyed existing practice at the time and determined that the most common syntax for defining a member of an explicit specialization used the template<> prefix. This approach, however, does not seem consistent, since such a definition is not itself an explicit specialization.
The status of an example like the following is not clear:
struct S { template <int N> static constexpr inline int m = N; }; template <> constexpr inline int S::m<5>;
Some implementations accept this, apparently on the basis of allowing and ignoring a redeclaration of a constexpr static data member outside its class, although there is implementation divergence. Most or all implementations, however, diagnose an attempt to use such a specialization in a constant context.
Should it be required to have a definition of the explicit specialization in order to declare it outside the class in such cases?
In addition, most or all implementations accept a version of the example in which the explicit specialization contains an initializer, including allowing its use in constant contexts:
template <> constexpr inline int S::m<5> = 2;
This would seem to be disallowed both by 11.4.9.3 [class.static.data] paragraph 3,
An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer. If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see _N4778_.D.4 [depr.static_constexpr]).
which prohibits an initializer, and 13.9.4 [temp.expl.spec] paragraph 2,
An explicit specialization may be declared in any scope in which the corresponding primary template may be defined (_N4868_.9.8.2.3 [namespace.memdef], 11.4 [class.mem], 13.7.3 [temp.mem]).
since the definition of a constexpr static data member is inside the class.
Notes from the May, 2019 teleconference:
These examples should behave in the same way as if the class were templated: instantiate the declaration and the definition of the static data member separately. The first example should be ill-formed, because the explicit specializaation does not have an initializer.
According to 13.10.2 [temp.arg.explicit] paragraph 3,
Trailing template arguments that can be deduced (13.10.3 [temp.deduct]) or obtained from default template-arguments may be omitted from the list of explicit template-arguments. A trailing template parameter pack (13.7.4 [temp.variadic]) not otherwise deduced will be deduced to an empty sequence of template arguments. If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted. In contexts where deduction is done and fails, or in contexts where deduction is not done, if a template argument list is specified and it, along with any default template arguments, identifies a single function template specialization, then the template-id is an lvalue for the function template specialization.
It is not clear that this permits an example like:
template<typename... T> void f(typename T::type...) { } int main() { f<>(); }
See also issue 2105.
There are certain constructs that are not covered by the existing categories of “type dependent” and “value dependent.” For example, the expression sizeof(sizeof(T())) is neither type-dependent nor value-dependent, but its validity depends on whether T can be value-constructed. We should be able to overload on such characteristics and select via deduction failure, but we need a term like “instantiation-dependent” to describe these cases in the Standard. The phrase “expression involving a template parameter” seems to come pretty close to capturing this idea.
Notes from the November, 2010 meeting:
The CWG favored extending the concepts of “type-dependent” and “value-dependent” to cover these additional cases, rather than adding a new concept.
Notes from the March, 2011 meeting:
The CWG reconsidered the direction from the November, 2010 meeting, as it would make more constructs dependent, thus requiring more template and typename keywords, resulting in worse error messages, etc.
Notes from the August, 2011 meeting:
The following example (from issue 1273) was deemed relevant for this issue:
template <class T> struct C; class A { int i; friend struct C<int>; } a; class B { int i; friend struct C<float>; } b; template <class T> struct C { template <class U> decltype (a.i) f() { } // #1 template <class U> decltype (b.i) f() { } // #2 }; int main() { C<int>().f<int>(); // calls #1 C<float>().f<float>(); // calls #2 }
The discussion of issue 1001 seemed to have settled on the approach of doing the 9.3.4.6 [dcl.fct] transformations immediately to the function template declaration, so that the original form need not be remembered. However, the example in 13.10.3 [temp.deduct] paragraph 8 suggests otherwise:
template <class T> int f(T[5]);
int I = f<int>(0);
int j = f<void>(0); // invalid array
One way that might be addressed would be to separate the concepts of the type of the template that participates in overload resolution and function matching from the type of the template that is the source for template argument substitution. (See also the example in paragraph 3 of the same section.)
Notes, January, 2012:
According to 13.10.3 [temp.deduct] paragraph 5,
The resulting substituted and adjusted function type is used as the type of the function template for template argument deduction. If a template argument has not been deduced and its corresponding template parameter has a default argument, the template argument is determined by substituting the template arguments determined for preceding template parameters into the default argument. If the substitution results in an invalid type, as described above, type deduction fails.
This leaves the impression that default arguments are used after deduction failure leaves an argument undeduced. For example,
template<typename T> struct Wrapper; template<typename T = int> void f(Wrapper<T>*); void g() { f(0); }
Deduction fails for T, so presumably int is used. However, some implementations reject this code. It appears that the intent would be better expressed as something like
...If a template argument is used only in a non-deduced context and its corresponding template parameter has a default argument...
Rationale (November, 2013):
CWG felt that this issue should be considered by EWG in a broader context before being resolved.
Additional note, April, 2015:
EWG has requested that CWG resolve this issue along the lines discussed above.
Notes from the May, 2015 meeting:
CWG agreed that a default template argument should only be used if the parameter is not used in a deducible context. See also issue 2092.
According to 13.10.3.2 [temp.deduct.call] paragraph 1,
If removing references and cv-qualifiers from P gives std::initializer_list<P'> for some P' and the argument is an initializer list (9.4.5 [dcl.init.list]), then deduction is performed instead for each element of the initializer list, taking P' as a function template parameter type and the initializer element as its argument. Otherwise, an initializer list argument causes the parameter to be considered a non-deduced context (13.10.3.6 [temp.deduct.type]).
It is not entirely clear whether the deduction for an initializer list meeting a std::initializer_list<T> is a recursive subcase, or part of the primary deduction. A relevant question is: if the deduction on that part fails, does the entire deduction fail, or is the parameter to be considered non-deduced?
See also issue 2326.
Notes from the October, 2012 meeting:
CWG determined that the entire deduction fails in this case.
It is not clear whether the following is well-formed or not:
void foo(){} template<class T> void deduce(const T*) { } int main() { deduce(foo); }
Implementations vary in their treatment of this example.
Proposed resolution (April, 2013):
Change 13.10.3.6 [temp.deduct.type] paragraph 18 as follows:
A template-argument can be deduced from a function, pointer to function, or pointer to member function type. [Note: cv-qualification of a deduced function type is ignored; see 9.3.4.6 [dcl.fct]. —end note] [Example:
template<class T> void f(void(*)(T,int)); template<class T> void f2(const T*); template<class T> void foo(T,int); void g(int,int); void g(char,int); void g2(); void h(int,int,int); void h(char,int); int m() { f(&g); // error: ambiguous f(&h); // OK: void h(char,int) is a unique match f(&foo); // error: type deduction fails because foo is a template f2(g2); // OK: cv-qualification of deduced function type ignored }—end example]
Additional note, November, 2014:
Concern was expressed regarding the proposed resolution over its treatment of an example like the following:
template<typename T> struct tuple_size {}; template<typename T> struct tuple_size<T const>: tuple_size<T> {}; tuple_size<void()> t;
In this case T const is always considered to be more specialized for void(), leading to infinite self-derivation.
The issue has been returned to "open" status for further consideration.
Notes from the May, 2015 meeting:
The consensus of CWG was that the cv-qualification of the argument and parameter must match, so the original example should be rejected.
The rules for deducing template arguments when taking the address of a function template in 13.10.3.3 [temp.deduct.funcaddr] do not appear to allow for a base-to-derived conversion in a case like:
struct Base { template<class U> void f(U); }; struct Derived : Base { }; int main() { void (Derived::*pmf)(int) = &Derived::f; }
Most implementations appear to allow this adjustment, however.
Given
template<class C> void foo(const C* val) {} template<int N> void foo(const char (&t)[N]) {}
it is intuitive that the second template is more specialized than the first. However, the current rules make them unordered. In 13.10.3.5 [temp.deduct.partial] paragraph 4, we have P as const C* and A as const char (&)[N]. Paragraph 5 transforms A to const char[N]. Finally, paragraph 7 removes top-level cv-qualification; since a cv-qualified array element type is considered to be cv-qualification of the array (6.8.5 [basic.type.qualifier] paragraph 5, cf issue 1059), A becomes char[N]. P remains const C*, so deduction fails because of the missing const in A.
Notes from the April, 2013 meeting:
CWG agreed that the const should be preserved in the array type.
The presentation style of 13.10.3.6 [temp.deduct.type] paragraph 8 results in a specification that is unclear, needlessly verbose, and incomplete. Specific problems include:
What does it mean for P and A to have one of a set of forms? Do they both have to have that form? (That doesn't happen; typically, only P contains template parameters)
In the introductory sentence, aren't T, TT, and i supposed to be the names of template parameters rather than template arguments?
In T[i], it appears we can deduce i, but not T (T can only be deduced in the form T[integer-constant]
What is an integer-constant supposed to be?
What is a cv-list?
Why can we not deduce const T from T? (Apparently you only get to deduce if both or neither type have a cv-list, whatever a cv-list is.)
We have extreme redundancy because, for instance, there is no way to say “in T (T::*)(T), you can deduce any of those Ts, and it's OK if some of the positions don't have a T”. So we have seven (!) forms of that construct, for all cases except the one where none of the three positions contain a T.
We have special case rules for pointers to member functions, even though they're not a special case and should be covered by the rule for pointers to members and the rule for functions.
We do not allow deducing a template template parameter's value from a template template argument — there is a TT<T> form, a TT<i> form, a template-name<T> form, and a template-name<i> form, but no TT<TT> form nor template-name<TT> form.
During the discussion of issue 2098 it was observed that multiple exceptions may share a single exception object via std::exception_ptr. It is not clear that the current wording handles that case correctly.
Consider the following example:
#include <cstdio> #include <cstdlib> void f() { struct X { ~X() { std::puts("unwound"); std::exit(0); } } x; throw 0; } int main(int argc, char**) { try { f(); } catch (int) { std::puts("caught"); } }
According to the Standard, this should print unwound and exit. Current optimizing implementations call terminate(), because:
f() obviously never unwinds (all paths out of it call exit)
Therefore the catch(int) clause is unreachable
Therefore the catch(int) clause is removed
When countering the throw 0; statement, the search for a handler finds nothing, so terminate is called without unwinding the stack.
More abstractly, before calling terminate, we're required to check whether there is an active handler for an exception of type int, and in some sense there is not (because the handler in main is dynamically unreachable).
There seem to be three possible solutions:
Change the standard to say that terminate() is a valid response to this situation [this seems problematic, as any non-returning destructor now risks program termination, but is in fact the status quo on multiple implementations and does not seem to have resulted in any bug reports]
Always fully unwind before calling terminate() [this significantly harms debugability of exceptions]
Teach the compilers to not optimize out unreachable exception handlers [for some implementations, this is “remove, redesign and reimplement middle-end support for EH”-level difficult, and harms the ability to optimize code involving catch handlers]
Although it seems to be common implementation practice to reject a macro invocation that begins in a header file and whose closing right parenthesis appears in the file that included it, there does not seem to be a prohibition of this case in the specification of function-style macros. Should this be accepted?
Notes from the February, 2014 meeting:
CWG agreed that macro invocations spanning file boundaries should be prohibited. Resolution of this issue should be coordinated with WG14.
According to 15.6 [cpp.replace] paragraph 4,
If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments (including those arguments consisting of no preprocessing tokens) in an invocation of a function-like macro shall equal the number of parameters in the macro definition.
That is, a sequence of no preprocessing tokens counts as an argument. That phrasing has problems with zero-argument function-like macros, e.g.,
#define M() M();
M is defined as having no parameters but the invocation has one (empty) argument, which does not match the number of parameters in the definition.
Stringizing a raw string literal containing a newline produces an invalid (unterminated) string literal and hence results in undefined behavior. It should be specified that a newline in a string literal is transformed to the two characters '\' 'n' in the resulting string literal.
A slightly related case involves stringizing a bare backslash character: because backslashes are only escaped within a string or character literal, a stringized bare backslash becomes "\", which is invalid and hence results in undefined behavior.
According to 15.9 [cpp.pragma] paragraph 1, the effect of a #pragma is to cause
the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner.
It should be clarified that the extent of the non-conformance is limited to the implementation-defined behavior.
A number of differences between C++03 and C++11 were omitted from C.6 [diff.cpp03]:
New keywords. Although these are said in C.6.2 [diff.cpp03.lex] only to invalidate C++03 code, they can also change the meaning, e.g., thread_local x(y), which would declare a variable thread_local initialized with y in C++03 and a thread-local variable y in C++11.
New deduction rules.
Removal of the deprecated string literal conversion.
When a friend function defined in a class template is actually defined (i.e., with each instantiation or only when odr-used).
Removal of access declarations.
Use of the injected-class-name of a class template as a template template-argument.
Additional note (January, 2012):
In addition to the items previously mentioned, access declarations were removed from C++11 but are not mentioned in C.6 [diff.cpp03].
Proposed (partial) resolution (February, 2012):
Add the following as a new section in C.6 [diff.cpp03]:
C.2.5 11.8 [class.access]: member access control pdiff.cpp03.class.access Change: Remove access declarations.
Rationale: Removal of feature deprecated since C++ 1998.
Effect on original feature: Valid C++ 2003 code that uses access declarations is ill-formed in this International Standard. Instead, using-declarations (9.9 [namespace.udecl]) can be used.
The definition of an argument does not seem to cover many assumed use cases, and we believe that is not intentional. There should be answers to questions such as: Are lambda-captures arguments? Are type names in a throw-spec arguments? “Argument” to casts, typeid, alignof, alignas, decltype and sizeof? why in x[arg] arg is not an argument, but the value forwarded to operator[]() is? Does not apply to operators as call-points not bounded by parentheses? Similar for copy initialization and conversion? What are deduced template “arguments?” what are “default arguments?” can attributes have arguments? What about concepts, requires clauses and concept_map instantiations? What about user-defined literals where parens are not used?
According to 4.1 [intro.compliance] paragraph 7,
A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries (16.4.2.5 [compliance]).
This definition links two relatively separate topics: the lack of an operating system and the minimal set of libraries. Furthermore, 6.9.3.1 [basic.start.main] paragraph 1 says:
[Note: in a freestanding environment, start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. —end note]
It would be helpful if the two characteristics (lack of an operating system and restricted set of libraries) were named separately and if these statements were clarified to identify exactly what is implementation-defined.
Notes from the October, 2009 meeting:
The CWG felt that it needed a specific proposal in a paper before attempting to resolve this issue.
Are there behavioral bounds in case an implementation limit is encountered? For example, a constant expression may fail to be recognized as such due to an implementation limit (e.g. a recursion or step limit), causing overload resolution to pick a different overload. Is that acceptable?
Possible resolution:
Change in 4.1.1 [intro.compliance.general] paragraph 2 as follows:
Although this document states only requirements on C++ implementations, those requirements are often easier to understand if they are phrased as requirements on programs, parts of programs, or execution of programs as specified in the rules in Clause 5 [lex] through Clause 33 [thread] and Annex D. Such requirements have the following meaning:
If a program contains a violation of a rule for which no diagnostic is required, this document places no requirement on implementations with respect to that program.
Otherwise, if an implementation limit (Annex B) is reached during translation, an implementation shall reject the program.
Recommended practice: An implementation should document limits where they are known, and should issue a diagnostic in cases where it can reasonably detect that the implementation limit is reached.
If a program contains no violations of the rules in Clause 5 [lex] through Clause 33 [thread] and Annex D, a conforming implementation shall, within its resource limits as described in Annex B, accept and correctly execute [ Footnote: ... ] that program.If a program contains a violation of a rule for which no diagnostic is required, this document places no requirement on implementations with respect to that program.- Otherwise, if
athat program contains a violation of any diagnosable rule or an occurrence of a construct described in this document as “conditionally-supported” when the implementation does not support that construct, a conforming implementation shall issue at least one diagnostic message.- Otherwise, a conforming implementation shall accept and correctly execute that program. [ Note: "Correct execution" can include undefined behavior, depending on the data being processed; see Clause 3 [intro.defs] and 4.1.2 [intro.abstract]. -- end note ]
Add a new paragraph after 4.1.2 [intro.abstract] paragraph 6 as follows:
[Note 2 : More stringent correspondences between abstract and actual semantics can be defined by each implementation. -- end note]
If an implementation limit (Annex B) is reached, the behavior is undefined.
Recommended practice: An implementation should document limits where they are known and abort the execution of the program in cases where it can reasonably detect that the implementation limit is reached.
Change in 7.7 [expr.const] paragraph 5 as follows:
- ...
an expression that would exceed the implementation-defined limits (see Annex B);- an operation that would have undefined behavior as specified in Clause 4 [intro] through Clause 15 [cpp], excluding 9.12.3 [dcl.attr.assume] and exceeding an implementation limit (4.1.2 [intro.abstract]); [ Footnote: ... ]
[ Example:constexpr bool f(int x) { return (x == 0) || f(x-1); } const bool g = f(60000); // well-formed static initialization (6.9.3.2 [basic.start.static]); // program can be rejected because implementation limit is reached-- end example ]- ...
Change in 13.9.2 [temp.inst] paragraph 16 as follows:
There is an implementation-defined quantity that specifies the limit on the total depth of recursive instantiations (Annex B), which could involve more than one template.The result of an infinite recursion in instantiation is undefined.
CWG 2023-08-25
For the example in change 3, implementations currently switch to dynamic initialization when their implementation limits related to constant evaluation are exceeded. This might be surprising to users, because dynamic initialization can exhibit order-of-initialization issues. It might be less harmful to reject such situations as ill-formed. Furthermore, it is unclear whether implementations can reliably determine that an implementation limit is reached, before crashing.
CWG is requesting EWG guidance via paper issue #1631.
EWG 2023-11-07
EWG is soliciting a paper to make this either ill-formed, no diagnostic required, or well-formed.
There are no restrictions on the implementation's choice of ordinary literal encoding. However, there is an implicit assumption that a code unit value must fit into a char.
Tangentially related to that, "cannot be encoded as a single code unit" could be interpreted as referring to the values of the code units as opposed to the fact that multiple code units might be needed.
Possible resolution:
Change in 5.3 [lex.charset] paragraph 8 as follows and add to the index of implementation-defined behavior:
A code unit is an integer value of character type (6.8.2 [basic.fundamental]). Characters in a character-literal other than a multicharacteror non-encodable characterliteral or in a string-literal are encoded as a sequence of one or more code units, as determined by the encoding-prefix (5.13.3 [lex.ccon], 5.13.5 [lex.string]); this is termed the respective literal encoding. The ordinary literal encoding is the implementation-defined encoding applied to an ordinary character or string literal; its code units are of type unsigned char. The wide literal encoding is the implementation-defined encoding applied to a wide character or string literal; its code units are of type wchar_t.
Change in 5.13.3 [lex.ccon] bullet 3.1 as follows:
- A character-literal with a c-char-sequence consisting of a single basic-c-char , simple-escape-sequence, or universal-character-name is the code unit value of the specified character as encoded in the literal's associated character encoding. If the specified character lacks representation in the literal's associated character encoding or if it
cannot be encoded as a single code unitis encoded with multiple code units, then the program is ill-formed.- ...
Consider:
int main() { constexpr auto x = 3.14f; assert( x == 3.14f ); // can fail? static_assert( x == 3.14f ); // can fail? }
Can a conforming implementation represent a floating-point literal with excess precision, causing the comparisons to fail?
Subclause 5.13.4 [lex.fcon] paragraph 3 specifies:
If the scaled value is not in the range of representable values for its type, the program is ill-formed. Otherwise, the value of a floating-point-literal is the scaled value if representable, else the larger or smaller representable value nearest the scaled value, chosen in an implementation-defined manner.
This phrasing leaves little leeway for excess precision. In contrast, C23 (WG14 N3096) specifies in section 6.4.4.2 paragraph 6:
The values of floating constants may be represented in greater range and precision than that required by the type (determined by the suffix); the types are not changed thereby. ...
Subclause 7.1 [expr.pre] paragraph 6 allows excess precision for floating-point computations (including their operands):
The values of the floating-point operands and the results of floating-point expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby. [ Footnote: The cast and assignment operators must still perform their specific conversions as described in 7.6.1.4 [expr.type.conv], 7.6.3 [expr.cast], 7.6.1.9 [expr.static.cast] and 7.6.19 [expr.ass]. -- end footnote ]
Taken together, that means that 314.f / 100.f can be computed and represented more precisely than 3.14f, which is hard to justify. The footnote appears to imply that (float)3.14f is required to yield a value with float precision, but that conversion (eventually) ends up at 9.4.1 [dcl.init.general] bullet 16.9:
- ...
- Otherwise, the initial value of the object being initialized is the (possibly converted) value of the initializer expression. ...
This phrasing leaves no permission to discard excess precision when converting from a float value to type float ("... is the value...").
However, if initialization is intended to drop excess precision, then an overloaded operator returning float can never behave like a built-in operation with excess precision, because returning a value means initializing the return value.
The C++ standard library inherits the FLT_EVAL_METHOD macro from the C standard library. C23 (WG14 N3096) specifies it as follows in section 5.2.4.2.2:
0 evaluate all operations and constants just to the range and precision of the type; 1 evaluate operations and constants of type float and double to the range and precision of the double type, evaluate long double operations and constants to the range and precision of the long double type; 2 evaluate all operations and constants to the range and precision of the long double type.
Taken together, a conforming C++ implementation cannot define FLT_EVAL_METHOD to 1 or 2, because literals (= "constants") cannot be represented with excess precision in C++.
Additional notes (June, 2023)
Forwarded to EWG via cplusplus/papers#1584, by decision of the CWG chair.
Prior to the resolution of issue 1823 (incorporated into C++17), the specification required in 9.2.3 [dcl.fct.spec] paragraph 4:
A string literal in the body of an extern inline function is the same object in different translation units.
That was changed to the following specification in 5.13.5 [lex.string] paragraph 9:
Evaluating a string-literal results in a string literal object with static storage duration (6.7.5 [basic.stc]). ... whether successive evaluations of a string-literal yield the same or a different object is unspecified.
The treatment of string literal objects was harmonized with backing arrays in P2752R3 (approved in June, 2023) and issue 2753.
The rationale for the change made by issue 1823 was deviating implementation practice: Implementations do not unify string literal objects in inline functions or template instantiations across different translation units.
However, that was a silent behavior change for examples such as:
const char *f() { return "abc"; } bool b = f() == f(); // guaranteed true prior to CWG1823, now unspecified
The matter should be revisited in light of this new information.
The decimal-literal in a user-defined-integer-literal might be too large for an unsigned long long to represent (in implementations with extended integer types). In such cases, the original intent appears to have been to call a raw literal operator or a literal operator template; however, the existing wording of 5.13.9 [lex.ext] paragraph 3 always calls the unsigned long long literal operator if it exists, regardless of the value of the decimal-literal.
Consider the following complete program:
void f(); template<typename T> void g() { f(); } int main() { }
Must f() be defined to make this program well-formed? The current wording of 6.3 [basic.def.odr] does not make any special provision for expressions that appear only in uninstantiated template definitions.
(See also issue 1254.)(From submission #402.)
Consider the following tokens appearing in multiple translation units:
inline void f() { int a; int b = a; }
Does f violate the one-definition rule?
We have this check in 6.3 [basic.def.odr] paragraph 14.5:
- ...
- In each such definition, corresponding names, looked up according to 6.5 [basic.lookup], shall refer to the same entity, ...
- ...
Does a refer to the same entity in the several definitions of f? Subclause 6.3 [basic.def.odr] paragraph 16 specifies:
These requirements also apply to corresponding entities defined within each definition of D (including the closure types of lambda-expressions, but excluding entities defined within default arguments or default template arguments of either D or an entity not defined within D). For each such entity and for D itself, the behavior is as if there is a single entity with a single definition, including in the application of these requirements to other entities.
Thus, the requirements apply recursively to the definitions of a, but it is unclear whether the conclusion "as if there is a single entity with a single definition" is reached at each level of the recursion separately.
Suggested resolution:
Change in 6.3 [basic.def.odr] paragraph 16 as follows:
If these requirements are satisfied, the behavior for D is as if there is a single entity with a single definition, including in the application of these requirements to other entities.These requirementsThis behavior alsoapplyapplies to corresponding entities defined within each definition of D (including the closure types of lambda-expressions, but excluding entities defined within default arguments or default template arguments of either D or an entity not defined within D).For each such entity and for D itself, the behavior is as if there is a single entity with a single definition, including in the application of these requirements to other entities.
Consider:
inline auto lambda = []{}; // same in different translation units or not?
This can be observed, for example, through the following variable template:
template<class T> int v;
Is &v<decltype(lambda)> the same address in every translation unit?
Possible resolution:
Change in 6.3 [basic.def.odr] paragraph 14 as follows:
For any definable item D with definitions in multiple translation units,the program is ill-formed; a diagnostic is required only if the definable item is attached to a named module and a prior definition is reachable at the point where a later definition occurs. Given such an item, for all definitions of D, or, if D is an unnamed enumeration, for all definitions of D that are reachable at any given program point, the following requirements shall be satisfied, where the definition of a closure type is considered to consist of the sequence of tokens of the corresponding lambda-expression.
- if D is a non-inline non-templated function or variable, or
- if the definitions in different translation units do not satisfy the following requirements,
- ...
- Each such definition shall consist of the same sequence of tokens
, where the definition of a closure type is considered to consist of the sequence of tokens of the corresponding lambda-expression.- ...
Add another example after example 6, immediately before 6.3 [basic.def.odr] paragraph 18:
[ Example:inline decltype([]{}) v1; inline auto v2 = []{};If the definition of v1 appears in multiple translation units, the program is ill-formed, no diagnostic required, because each definition declares v1 to have a different type. If the definition of v2 appears in multiple translation units, the behavior of the program is as if there is only one definition, and only a definition can supply an initializer; therefore the behavior is as if there is only one initializer. Therefore, v2 has the same type in every translation unit. -- end example]
If, at any point in the program, there is more than one reachable unnamed enumeration definition in the same scope...
(From submission #561.)
Consider:
bool f() { constexpr int z = 42; return requires { sizeof(int [*&z]); } && requires (int x) { sizeof(int [*&z]); }; }
The second requires-expression introduces a function parameter scope according to 6.4.4 [basic.scope.param]. This affects odr-usability as specified in 6.3 [basic.def.odr] paragraph 10, but the two requires-expression in the example ought to actually behave the same.
Suggested resolution:
Change in 6.3 [basic.def.odr] bullet 10.2.2 as follows:
A local entity (6.1 [basic.pre]) is odr-usable in a scope (6.4.1 [basic.scope.scope]) if:
- ...
- for each intervening scope (6.4.1 [basic.scope.scope]) between the point at which the entity is introduced and the scope (where *this is considered to be introduced within the innermost enclosing class or non-lambda function definition scope), either:
- the intervening scope is a block scope, or
- the intervening scope is the function parameter scope of a requires-expression or a lambda-expression that has a simple-capture naming the entity or has a capture-default, and the block scope of the lambda-expression is also an intervening scope.
According to 6.4.1 [basic.scope.scope] paragraph 3,
Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless
...
each declares a function or function template, except when
both declare functions with the same parameter-type-list,21 equivalent (13.7.7.2 [temp.over.link]) trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]), and, if both are non-static members, the same cv-qualifiers (if any) and ref-qualifier (if both have one), or
...
This would indicate that a virtual function (which cannot have a trailing requires-clause, per 11.7.3 [class.virtual] paragraph 6) can be overloaded with a non-virtual member function with the same parameter type list but with a trailing requires-clause. However, this is not implementable on some ABIs, since the mangling of the two functions would be the same. For example:
#include <type_traits> template<class T> struct Foo { virtual void fun() const {} void fun() const requires std::is_object_v<T> {} }; int main() { Foo<int>{}.fun(); }
Should such overloading be ill-formed or conditionally-supported, or should the current rules be kept?
Rationale (August, 2021):
CWG felt that the current rules are correct; it simply means that only the virtual function can be called, and all other references are simply ambiguous. (See also issue 2501 for a related question dealing with explicit instantiation.
Notes from the November, 2021 teleconference:
The issue has been reopened in response to additional discussion.
CWG 2022-11-11
This is related to issue 2501. CWG solicits a paper to address this issue.
Consider:
struct S { void f() &; }; void S::f(this S&) {}
Both declarations of S::f correspond (6.4.1 [basic.scope.scope] paragraph 4), but are ill-formed according to 6.6 [basic.link] paragraph 1, because the functions are not of the same type:
For any two declarations of an entity E:
- If one declares E to be a variable or function, the other shall declare E as one of the same type.
- ...
A similar situation arises for the following example:
struct S { void g() &; }; void S::g() { }
Possible resolution:
Add a note in 6.4.1 [basic.scope.scope] paragraph 4 as follows:
[ Note: Two function declarations with different ref-qualifiers or parameter-type-lists do not have the same type even if they correspond (6.6 [basic.link]). -- end note ]
[ Example 2:typedef int Int; ... void m(); void n(); }; X::m() & {} // error: redeclaration of X::m with a different type X::n(this X&) {} // error: redeclaration of X::n with a different type
The wording for name-independent declarations applied by P2169R4 (A nice placeholder with no name) apparently fails to implement the intent.
Consider:
void foo () { auto c = [_ = 2, _ = 3] () {}; } // duplicate identifier in lambda-capture struct S { int _; int _; }; // repeated member declaration void f() { if (int _ = 5) { // #1 int _ = 6; // #2 int k = _; // ambiguous lookup despite #1 and #2 in different scopes? } else { int _ = 7; } }
Possible resolution (first two issues):
Change in 7.5.6.3 [expr.prim.lambda.capture] paragraph 2 as follows:
... A capture name is an identifier or this. Ignoring appearances in initializers of init-captures,an identifier or this shall not appear more than oncethe program is ill formed if the appearance of a capture name precedes another appearance of the same capture name in a lambda-capture, and the latter is not a name-independent declaration (6.4.1 [basic.scope.scope]). [ Example 1:struct S2 { void f(int i); }; void S2::f(int i) { [&, i]{ }; // OK [&, this, i]{ }; // OK, equivalent to [&, i] [&, &i]{ }; // error: i preceded by & when & is the default [=, *this]{ }; // OK [=, this]{ }; // OK, equivalent to [=] [i, i]{ }; // error: i repeated int _ = 17; [_, _ = 42]{ }; // OK [this, *this]{ }; // error: this appears twice }--end example ]
Change in 11.4.1 [class.mem.general] paragraph 5 as follows:
A member shall not be declared twice in the member-specification, except that[Note 3: A single name can denote several member functions provided their types are sufficiently different (6.4.1 [basic.scope.scope]). Repeated name-independent declarations declare different members. —end note]
- a nested class or member class template can be declared and then later defined, and
- an enumeration can be introduced with an opaque-enum-declaration and later redeclared with an enum-specifier.
CWG 2023-12-15
Having a lookup ambiguity for the third case (int k = _;) seems consistent with the idea that #1 and #2 are considered to be in the same scope as far as name conflict detection is concerned. However, that would need new rules, given that name lookup does not proceed to outer scopes when a name is found in an inner scope. Similar lookup ambiguities across nested scopes can arise among lambda captures, template parameters of a lambda, lambda function parameters, and declartions in the outermost block scope of a lambda.
(From submission #475.)
Consider:
void foo () { auto f = [i = 5] () { int i; return 0; }; }
Before P2579R0 and P2036R3, this was ill-formed, because the capture inhabited the same scope as int i. Subclause 6.4.3 [basic.scope.block] paragraph 2 relies on the "parent" scope:
If a declaration that is not a name-independent declaration and whose target scope is the block scope S of apotentially conflicts with a declaration whose target scope is the parent scope of S, the program is ill-formed.
- compound-statement of a lambda-expression, function-body, or function-try-block,
- substatement of a selection or iteration statement that is not itself a selection or iteration statement, or
- handler of a function-try-block
However, after P2579R0, the init-capture inhabits the lambda scope (6.4.5 [basic.scope.lambda]), which is the parent of the parameter scope (6.4.4 [basic.scope.param]), which is the parent of the compound-statement scope, thus the provision does not apply. For a generic lambda, there's a template parameter scope (6.4.9 [basic.scope.temp]) in between, too:
auto f = [i = 5] <int N> () { int i; return 0; };
Possible resolution:
Change in 6.4.3 [basic.scope.block] paragraph 2 as follows, adding bullets:
If a declaration that is not a name-independent declaration and whose target scope is the block scope S of apotentially conflicts with a declaration whose target scope is
- compound-statement of a lambda-expression, function-body, or function-try-block,
- substatement of a selection or iteration statement that is not itself a selection or iteration statement, or
- handler of a function-try-block
the program is ill-formed.
- the parent scope of S or,
- if S is the block scope of a compound-statement of a lambda-expression, the nearest enclosing lambda scope of S,
The term "ambiguous base class" doesn't seem to be actually defined anywhere. 6.5.2 [class.member.lookup] paragraph 7 seems like the place to do it.
In an example like
template<typename T> void f(T p)->decltype(p.T::x);
The nested-name-specifier T:: looks like it refers to the template parameter. However, if this is instantiated with a type like
struct T { int x; }; struct S: T { };
the reference will be ambiguous, since it is looked up in both the context of the expression, finding the template parameter, and in the class, finding the base class injected-class-name, and this could be a deduction failure. As a result, the same declaration with a different parameter name
template<typename U> void f(U p)->decltype(p.U::x);
is, in fact, not a redeclaration because the two can be distinguished by SFINAE.
It would be better to add a new lookup rule that says that if a name in a template definition resolves to a template parameter, that name is not subject to further lookup at instantiation time.
Additional note (November, 2020):
Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.
CWG 2023-12-01
Per the status quo rules, T::x is a dependent name and thus no lookup occurs at template definition time. At template instantiation time, T is first looked up in the class of p and, if not found, the template parameter T is found. This, in turn, implies that the two templates discussed above are not equivalent, because the first potentially references a T member, the other a U member of p.
Subclause 13.7.7.2 [temp.over.link] paragraph 5 is not in conflict with this interpretation, because no lookup occurs for the T token inside the decltype at template definition time at all, thus (reliably) "name a template parameter" is not satisfied at that point.
Two expressions involving template parameters are considered equivalent if two function definitions containing the expressions would satisfy the one-definition rule (6.3 [basic.def.odr]), except that the tokens used to name the template parameters may differ as long as a token used to name a template parameter in one expression is replaced by another token that names the same template parameter in the other expression. ...
In order to clarify this, suggestions for a note including the example from this issue are welcome.
Subclause 6.6 [basic.link] paragraph 1 specifies:
A program consists of one or more translation units (5.1 [lex.separate]) linked together. A translation unit consists of a sequence of declarations.
Subclause 5.1 [lex.separate] paragraph 1 defines "translation unit":
A source file together with all the headers (16.4.2.3 [headers]) and source files included (15.3 [cpp.include]) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (15.2 [cpp.cond]) preprocessing directives, is called a translation unit.
Subclause 5.2 [lex.phases] paragraph 7 first mentions the "translation" of translation units:
Whitespace characters separating tokens are no longer significant. Each preprocessing token is converted into a token (5.6 [lex.token]). The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.
However, 5.2 [lex.phases] paragraph 8 introduces "instantiation units" and 5.2 [lex.phases] paragraph 9 specifies:
Translated translation units and instantiation units are combined as follows:
and 5.2 [lex.phases] paragraph 9 specifies:
All external entity references are resolved. Library components are linked to satisfy external references to entities not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment.
The term "linking" in 5.2 [lex.phases] comes after translated translation units and instantiation units are combined, yet 6.6 [basic.link] paragraph 1 states that (untranslated) translation units are linked to form a program. That seems inconsistent.
Additional notes (February, 2023)
See also issue 2518, which introduced the term "preprocessing translation unit" to remove the conflict between 6.6 [basic.link] paragraph 1 and 5.1 [lex.separate] paragraph 1.
Consider:
struct A { int a; }; struct B { int b; }; auto&& [x] = A{}; //#1 auto&& [x] = B{}; //#2
A rule is missing to make such repeated structured binding declarations ill-formed.
Suggested resolution:
Change in 6.6 [basic.link] paragraph 11 as follows:
For any two declarations of an entity E:
- If one declares E to be a variable or function, the other shall declare E as one of the same type.
- If one declares E to be an enumerator, the other shall do so.
- If one declares E to be a structured binding, the program is ill-formed.
- ...
According to 6.7.1 [intro.memory] paragraph 3,
A memory location is either an object of scalar type or a maximal sequence of adjacent bit-fields all having non-zero width. [Note: Various features of the language, such as references and virtual functions, might involve additional memory locations that are not accessible to programs but are managed by the implementation. —end note] Two or more threads of execution (6.9.2 [intro.multithread]) can update and access separate memory locations without interfering with each other.
It is not clear how this relates to the permission granted in 11.4 [class.mem] paragraph 18 to inspect the common initial sequence of standard-layout structs that are members of a standard-layout union. If one thread is writing to the common initial sequence and another is reading from it via a different struct, that should constitute a data race, but the current wording does not clearly state that.
The list of ways that an object may be created in 6.7.2 [intro.object] paragraph 1 does not include creation of type_info objects by typeid expressions, but 7.6.1.8 [expr.typeid] does not appear to require that such objects exist before they are referenced. Should the list in 6.7.2 [intro.object] be extended to include this case?
(From thread beginning here.)
Consider:
#include <new>
struct A { unsigned char buf[1]; };
static_assert(sizeof(A) == 1); // A can fit within A::buf
int main()
{
A x{};
new (x.buf) A{};
}
A::buf provides storage for another A object. Thus, there are now two objects of type A within lifetime, which is inconsistent with the goal expressed by 6.7.2 [intro.object] paragraph 9.
Suggested resolution:
Change in 6.7.2 [intro.object] paragraph 3 as follows:
If a complete object of type T is created (7.6.2.8 [expr.new]) in storage associated with another object e of type “array of N unsigned char” or of type “array of N std::byte” (17.2.1 [cstddef.syn]), that array provides storage for the created object if:
- the lifetime of e has begun and not ended, and
- the storage for the new object fits entirely within e, and
- e is not and is not nested within an object of type similar (7.3.6 [conv.qual]) to T that is within its lifetime, and
- there is no array object that satisfies these constraints nested within e.
The (arguably) expanded treatment of backing arrays and string literals as potentially non-unique objects in issue 2753 lead to the question how the resulting address comparisons are treated during constant evaluation.
Subclause 7.7 [expr.const] bullet 5.24 specifies:
An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine (6.9.1 [intro.execution]), would evaluate one of the following:
- ...
- a three-way comparison (7.6.8 [expr.spaceship]), relational (7.6.9 [expr.rel]), or equality (7.6.10 [expr.eq]) operator where the result is unspecified;
- ...
This phrasing is understood to refer to explicitly unspecified outcomes only. The treatment of an example such as
constexpr bool b = "abc" == "abc";
is unclear, given that identical string literals may or may not yield distinct string literal objects.
The assumption that equality comparison of std::string_view would compare addresses as a short-cut before comparing the character sequence could not be confirmed (23.2.2 [char.traits.require], 23.3.3.8 [string.view.ops] paragraph 12).
CWG in Tokyo 2024-03-22
Different approaches are feasible:
In the latter cases, tag values can be preserved when performing pointer arithmetic.
Consider
extern "C" int printf (const char *,...); struct Base { Base();}; struct Derived: virtual public Base { Derived() {;} }; Derived d; extern Derived& obj = d; int i; Base::Base() { if ((Base *) &obj) i = 4; printf ("i=%d\n", i); } int main() { return 0; }
11.9.5 [class.cdtor] paragraph 2 makes this valid, but 6.7.3 [basic.life] paragraph 5 implies that it isn't valid.
Steve Adamczyk: A second issue:
extern "C" int printf(const char *,...); struct A { virtual ~A(); int x; }; struct B : public virtual A { }; struct C : public B { C(int); }; struct D : public C { D(); }; int main() { D t; printf("passed\n");return 0; } A::~A() {} C::C(int) {} D::D() : C(this->x) {}
Core issue 52 almost, but not quite, says that in evaluating "this->x" you do a cast to the virtual base class A, which would be an error according to 11.9.5 [class.cdtor] paragraph 2 because the base class B constructor hasn't started yet. 7.6.1.5 [expr.ref] should be clarified to say that the cast does need to get done.
James Kanze submitted the same issue via comp.std.c++ on 11 July 2003:
Richard Smith: Nonsense. You can use "this" perfectly happily in a constructor, just be careful that (a) you're not using any members that are not fully initialised, and (b) if you're calling virtual functions you know exactly what you're doing.
In practice, and I think in intent, you are right. However, the standard makes some pretty stringent restrictions in 6.7.3 [basic.life]. To start with, it says (in paragraph 1):
The lifetime of an object is a runtime property of the object. The lifetime of an object of type T begins when:(Emphasis added.) Then when we get down to paragraph 5, it says:The lifetime of an object of type T ends when:
- storage with the proper alignment and size for type T is obtained, and
- if T is a class type with a non-trivial constructor, the constructor calls has COMPLETED.
- if T is a class type with a non-trivial destructor, the destructor call STARTS, or
- the storage which the object occupies is reused or released.
Before the lifetime of an object has started but after the storage which the object will occupy has been allocated [which sounds to me like it would include in the constructor, given the text above] or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that refers to the storage location where the object will be or was located may be used but only in limited ways. [...] If the object will be or was of a non-POD class type, the program has undefined behavior if:
[...]
- the pointer is implicitly converted to a pointer to a base class type, or [...]
I can't find any exceptions for the this pointer.
Note that calling a non-static function in the base class, or even constructing the base class in initializer list, involves an implicit conversion of this to a pointer to the base class. Thus undefined behavior. I'm sure that this wasn't the intent, but it would seem to be what this paragraph is saying.
What happens if the storage for an object is deallocated in its period of destruction? Consider:
struct Base { virtual ~Base() { ::operator delete(this); } void operator delete(void*) {} }; struct Derived : Base {}; int main() { delete new Derived; }
This ought to be undefined behavior, but the standard is silent on the matter.
Notes from the December, 2016 teleconference:
The consensus view was that this should be undefined behavior.
Additional notes (July, 2023)
This issue is a subset of issue 2757.
The change in C++20 for RU007 allows transparently replacing a const subobject whose complete object is not const, and the new object can be non-const. However, if the reuse of the object has not happened, modifying such subobjects is still undefined behavior.
This restriction causes problems in the implementation of std::map and std::unordered_map; see this bug report. Here, the key_type objects in map containers are const, and implementations generally can't replace these objects after construction.
Perhaps these restrictions can be relaxed to assist in this case: if
the complete object of a key_type subobject in a container is not const (as the mapped_type subobject is not const), or
the complete object has dynamic storage duration
a const subobject could be modified.
(Is it meaningful to allow a new-expression like new const int(42) to create cv-qualified objects? Perhaps such objects should be unqualified, while maintaining the cv-qualification in the type of the expression?)
Notes from the November, 2022 meeting
The advice of SG12 is solicited; see cplusplus/papers#1395.
Base subobjects cannot be transparently replaced with complete objects, as specified in 6.7.3 [basic.life] bullet 8.4:
An object o1 is transparently replaceable by an object o2 if:
- ...
- neither o1 nor o2 is a potentially-overlapping subobject (6.7.2 [intro.object]), and
- ...
However, that bullet is over-reaching, because it disallows:
struct A { int n; }; struct B : A {}; B b; new (&b) B { {5} }; // New A base class does not transparently replace existing A base class due to /8.4. int k = b.n; // UB: member n of A base class is outside its lifetime
See issue 2677 for a suggested resolution.
Additional notes (February, 2023)
Consider this example:
struct A { int n; char c; // tail padding }; struct B { [[no_unique_address]] A a; char in_tail_padding[3]; }; B b; void f() { // Transparently replaces old member, potentially overwriting the data in the tail padding! new (&b.a) A{}; }
The suggestions do not address this example.
There are several problems with 6.7.5 [basic.stc]:
6.7.5 [basic.stc] paragraph 2 says that "Static and automatic storage durations are associated with objects introduced by declarations (6.2 [basic.def]) and implicitly created by the implementation (6.7.7 [class.temporary])."
In fact, objects "implicitly created by the implementation" are the temporaries described in (6.7.7 [class.temporary]), and have neither static nor automatic storage duration, but a totally different duration, described in 6.7.7 [class.temporary].
6.7.5 [basic.stc] uses the expression "local object" in several places, without ever defining it. Presumably, what is meant is "an object declared at block scope", but this should be said explicitly.
In a recent discussion in comp.lang.c++.moderated, on poster interpreted "local objects" as including temporaries. This would require them to live until the end of the block in which they are created, which contradicts 6.7.7 [class.temporary]. If temporaries are covered by this section, and the statement in 6.7.5 [basic.stc] seems to suggest, and they aren't local objects, then they must have static storage duration, which isn't right either.
I propose adding a fourth storage duration to the list after 6.7.5 [basic.stc] paragraph 1:
And rewriting the second paragraph of this section as follows:
Temporary storage duration is associated with objects implicitly created by the implementation, and is described in 6.7.7 [class.temporary]. Static and automatic storage durations are associated with objects defined by declarations; in the following, an object defined by a declaration with block scope is a local object. The dynamic storage duration is associated with objects created by the operator new.
Steve Adamczyk: There may well be an issue here, but one should bear in mind the difference between storage duration and object lifetime. As far as I can see, there is no particular problem with temporaries having automatic or static storage duration, as appropriate. The point of 6.7.7 [class.temporary] is that they have an unusual object lifetime.
Notes from Ocrober 2002 meeting:
It might be desirable to shorten the storage duration of temporaries to allow reuse of them. The as-if rule allows some reuse, but such reuse requires analysis, including noting whether the addresses of such temporaries have been taken.
Notes from the August, 2011 meeting:
The CWG decided that further consideration of this issue would be deferred until someone produces a paper explaining the need for action and proposing specific changes.
See also issue 1634.
Requirements for allocation functions are given in 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 1:
An allocation function can be a function template. Such a template shall declare its return type and first parameter as specified above (that is, template parameter types shall not be used in the return type and first parameter type). Template allocation functions shall have two or more parameters.
There are a couple of problems with this description. First, it is instances of function templates that can be allocation functions, not the templates themselves (cf 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2, which uses the correct terminology regarding deallocation functions).
More importantly, this specification was written before template metaprogramming was understood and hence prevents use of SFINAE on the return type or parameter type to select among function template specializations. (The parallel passage for deallocation functions in 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2 shares this deficit.)
(See also issue 1628.)
When an object is deleted, 6.7.5.5.3 [basic.stc.dynamic.deallocation] says that the deallocation “[renders] invalid all pointers referring to any part of the deallocated storage.” According to 6.8.4 [basic.compound] paragraph 3, a pointer whose address is one past the end of an array is considered to point to an unrelated object that happens to reside at that address. Does this need to be clarified to specify that the one-past-the-end pointer of an array is not invalidated by deleting the following object? (See also 7.6.2.9 [expr.delete] paragraph 4, which also mentions that the system deallocation function renders a pointer invalid.)
(From submission #412.)
Consider:
struct S { long double d; };
There appears to be no rule that prevents S from having an extended alignment. (See also WG14 DR445.)
Possible resolution:
Change in 6.7.6 [basic.align] paragraph 2 as follows:
A fundamental alignment is represented by an alignment less than or equal tothe greatest alignment supported by the implementation in all contexts, which is equal toalignof(std::max_align_t) (17.2 [support.types]). Fundamental alignments shall be supported by the implementation for objects of all storage durations (6.7.5 [basic.stc]). The alignment required for a type may be different when it is used as the type of a complete object and when it is used as the type of a subobject. ...
Change in 6.7.6 [basic.align] paragraph 3 as follows:
An extended alignment is represented by an alignment greater than alignof(std::max_align_t). It is implementation-defined whether any extended alignments are supported and thecontexts instorage durations for which they are supported (9.12.2 [dcl.align]). A type having an extended alignment requirement is an over-aligned type. [Note 1: Every over-aligned type is or contains a class type to which extended alignment applies (possibly through a non-static data member). —end note] [Note 2: The strictest supported alignment can be different for objects with different storage durations. —end note]
Insert a new paragraph before 6.7.6 [basic.align] paragraph 4 and change as follows:
Each of the following types has a fundamental alignment requirement:
- a scalar type (6.8.1 [basic.types.general]),
- an array type (9.3.4.5 [dcl.array]) whose element type has a fundamental alignment requirement, and
- a class type (Clause 11 [class]) where all base classes and non-static data members have a type with a fundamental alignment requirement and no non-static data member is declared with an alignment-specifier (9.12.2 [dcl.align]) specifying an extended alignment.
Alignments are represented as values of the type std::size_t. Valid alignments include only
those values returned by an alignof expression for thefundamentaltypesalignments plus an additional implementation-defined set of values, which may be empty. Every alignment value shall be a non-negative integral power of two.
Change in 9.12.2 [dcl.align] paragraph 2 as follows:
When the alignment-specifier is of the form alignas( constant-expression ):
- the constant-expression shall be an integral constant expression and
- if the constant expression does not evaluate to an alignment value (6.7.6 [basic.align]), or evaluates to an extended alignment and the implementation does not support that alignment
in the context of the declarationfor the storage duration (if any) of the entity being declared, the program is ill-formed.
Consider:
struct S { S(int) {} }; const S& r1 = 0; // #1 const S& r2 = static_cast<const S&>(S(0)); // #2 const S& r3 = static_cast<const S&>(0); // #3 // Are r1, r2, and r3 dangling past this point?
For #1, a temporary object is materialized and the reference binds to the result of the materialization conversion. This results in lifetime extension per 6.7.7 [class.temporary] bullet 6.1.
For #2, 6.7.7 [class.temporary] bullet 6.6 requires the cast to convert "without a user-defined conversion", but S(0) is such.
For #3, a user-defined conversion is clearly required.
However, implementations treat all three cases the same and do extend the lifetime of the temporary.
Suggested resolution:
Change in 6.7.7 [class.temporary] bullet 6.6 as follows:
- ...
- a
- const_cast (7.6.1.11 [expr.const.cast]),
- static_cast (7.6.1.9 [expr.static.cast]),
- dynamic_cast (7.6.1.7 [expr.dynamic.cast]), or
- reinterpret_cast (7.6.1.10 [expr.reinterpret.cast])
converting, without a user-defined conversion, a glvalue operand that is one of these expressions to a glvalue that refers to the object designated by the operand, or to its complete object or a subobject thereof, where
- the cast does not invoke a user-defined conversion, or
- the cast invokes a user-defined conversion that yields a prvalue, and the glvalue result of the cast is bound directly (9.4.4 [dcl.init.ref]) to that prvalue,
- ...
(From editorial issue 6646.)
Both 7.3.1 [conv.general] and 7.6.1.9 [expr.static.cast] introduce "invented temporary variables", but it is unclear whether the corresponding objects are temporary objects with lifetimes as specified in 6.7.7 [class.temporary].
Possible resolution:
Change in 6.7.7 [class.temporary] as follows:
Change in 7.3.1 [conv.general] paragraph 3 and paragraph 4 as follows, moving the declarations into a code block:
An expression E can be implicitly converted to a type T if and only if the declaration
T t = E;is well-formed, for some invented temporary variable t (6.7.7 [class.temporary], 9.4 [dcl.init]).Certain language constructs require that an expression be converted to a Boolean value. An expression E appearing in such a context is said to be contextually converted to bool and is well-formed if and only if the declaration
bool t(E);is well-formed, for some invented temporary variable t (6.7.7 [class.temporary], 9.4 [dcl.init]).
Change in 7.6.1.9 [expr.static.cast] as follows:
... If T is a reference type, the effect is the same as performing the declaration and initializationT t(E);for some invented temporary variable t (6.7.7 [class.temporary], 9.4 [dcl.init]) and then using the temporary variable as the result of the conversion. ...
Consider:
struct A { A *children; long long arr[100]; A() : children() {} A(int) : children(this) {} }; __attribute__((noinline)) A Foo() { return A(0); // #1 } A x[3] = {}; void Bar(int n) { A a = Foo(); for (int i = 0; i < n; i++) { a.children[i].children = x; // #2 } } int main() { Bar(3); return x[0].children || !x[1].children || !x[2].children; }
It ought to be valid to hoist the load of a.children out of the loop. However, guaranteed copy elision is required to apply to #1, making the A objects in Foo and Bar be the same object. An additional temporary copy can be made per 6.7.7 [class.temporary] paragraph 3, but that does not cause the children member of the a variable to become invalid.
Suggested resolution:
Change in 6.7.7 [class.temporary] paragraph 3 as follows:
When an object of class type X is passed to or returned from a function, if X has at least one eligible copy or move constructor (11.4.4 [special]), each such constructor is trivial, and the destructor of X is either trivial or deleted, implementations are permitted to create a temporary object to hold the function parameter or result object. The temporary object is constructed from the function argument or return value, respectively, and the function's parameter or return object is initialized as if by using the eligible trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object). The temporary object may occupy the storage that the parameter or return object is going to occupy, but pointers that pointed to the temporary object become invalid pointer values (6.8.4 [basic.compound]) when the parameter or return object is constructed and do not point to the parameter or return object.
CWG 2024-05-03
For return values, CWG favors replacing the exception in 6.7.7 [class.temporary] paragraph 3 with an amendment to the specification of guaranteed copy elision in 9.4.1 [dcl.init.general] bullet 16.6.1. It is unclear whether parameter values should be treated the same.
Possible resolution:
Change in 6.7.7 [class.temporary] paragraph 3 as follows:
When an object ofclasstrivially returnable (8.7.4 [stmt.return]) type X is passed toor returned froma function,if X has at least one eligible copy or move constructor (11.4.4 [special]), each such constructor is trivial, and the destructor of X is either trivial or deleted,implementations are permitted to create a temporary object to hold the function parameteror result object. The temporary object is constructed from the function argumentor return value, respectively,and the function's parameteror return objectis initialized as if by using the eligible trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object).
Split and change 8.7.4 [stmt.return] paragraph 2 as follows:
The expr-or-braced-init-list of a return statement is called its operand. A return statement with no operand shall be used only in a function whose return type is cv void, a constructor (11.4.5 [class.ctor]), or a destructor (11.4.7 [class.dtor]). A return statement with an operand of type void shall be used only in a function that has a cv void return type. A return statement with any other operand shall be used only in a function that has a return type other than cv void
; the return statement initializes the returned reference or prvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand. [Note 1: A constructor or destructor does not have a return type. —end note]A type T is trivially returnable if T is a class type that has at least one eligible copy or move constructor (11.4.4 [special]), each such constructor is trivial, and the destructor of T is either trivial or deleted. If a function has a return type R other than cv void:
[ Note 2: A return statement can involve an invocation of a constructor to perform a copy or move of the operand
- If R is trivially returnable, the return statement initializes a first temporary object from the operand (7.3.5 [conv.rval]). Then, a second temporary object is constructed from the first temporary object, using an eligible trivial constructor for the copy. Then, the first temporary object is destroyed and the duration of its storage ends. [ Note: A pointer value pointing to the first temporary object becomes an invalid pointer value (6.8.4 [basic.compound]). -- end note ] Finally, the prvalue result object of the function call is initialized from the second temporary object, using an eligible trivial constructor for the copy, and the second temporary object is destroyed. In both cases, an eligible trivial constructor is used even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object.
- Otherwise, the return statement initializes the returned reference or prvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand.
if it is not a prvalue or if its type differs from the return type of the function. A copy operation associated with a return statement can be elided or converted to a move operation if an automatic storage duration variable is returned (11.9.6 [class.copy.elision]). —end note]
Sent in by David Abrahams:
Yes, and to add to this tangent, 6.8.2 [basic.fundamental] paragraph 1 states "Plain char, signed char, and unsigned char are three distinct types." Strangely, 6.8 [basic.types] paragraph 2 talks about how "... the underlying bytes making up the object can be copied into an array of char or unsigned char. If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value." I guess there's no requirement that this copying work properly with signed chars!
Notes from October 2002 meeting:
We should do whatever C99 does. 6.5p6 of the C99 standard says "array of character type", and "character type" includes signed char (6.2.5p15), and 6.5p7 says "character type". But see also 6.2.6.1p4, which mentions (only) an array of unsigned char.
Proposed resolution (April 2003):
Change 6.7.3 [basic.life] bullet 5.3 from
to
Change 6.7.3 [basic.life] bullet 6.3 from
to
Change the beginning of 6.8 [basic.types] paragraph 2 from
For any object (other than a base-class subobject) of POD type T, whether or not the object holds a valid value of type T, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of char or unsigned char.
to
For any object (other than a base-class subobject) of POD type T, whether or not the object holds a valid value of type T, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of byte-character type.
Add the indicated text to 6.8.2 [basic.fundamental] paragraph 1:
Objects declared as characters (char) shall be large enough to store any member of the implementation's basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types, called the byte-character types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (6.8 [basic.types]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. For unsigned byte-character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain char object can take on either the same values as a signed char or an unsigned char; which one is implementation-defined.
Change 7.2.1 [basic.lval] paragraph 15 last bullet from
to
Notes from October 2003 meeting:
It appears that in C99 signed char may have padding bits but no trap representation, whereas in C++ signed char has no padding bits but may have -0. A memcpy in C++ would have to copy the array preserving the actual representation and not just the value.
March 2004: The liaisons to the C committee have been asked to tell us whether this change would introduce any unnecessary incompatibilities with C.
Notes from October 2004 meeting:
The C99 Standard appears to be inconsistent in its requirements. For example, 6.2.6.1 paragraph 4 says:
The value may be copied into an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is called the object representation of the value.
On the other hand, 6.2 paragraph 6 says,
If a value is copied into an object having no declared type using memcpy or memmove, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one.
Mike Miller will investigate further.
Proposed resolution (February, 2010):
Change 6.7.3 [basic.life] bullet 5.4 as follows:
...The program has undefined behavior if:
...
the pointer is used as the operand of a static_cast (7.6.1.9 [expr.static.cast]) (except when the conversion is to cv void*, or to cv void* and subsequently to
char*, or unsigned char*a pointer to a cv-qualified or cv-unqualified byte-character type (6.8.2 [basic.fundamental])), or...
Change 6.7.3 [basic.life] bullet 6.4 as follows:
...The program has undefined behavior if:
...
the lvalue is used as the operand of a static_cast (7.6.1.9 [expr.static.cast]) except when the conversion is ultimately to
cv char& or cv unsigned char&a reference to a cv-qualified or cv-unqualified byte-character type (6.8.2 [basic.fundamental]) or an array thereof, or...
Change 6.8 [basic.types] paragraph 2 as follows:
For any object (other than a base-class subobject) of trivially copyable type T, whether or not the object holds a valid value of type T, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array ofchar or unsigned chara byte-character type (6.8.2 [basic.fundamental]).39 If the content ofthethat arrayof char or unsigned charis copied back into the object, the object shall subsequently hold its original value. [Example:...
Change 6.8.2 [basic.fundamental] paragraph 1 as follows:
...Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types, called the byte-character types. A char, a signed char, and an unsigned char occupy the same amount of storage and have the same alignment requirements (6.7.6 [basic.align]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. Forunsigned character typesunsigned char, all possible bit patterns of the value representation represent numbers...
Change 7.2.1 [basic.lval] paragraph 15 final bullet as follows:
If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined 52
...
a
char or unsigned charbyte-character type (6.8.2 [basic.fundamental]).
Change 6.7.6 [basic.align] paragraph 6 as follows:
The alignment requirement of a complete type can be queried using an alignof expression (7.6.2.6 [expr.alignof]). Furthermore, the byte-character types (6.8.2 [basic.fundamental])char, signed char, and unsigned charshall have the weakest alignment requirement. [Note: this enables the byte-character types to be used as the underlying type for an aligned memory area (9.12.2 [dcl.align]). —end note]
Change 7.6.2.8 [expr.new] paragraph 10 as follows:
...For arrays ofchar and unsigned chara byte-character type (6.8.2 [basic.fundamental]), the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the strictest fundamental alignment requirement (6.7.6 [basic.align]) of any object type whose size is no greater than the size of the array being created. [Note: Because allocation functions are assumed to return pointers to storage that is appropriately aligned for objects of any type with fundamental alignment, this constraint on array allocation overhead permits the common idiom of allocating byte-character arrays into which objects of other types will later be placed. —end note]
Notes from the March, 2010 meeting:
The CWG was not convinced that there was a need to change the existing specification at this time. Some were concerned that there might be implementation difficulties with giving signed char the requisite semantics; implementations for which that is true can currently make char equivalent to unsigned char and avoid those problems, but the suggested change would undermine that strategy.
Additional note, November, 2014:
There is now the term “narrow character type” that should be used instead of “byte-character type”.
6.8.2 [basic.fundamental] does not impose a requirement on the floating point types that there be an exact representation of the value zero. This omission is significant in 7.3.14 [conv.fctptr] paragraph 1, in which any non-zero value converts to the bool value true.
Suggested resolution: require that all floating point types have an exact representation of the value zero.
6.8.2 [basic.fundamental] paragraph 2 says that
There are four signed integer types: "signed char", "short int", "int", and "long int."
This would indicate that const int is not a signed integer type.
Notes from the June, 2016 meeting:
See issue 2185.
6.8.4 [basic.compound] paragraph 3 states:
A value of a pointer type that is a pointer to or past the end of an object represents the address of the first byte in memory (6.7.1 [intro.memory]) occupied by the object [ Footnote: ... ] or the first byte in memory after the end of the storage occupied by the object, respectively.
A potentially-overlapping subobject of type T may occupy fewer bytes than indicated by sizeof(T), yet pointer arithmetic will only consider sizeof(T), not the number of actually occupied bytes. For example,
struct X { X() = default; int x; short y; }; struct S { [[no_unique_address]] X x; short z; }; static_assert(sizeof(X) == sizeof(S));
On a popular implementation, z is actually put into the tail padding of x, and thus &S().x + 1 does not actually point to "the first byte in memory after the end of the storage occupied by" x.
Suggested resolution (amended 2022-03-10):
Change in 6.8.4 [basic.compound] paragraph 3 as follows:
A value V of a pointer type
that is a pointer to or past the end of an objectrepresents the addressof the first byte in memory (6.7.1 [intro.memory]) occupied by the objectA as follows:
- If V is a pointer to an object, A is the address of the object as specified in 6.7.2 [intro.object] [ Footnote: ... ]
or the first byte in memory after the end of the storage occupied by the object, respectively.- If V is a pointer past the end of an object that is not a potentially-overlapping subobject, A is the address of the first byte in memory after the end of the storage occupied by the object.
- If V is a pointer past the end of a potentially-overlapping subobject, A is an implementation-defined choice of an address within the complete object or the address of the first byte in memory after the end of the storage occupied by the complete object.
- If V is a null pointer value or an invalid pointer value, V does not represent an address.
According to 6.9.1 [intro.execution] paragraph 14, “sequenced before” is a relation between “evaluations.” However, 6.9.3.3 [basic.start.dynamic] paragraph 3 says,
If the completion of the initialization of a non-local object with static storage duration is sequenced before a call to std::atexit (see <cstdlib>, 17.5 [support.start.term]), the call to the function passed to std::atexit is sequenced before the call to the destructor for the object. If a call to std::atexit is sequenced before the completion of the initialization of a non-local object with static storage duration, the call to the destructor for the object is sequenced before the call to the function passed to std::atexit. If a call to std::atexit is sequenced before another call to std::atexit, the call to the function passed to the second std::atexit call is sequenced before the call to the function passed to the first std::atexit call.
Except for the calls to std::atexit, these events do not correspond to “evaluation” of expressions that appear in the program. If the “sequenced before” relation is to be applied to them, a more comprehensive definition is needed.
According to 6.9.2 [intro.multithread] paragraph 9,
An evaluation A carries a dependency to an evaluation B if
the value of A is used as an operand of B, unless:
...
A is the left operand of a built-in logical AND (&&, see 7.6.14 [expr.log.and]) or logical OR (||, see 7.6.15 [expr.log.or]) operator, or
...
The intent is that this does not apply to the second operands of such operators if the first operand is such that they are not evaluated, but the wording is not clear to that effect. (A similar question applies to the non-selected operand of the conditional operator ?:.)
Notes from the October, 2015 meeting:
It appears likely that the text involved will be removed by a revision to the memory_order_consume specification.
Notes from the February, 2016 meeting:
Action on this issue will be deferred until the specification for memory_order_consume is complete; it should not currently be used.
It is not sufficiently clear that the only atomic operations are the ones defined in 33.5 [atomics] by the library. The intent is that no accesses are atomic unless the Standard describes them as such.
An additional problem is that, e.g., new and delete are defined to be synchronization operations, but they are not defined in Clauses 33.5 [atomics] and Clause 33 [thread].
Suggested resolution:
Change 6.9.2.2 [intro.races] paragraph 3 as follows:
The library definesa numberthe set of atomic operations (33.5 [atomics])and operations on mutexes (Clause 33 [thread]) that. Some of these, and some other library operations, such as those on mutexes ( Clause 33 [thread]) are specially identified as synchronization operations. These operations...
Notes from the April, 2017 teleconference:
CWG determined that this issue should be handled editorially; it will be in "review" status until the change has been made and verified. See editorial issue 1611.
Additional notes, October, 2018:
This is also library issue 2506. SG1 has requested a paper to deal with this issue, so it is no longer considered editorial.
Section 6.9.2.2 [intro.races] uses the terms “action” and “expression evaluation” interchangeably. “Sequenced before” is defined on expression evaluations. Probably none of those is correct.
We should really be talking about individual accesses to “memory locations”. Talking about larger “expression evaluations” is incorrect, since they may include internal synchronization. Thus concurrent evaluation of large conflicting expression evaluations may not actually correspond to a data race. I'm not sure what term we should be using instead of “expression evaluation” to denote such individual accesses. Call it X for now.
There is also an issue with the fact that “sequenced before” is defined on expression evaluation. “Sequenced before” should also be defined on Xs. It doesn't make any sense to talk about “sequenced before” ordering on two evaluations when one includes the other. Whenever we say “A is sequenced before B”, we probably really mean that all Xs in A are sequenced before all Xs in B. We could probably just include a blanket statement to that effect.
Additional notes (April, 2022)
Forwarded to SG1 with paper issue 1234, reflecting the former "concurrency" status of this issue.
Is a compiler allowed to interleave constructor calls when performing dynamic initialization of nonlocal objects? What I mean by interleaving is: beginning to execute a particular constructor, then going off and doing something else, then going back to the original constructor. I can't find anything explicit about this in 6.9.3.2 [basic.start.static].
I'll present a few different examples, some of which get a bit wild. But a lot of what this comes down to is exactly what the standard means when it talks about the order of initialization. If it says that some object x must be initialized before a particular event takes place, does that mean that x's constructor must be entered before that event, or does it mean that it must be exited before that event? If object x must be initialized before object y, does that mean that x's constructor must exit before y's constructor is entered?
(The answer to that question might just be common sense, but I couldn't find an answer in 6.9.3.2 [basic.start.static]. Actually, when I read 6.9.3.2 [basic.start.static] carefully, I find there are a lot of things I took for granted that aren't there.)
OK, so a few specific scenerios.
<runtime gunk> <Enter A's constructor> <Enter f> <runtime gunk> <Enter B's constructor> <Enter f> <Leave f> <Leave B's constructor> <Leave f> <Leave A's constructor>The implication of a 'yes' answer for users is that any function called by a constructor, directly or indirectly, must be reentrant.
At this point, you might be thinking we could avoid all of this nonsense by removing compilers' freedom to defer initialization until after the beginning of main(). I'd resist that, for two reasons. First, it would be a huge change to make after the standard has been out. Second, that freedom is necessary if we want to have support for dynamic libraries. I realize we don't yet say anything about dynamic libraries, but I'd hate to make decisions that would make such support even harder.
According to 6.9.3.2 [basic.start.static] paragraph 3,
An implementation is permitted to perform the initialization of a non-local variable with static storage duration as a static initialization even if such initialization is not required to be done statically, provided that
the dynamic version of the initialization does not change the value of any other object of namespace scope prior to its initialization, and
the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically.
This does not consider side effects of the initialization in this determination, only the values of namespace-scope variables.
CWG 2022-11-11
The precise normative identification of side effects relevant for the rule remains open. An approach similar to the constexpr model of considering the transitive hull of evaluations might be applicable.
According to 6.9.3.2 [basic.start.static] paragraph 5,
It is implementation-defined whether the dynamic initialization of a non-local variable with static or thread storage duration is done before the first statement of the initial function of the thread. If the initialization is deferred to some point in time after the first statement of the initial function of the thread, it shall occur before the first odr-use (6.3 [basic.def.odr]) of any variable with thread storage duration defined in the same translation unit as the variable to be initialized.
This doesn't consider that initialization of instantiations of static data members of class templates (which can be thread_local) are unordered. Presumably odr-use of such a static data member should not trigger the initialization of any thread_local variable other than that one?
Subclause 6.9.3.3 [basic.start.dynamic] paragraph 7 specifies:
It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred. If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration defined in the same translation unit as the variable to be initialized. ...
How does the rule quoted above affect variables declared constinit? For example:
extern thread_local constinit int x;
Clang and gcc do not emit suitable wrapper code to allow for deferred initialization of x, which is non-conforming. Should this be allowed?
Suggested resolution:
Change in 6.9.3.3 [basic.start.dynamic] paragraph 7 as follows:
It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred. If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration and dynamic initialization defined in the same translation unit as the variable to be initialized. ...
(From submission #548.)
Subclause 7.2.1 [basic.lval] paragraph 11 specifies:
... If a program attempts to access (3.1 [defns.access]) the stored value of an object through a glvalue through which it is not type-accessible, the behavior is undefined. ...
Thus, access (read or write) to an int object using an lvalue of type unsigned int is valid. However, 7.3.2 [conv.lval] bullet 3.4 does not specify the resulting value when, for example, the object contains the value -1:
- Otherwise, the object indicated by the glvalue is read (3.1 [defns.access]), and the value contained in the object is the prvalue result. ...
Similarly, 7.6.19 [expr.ass] paragraph 2 is silent for the assignment case:
In simple assignment (=), the object referred to by the left operand is modified (3.1 [defns.access]) by replacing its value with the result of the right operand.
Any concerns about accesses to the object representation are handled in the context of P1839.
Possible resolution:
Change in 7.3.2 [conv.lval] bullet 3.4 as follows:
- Otherwise, the object indicated by the glvalue is read (3.1 [defns.access]), and the value congruent (6.8.2 [basic.fundamental]) to the value contained in the object is the prvalue result. ...
Change in 7.6.19 [expr.ass] paragraph 2 as follows:
In simple assignment (=), the object referred to by the left operand is modified (3.1 [defns.access]) by replacing its value with the value congruent (6.8.2 [basic.fundamental]) to the result of the right operand.
(From editorial issue #7051.)
Consider:
static_assert(sizeof(bool) == 1); // assumption for the example bool f() { char c = 2; bool b = true; memcpy(&b, &c, 1); // #1 return b; // #2 }
At #1, the lifetime of the original b ends because its storage is reused by the memcpy (6.7.3 [basic.life] bullet 1.5). memcpy implicitly creates objects (23.5.3 [cstring.syn] paragraph 3). Creating a bool object gives the rest of the program defined behavior, as follows. b refers to the newly-created object, because scalars of the same type are transparently replaceable (6.7.3 [basic.life] paragraph 9). Assuming that false and true are represented as 0 and 1, the value representation of b now has a bad value. This should, but does not, result in undefined behavior during the lvalue-to-rvalue conversion at #2.
Possible resolution:
Change in 7.3.2 [conv.lval] paragraph 3 as follows:
The result of the conversion is determined according to the following rules:
- If T is cv std::nullptr_t, the result is a null pointer constant (7.3.12 [conv.ptr]). [Note 1: Since the conversion does not access the object to which the glvalue refers, there is no side effect even if T is volatile-qualified (6.9.1 [intro.execution]), and the glvalue can refer to an inactive member of a union (11.5 [class.union]). —end note]
- Otherwise, if T has a class type, the conversion copy-initializes the result object from the glvalue.
- Otherwise, if the object to which the glvalue refers contains an invalid pointer value (6.7.5.5.3 [basic.stc.dynamic.deallocation]), the behavior is implementation-defined.
- Otherwise, if the bits in the value representation of the object to which the glvalue refers are not valid for the object's type, the behavior is undefined. [ Example:
bool f() { bool b = true; char c = 42; memcpy(&b, &c, 1); // implicitly creates objects at b (23.5.3 [cstring.syn], 6.7.2 [intro.object]) return b; // undefined behavior if 42 is not a valid value representation for bool }-- end example ]- Otherwise, the object indicated by the glvalue is read (3.1 [defns.access]), and the value contained in the object is the prvalue result.
If the result is an erroneous value (6.7.4 [basic.indet]) and the bits in the value representation are not valid for the object's type, the behavior is undefined.
A type has multiple cv-decompositions, and 7.3.6 [conv.qual] paragraph 3 does not say which one to use when determining the cv-combined type. Should this be the longest decomposition that works, i.e., the greatest n for which you can decompose both types? (We used to refer to the cv-qualification signature, which implicitly meant to take the longest decomposition.)
When computing the cv-combined types of two types T1 and T2, if U1 and U2 are different, shouldn't we add const to all layers above that in the type?
cv03 is left unspecified by the wording in paragraph 3.
We are too eager to replace a Pi3 with “array of unknown bound of”. That should only happen if both Pi1 and Pi2 are array types, or we end up not forming a type T3 that is similar to T1. For example, the cv-combined type of int** and const int (*)[], when decomposed with n == 2, is required to be const int (*)[] by the bulleted rules, and that type is not similar to the original T1.
In various places, we have operators that say, “if one operand is of pointer type, apply array-to-pointer conversions, pointer conversions, and qualification conversions to bring the two operands to their composite pointer type,” but that doesn't work, because the definition of composite pointer type can't cope with one operand being a pointer and the other being an array. We either need to define the composite pointer type of a pointer and an array (and, if that's done in terms of computing the cv-combined type, be careful to ensure that computing the cv-combined type actually works in that case) or to perform the array-to-pointer conversion before considering the composite pointer type.
Consider:
template <typename T> concept C = requires (typename T::type x) { x + 1; }; static_assert(!C<int>);
All implementations accept this translation unit. However, the rule in 7.5.8.1 [expr.prim.req.general] paragraph 5 does not cover the parameter-declaration-clause::
The substitution of template arguments into a requires-expression may result in the formation of invalid types or expressions in its requirements or the violation of the semantic constraints of those requirements. In such cases, the requires-expression evaluates tofalse
; it does not cause the program to be ill-formed. The substitution and semantic constraint checking proceeds in lexical order and stops when a condition that determines the result of the requires-expression is encountered. If substitution (if any) and semantic constraint checking succeed, the requires-expression evaluates totrue
.
Proposed resolution (approved by CWG 2023-06-17):
Change in 7.5.8.1 [expr.prim.req.general] paragraph 5:
The substitution of template arguments into a requires-expression may result in the formation of invalid types or expressions in its parameter-declaration-clause (if any) or its requirements or the violation of the semantic constraints of those requirements. In such cases, ...
CWG 2023-11-09
The initial example is not the point of the issue, but the very similar following case:
template <typename T>
constexpr bool b = requires (T::type x) { x + 1; };
static_assert(!b<int>); // de-jure ill-formed, but widely accepted by implementations
Users were vocal they wanted this not to be an error, but just a substitution failure. Reportedly, the original design intent was that this situation is a hard error, though.
Soliciting guidance from EWG how to handle this issue via paper issue 1695.
EWG 2024-03-18
EWG invites a paper to propose a change.
(From submission #562.)
Subclause 7.5.8.1 [expr.prim.req.general] paragraph 2 specifies:
A requires-expression is a prvalue of type bool whose value is described below. Expressions appearing within a requirement-body are unevaluated operands (7.2.3 [expr.context]).
A constant-expression used as a non-type template argument "appearing within" the requirement-body should not be considered an "unevaluated operand". Similarly, bodies of lambda-expressions should not be in focus of "appearing within".
Suggested resolution:
Change in 7.2.3 [expr.context] paragraph 1 as follows:
In some contexts, unevaluated operands appear (7.5.8 [expr.prim.req]7.5.8.2 [expr.prim.req.simple], 7.5.8.4 [expr.prim.req.compound], 7.6.1.8 [expr.typeid], 7.6.2.5 [expr.sizeof], 7.6.2.7 [expr.unary.noexcept], 9.2.9.6 [dcl.type.decltype], 13.1 [temp.pre], 13.7.9 [temp.concept]). An unevaluated operand is not evaluated.
Change in 7.5.8.1 [expr.prim.req.general] paragraph 2 as follows:
A requires-expression is a prvalue of type bool whose value is described below.Expressions appearing within a requirement-body are unevaluated operands (7.2.3 [expr.context]).
Change in 7.5.8.2 [expr.prim.req.simple] paragraph 1 as follows:
A simple-requirement asserts the validity of an expression. The expression is an unevaluated operand. [Note 1: The enclosing requires-expression will evaluate to false if substitution of template arguments into the expression fails.The expression is an unevaluated operand (7.2.3 [expr.context]).—end note] ...
Change in 7.5.8.4 [expr.prim.req.compound] paragraph 1 as follows:
A compound-requirement asserts properties of the expression E. The expression is an unevaluated operand. Substitution of template arguments (if any) and verification of semantic properties proceed in the following order:
Consider:
struct NonConstant { static bool f() { return true; } };
struct True { static constexpr bool f() { return true; } };
struct False { static constexpr bool f() { return false; } };
template <class T>
concept C = requires { requires T::f(); };
static_assert(!C<NonConstant>); // #1
static_assert(C<True>);
static_assert(!C<False>);
clang accepts; gcc, MSVC, and EDG all consider #1 to be ill-formed. Subclause 7.5.8.5 [expr.prim.req.nested] paragraph 1 specifies:
The constraint-expression shall be satisfied (13.5.3 [temp.constr.decl]) by the substituted template arguments, if any. Substitution of template arguments into a nested-requirement does not result in substitution into the constraint-expression other than as specified in 13.5.2 [temp.constr.constr].
Neither C<NonConstant> nor C<False> are saisfied, but it is unclear whether those two cases should be treated differently. Subclause 13.5.2.3 [temp.constr.atomic] paragraph 3 might be relevant:
To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression, the constraint is not satisfied. Otherwise, the lvalue-to-rvalue conversion (7.3.2 [conv.lval]) is performed if necessary, and E shall be a constant expression of type bool.
An example from the standard library is:
template<class G> concept uniform_random_bit_generator = invocable<G&> && unsigned_integral<invoke_result_t<G&>> && requires { { G::min() } -> same_as<invoke_result_t<G&>>; { G::max() } -> same_as<invoke_result_t<G&>>; requires bool_constant<(G::min() < G::max())>::value; };
Is the bool_constant part necessary to ensure that G::min() < G::max() is a constant expression?
As of P0400R0 (Wording for Order of Evaluation of Function Arguments), we have in subclause 7.6.1.3 [expr.call] paragraph 8:
The postfix-expression is sequenced before each expression in the expression-list and any default argument. The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter.
What about the case when the element in the expression-list is a braced-init-list rather than an expression? The braced-init-list is certainly evaluated left-to-right, but is that required to happen after we evaluate the postfix-expression?
The editorial change referred to in the resolution of issue 2495 updated the terminology used to describe the return statement to allow for the fact that the operand could be a braced-init-list instead of an expression. A similar problem exists describing the result of a function call in 7.6.1.3 [expr.call] paragraph 9:
The result of a function call is the result of the possibly-converted operand of the return statement (8.7.4 [stmt.return]) that transferred control out of the called function
It's incorrect to refer to “converting” the operand when it is a braced-init-list.
Subclause 7.6.1.3 [expr.call] paragraph 7 specifies:
If the function is an implicit object member function, the this parameter of the function (7.5.3 [expr.prim.this]) is initialized with a pointer to the object of the call, converted as if by an explicit type conversion (7.6.3 [expr.cast]).
The term "this parameter" is undefined.
Suggested resolution:
Change in 7.6.1.3 [expr.call] paragraph 7 as follows:
If the function is an implicit object member function, thethisimplicit object parameter of the function (7.5.3 [expr.prim.this]) is initialized with a pointer to the object of the call, converted as if by an explicit type conversion (7.6.3 [expr.cast]).
Subclause 7.6.1.3 [expr.call] paragraphs 1 and 2 do not properly handle explicit object member functions. Such a function can be called via a function pointer, via an lvalue obtained by dereferencing a function pointer, and via a class member access expression.
Although value-initialization is defined for array types and the () initializer is permitted in a mem-initializer naming an array member of a class, the syntax T() (where is an array type) is explicitly forbidden by 7.6.1.4 [expr.type.conv] paragraph 2. This is inconsistent and the syntax should be permitted.
Rationale (July, 2009):
The CWG was not convinced of the utility of this extension, especially in light of questions about handling the lifetime of temporary arrays. This suggestion needs a proposal and analysis by the EWG before it can be considered by the CWG.
EWG 2022-11-11
This is a defect; a paper is needed. This is tracked in github issue cplusplus/papers#1372.
Consider:
struct A {int i;}; struct X : A {}; struct Y : A {}; struct S : X,Y {}; void f(S &s) {++s.X::i;}
There is a rule that &s be able to "be implicitly converted to a pointer to the naming class of the right operand" (11.8.3 [class.access.base] paragraph 6), which correctly selects X here, but 7.6.1.5 [expr.ref] bullet 6.2 merely says that
If E2 is a non-static data member [...], the expression designates the corresponding member subobject of the object designated by the first expression.
E2 (i.e. X::i) and Y::i equally denote A::i (since name lookup produces declarations), but there is no rule that indicates which A::i is selected as the "corresponding member subobject". (Note that s.X::A::i means the same as s.A::i and is ambiguous.)
Suggested resolution:
Change 11.8.3 [class.access.base] paragraph 6 to actually perform the conversion it requires, possibly moving it to 7.6.1.5 [expr.ref] since it is not about access control; the definition of "naming class" in 11.8.3 [class.access.base] paragraph 5 would then need to be italicized.
Given the following declarations:
struct S { signed long long sll: 3; }; S s = { -1 };
the expressions s.sll-- < 0u and s.sll < 0u have different results. The reason for this is that s.sll-- is an rvalue of type signed long long (7.6.1.6 [expr.post.incr]), which means that the usual arithmetic conversions (Clause 7 [expr] paragraph 10) convert 0u to signed long long and the result is true. s.sll, on the other hand, is a bit-field lvalue, which is promoted (7.3.7 [conv.prom] paragraph 3) to int; both operands of < have the same rank, so s.sll is converted to unsigned int to match the type of 0u and the result is false. This disparity seems undesirable.
The original proposed resolution for issue 160 included changing extended_type_info (7.6.1.8 [expr.typeid] paragraph 1, footnote 61) to std::extended_type_info. There was no consensus on whether this name ought to be part of namespace std or in a vendor-specific namespace, so the question was moved into a separate issue.
According to 7.6.1.9 [expr.static.cast] paragraph 1,
The static_cast operator shall not cast away constness (7.6.1.11 [expr.const.cast]).
However, this phrasing is problematic in the context of a C-style cast like the following:
const void *p; int *q = (int*)p;
The intent of 7.6.3 [expr.cast] is that this should be interpreted as a static_cast followed by a const_cast. However, because int* to const void* is a valid standard conversion, and 7.6.1.9 [expr.static.cast] paragraph 7 allows static_cast to perform the inverse of a standard conversion sequence, the C-style cast is interpreted as just a static_cast without a const_cast and is thus ill-formed.
Class types may have padding, influencing the result of sizeof. It is unclear whether the placement and amount of padding is implementation-defined, unspecified, or something else. If it is unspecified, the limits of permissible behavior are unclear. Empty classes might need special consideration.
Suggested resolution:
Change in 7.6.2.5 [expr.sizeof] paragraph 2
... When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects of that type in an array. The amount and placement of padding in a class type is unspecified. The result of applying sizeof to a potentially-overlapping subobject is the size of the type, not the size of the subobject. [ Footnote: ... ]
Subclause 7.6.2.5 [expr.sizeof] paragraph 1 specifies:
The sizeof operator yields the number of bytes occupied by a non-potentially-overlapping object of the type of its operand. The operand is either an expression, which is an unevaluated operand (7.2.3 [expr.context]), or a parenthesized type-id. ...
Since an abstract class can be used only in a situation where it is a potentially-overlapping object, it is unclear what its size is.
Requirements for the alignment of pointers returned by new-expressions are given in 7.6.2.8 [expr.new] paragraph 10:
For arrays of char and unsigned char, the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the most stringent alignment requirement (6.8 [basic.types]) of any object type whose size is no greater than the size of the array being created.
The intent of this wording is that the pointer returned by the new-expression will be suitably aligned for any data type that might be placed into the allocated storage (since the allocation function is constrained to return a pointer to maximally-aligned storage). However, there is an implicit assumption that each alignment requirement is an integral multiple of all smaller alignment requirements. While this is probably a valid assumption for all real architectures, there's no reason that the Standard should require it.
For example, assume that int has an alignment requirement of 3 bytes and double has an alignment requirement of 4 bytes. The current wording only requires that a buffer that is big enough for an int or a double be aligned on a 4-byte boundary (the more stringent requirement), but that would allow the buffer to be allocated on an 8-byte boundary — which might not be an acceptable location for an int.
Suggested resolution: Change "of any object type" to "of every object type."
A similar assumption can be found in 7.6.1.10 [expr.reinterpret.cast] paragraph 7:
...converting an rvalue of type "pointer to T1" to the type "pointer to T2" (where ... the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value...
Suggested resolution: Change the wording to
...converting an rvalue of type "pointer to T1" to the type "pointer to T2" (where ... the alignment requirements of T1 are an integer multiple of those of T2) and back to its original type yields the original pointer value...
The same change would also be needed in paragraph 9.
Additional note (June, 2022):
Subclause 6.7.6 [basic.align] paragraph 4 specifies:
... Every alignment value shall be a non-negative integral power of two.
Thus, the situation that a stricter alignment is not an integral multiple of a weaker alignment does not arise.
According to 7.6.2.8 [expr.new] paragraphs 18-20, an exception thrown during the initialization of an object allocated by a new-expression will cause a deallocation function to be called for the object's storage if a matching deallocation function can be found. The rules deal only with functions, however; nothing is said regarding a mechanism by which a deallocation function template might be instantiated to free the storage, although 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2 indicates that a deallocation function can be an instance of a function template.
One possibility for this processing might be to perform template argument deduction on any deallocation function templates; if there is a specialization that matches the allocation function, by the criteria listed in paragraph 20, that function template would be instantiated and used, although a matching non-template function would take precedence as is the usual outcome of overloading between function template specializations and non-template functions.
Another possibility might be to match non-template deallocation functions with non-template allocation functions and template deallocation functions with template allocation functions.
There is a slightly related wording problem in 7.6.2.8 [expr.new] paragraph 21:
If a placement deallocation function is called, it is passed the same additional arguments as were passed to the placement allocation function, that is, the same arguments as those specified with the new-placement syntax.
This wording ignores the possibility of default arguments in the allocation function, in which case the arguments passed to the deallocation function might be a superset of those specified in the new-placement.
(See also issue 1682.)
A pointer value must have one of the kinds specified in 6.8.4 [basic.compound] paragraph 3:
Every value of pointer type is one of the following:
- a pointer to an object or function (the pointer is said to point to the object or function), or
- a pointer past the end of an object (7.6.6 [expr.add]), or
- the null pointer value for that type, or
- an invalid pointer value.
When allocating an array with no elements, 7.6.2.8 [expr.new] paragraph 10 is silent on the kind of pointer value returned:
When the allocated type is “array of N T” (that is, the noptr-new-declarator syntax is used or the new-type-id or type-id denotes an array type), the new-expression yields a prvalue of type “pointer to T” that points to the initial element (if any) of the array. Otherwise, let T be the allocated type; the new-expression is a prvalue of type “pointer to T” that points to the object created.
Related to that, are p and q allowed to compare equal in the following example?
T *p = new T[0]; T *q = new T;
Some implementations return a pointer past the array cookie for empty arrays, which can compare equal to a pointer to an object obtained from an unrelated allocation. However, if new T[0] is specified to yield a pointer to an object, this behavior violates the rule that pointers to disjoint objects with overlapping lifetimes must not compare equal.
Subclause 7.6.2.8 [expr.new] has multiple references to "placement allocation function" and "placement deallocation function", but those terms are never defined. The term "usual deallocation function" is defined in 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3:
... A usual deallocation function is a deallocation function whose parameters after the first are
- optionally, a parameter of type std::destroying_delete_t, then
- optionally, a parameter of type std::size_t, [ Footnote: ... ] then
- optionally, a parameter of type std::align_val_t.
Possible resolution:
Split 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 1 and change it as follows:
... The value of the first parameter is interpreted as the requested size of the allocation. A usual allocation function is an allocation function with no parameters after the first or with a single parameter of type std::align_val_t after the first.
An allocation function can be a function template. ...
CWG 2023-06-17
Replace "placement allocation / deallocation function" with "not a usual allocation / deallocation function".
Consider:
#include <new> auto p = new (std::align_val_t{64}) T;
Is this example well-formed for T = int? This invokes the allocation function
::operator new(size_t, align_val_t);
If this is considered a "placement allocation function", the code is ill-formed per 7.6.2.8 [expr.new] paragraph 28, because deallocation function lookup finds a usual deallocation function. If this is considered a "usual allocation function", the rules find ::operator delete(void*) as the corresponding deallocation function.
There is implementation divergence: MSVC rejects; clang and gcc accept, but invoke ::operator delete(void*, align_val_t) if the constructor exits via an exception.
If T is a type with new-extended alignment, the preferred allocation function is operator new(std::size_t, std::align_val_t, std::align_val_t) or, as a fallback, operator new(std::size_t, std::align_val_t), removing the alignment information of T (7.6.2.8 [expr.new] paragraph 19).
As another concern, subclause 17.6.3.2 [new.delete.single] paragraph 11 disallows mixing aligned and unaligned allocation/deallocation functions, and implementations rely on that precondition. Thus, it is impossible to deallocate the memory obtained in the example using a delete-expression; an explicit deallocation function call appears to be required.
Possible resolution:
Disallow invocation of one of the allocation functions specified in 17.6.3.2 [new.delete.single] and 17.6.3.3 [new.delete.array] if the new-placement syntax is used with anything but a single expression of type std::nothrow_t.
Alternatively, make this conditionally-supported to offer a richer feature set on platforms where mixing aligned and unaligned allocation/deallocation works.
(From submission #560.)
Consider an implementation with a size_t smaller than long long. An implementation should flag too-large values for the size in an array "new", in the same manner it flags negative values. For example:
int *p = new int[ULLONG_MAX];
Possible resolution:
Change in 7.6.2.8 [expr.new] paragraph 8 as follows:
If the expression in a noptr-new-declarator is present, it is implicitly converted to std::size_t. The value of the expression is invalid if:
- the expression is of non-class type and its value before converting to std::size_t is less than zero or greater than the maximum value representable as a std::size_t;
- the expression is of class type and its value before application of the second standard conversion (12.2.4.2.3 [over.ics.user]) [ Footnote: ... ] is less than zero or greater than the maximum value representable as a std::size_t;
- its value is such that the size of the allocated object would exceed the implementation-defined limit (Annex B); or
- the new-initializer is a braced-init-list and the number of array elements for which initializers are provided (including the terminating '\0' in a string-literal (5.13.5 [lex.string])) exceeds the number of elements to initialize.
7.6.2.8 [expr.new] paragraph 10 says that the result of an array allocation function and the value of the array new-expression from which it was invoked may be different, allowing for space preceding the array to be used for implementation purposes such as saving the number of elements in the array. However, there is no corresponding description of the relationship between the operand of an array delete-expression and the argument passed to its deallocation function.
6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3 does state that
the value supplied to operator delete[](void*) in the standard library shall be one of the values returned by a previous invocation of either operator new[](std::size_t) or operator new[](std::size_t, const std::nothrow_t&) in the standard library.
This statement might be read as requiring an implementation, when processing an array delete-expression and calling the deallocation function, to perform the inverse of the calculation applied to the result of the allocation function to produce the value of the new-expression. (7.6.2.9 [expr.delete] paragraph 2 requires that the operand of an array delete-expression "be the pointer value which resulted from a previous array new-expression.") However, it is not completely clear whether the "shall" expresses an implementation requirement or a program requirement (or both). Furthermore, there is no direct statement about user-defined deallocation functions.
Suggested resolution: A note should be added to 7.6.2.9 [expr.delete] to clarify that any offset added in an array new-expression must be subtracted in the array delete-expression.
Consider:
#include <memory> struct B { void operator delete(B* ptr, std::destroying_delete_t); }; struct D : B { void operator delete(D* ptr, std::destroying_delete_t); using B::operator delete; }; void foo(D* ptr) { delete ptr; }
The selection rules in 7.6.2.9 [expr.delete] paragraph 10 do not disambiguate this case. There is implementation divergence.
A similar situation arises for the following example:
struct A { void operator delete(void *); }; struct B { void operator delete(void *); }; struct C : A, B { using A::operator delete; using B::operator delete; }; void foo(C* ptr) { delete ptr; }
CWG 2023-10-20
The ordered list of preferences should be retained. Eventual disambiguation should be via overload resolution, but it is unclear how to deal with the unspecified choice between overloads with and without a std::size_t parameter.
Subclause 7.6.2.9 [expr.delete] paragraph 6 specifies:
... The destructor shall be accessible from the point where the delete-expression appears. ...
This check cannot be performed in case the operand of the delete-expression is a pointer to incomplete type.
Possible resolution:
Change in 7.6.2.9 [expr.delete] paragraph 6 as follows:
...TheUnless the object being deleted has incomplete class type, the destructor shall be accessible from the point where the delete-expression appears. ...
(From submission #532.)
Subclause 7.6.2.9 [expr.delete] paragraph 6 specifies that the destructor must be accessible even if a destroying operator delete is used:
If the value of the operand of the delete-expression is not a null pointer value and the selected deallocation function (see below) is not a destroying operator delete, evaluating the delete-expression invokes the destructor (if any) for the object or the elements of the array being deleted. The destructor shall be accessible from the point where the delete-expression appears. In the case of an array, the elements are destroyed in order of decreasing address (that is, in reverse order of the completion of their constructor; see 11.9.3 [class.base.init]).
This does not seem to be intended.
Possible resolution:
Change in 7.6.2.9 [expr.delete] paragraph 6 as follows:
If thevaluetype of the operand of the delete-expressionis not a null pointer valueis a pointer to a class type or (possibly multidimensional) array thereof, and the selected deallocation function (see below) is not a destroying operator delete:, evaluating the delete-expression invokes the destructor (if any) for the object or the elements of the array being deleted. The destructor shall be accessible from the point where the delete-expression appears. In the case of an array, the elements are destroyed in order of decreasing address (that is, in reverse order of the completion of their constructor; see 11.9.3 [class.base.init]).
- The destructor is potentially invoked (11.4.7 [class.dtor]).
- If the pointer value is not a null pointer value, evaluating a single-object delete expression destroys the object pointed to by the operand, and evaluating an array delete expression destroys each element of the array pointed to by the operand in order of decreasing index.
CWG 2024-05-31
This area of the wording is affected by P3144 (Deprecate Delete of a Pointer to an Incomplete Type). The wording above requires a class definition due to the "potentially invoked" phrasing. See also issue 2880.
(From submission #522.)
Consider:
struct A {}; struct B : A {}; const B b; void f() { (A &&)b; // #1 const_cast<A &&>(static_cast<const A &>(b)); const_cast<A &&>(static_cast<const volatile A &&>(b)); const_cast<A &&>(static_cast<A>(b)); // slicing interpretation }
Is #1 ill-formed because of the three alternative valid interpretations shown in the following lines?
Also consider:
struct B { }; const B f(); B& b1 = const_cast<B&>(static_cast<const B&>(f())); // OK B& b2 = (B&)f(); // ???
There is implementation divergence: gcc and MSVC accept, clang and EDG accept b1 and reject b2. If f is changed to return non-const, gcc also rejects b2.
For another example:
struct A { operator const B() = delete; } a; B& b3 = const_cast<B&>(static_cast<const B&>(a)); // error, deleted B& b4 = (B&)a; // error or reinterpret_cast?
Implementations agree that b3 is ill-formed for selecting the deleted conversion operator function, but b4 is considered a reinterpret_cast by the majority of implementations.
CWG 2024-05-17
Casting away rvalueness in example b2 is surprising to perform in a C-style cast. The b4 example is also interesting, because the reinterpret_cast interpretation might be undesirable. EWG is requested to offer guidance, preferably in the form of a comprehensive mental model. The current model "static_cast, then const_cast" might not be appropriate.
See paper issue 1970.
Subclause 7.6.10 [expr.eq] bullet 3.1 specifies:
- If one pointer represents the address of a complete object, and another pointer represents the address one past the last element of a different complete object, [ Footnote: ... ] the result of the comparison is unspecified.
- ...
This phrasing does not properly handle the case where addresses of subobjects are compared, yet those subobjects happen to have the same address as their respective complete objects.
The rule in question was introduced by issue 1652 for purposes of constant evaluation: comparing a pointer to an object X with a pointer past the end of another object Y ought not to be possible during contant evaluation. However, that issue resolution also caused a change to runtime behavior, departing from the prior "address comparison" model. It turns out that implementations nowadays rely on that rule for optimization purposes (see the reflector discussion).
Possible resolution:
Change in 7.6.10 [expr.eq] bullet 3.1 as follows:
- If one pointer points to an object and represents the address of the first byte in memory occupied by that object's
acomplete object, and another pointer points past the end of an object whose complete object is different and represents the address of the first byte in memory after the end of the storage occupied by that object'sone past the last element of a differentcomplete object, the result of the comparison is unspecified.- ...
The current definition of constant expressions appears to make unevaluated operands constant expressions; for example, new char[10] would seem to be a constant expression if it appears as the operand of sizeof. This seems wrong.
Notes from the November, 2016 meeting:
CWG did not wish to require implementations to detect this kind of undefined behavior in determining whether an expression is constant or not, but an implementation should be permitted to reject such expressions. These should be indeterminately sequenced, not unsequenced.
Consider the following example:
union A { constexpr A(int) : x(++x) { } int x; char* y; }; union B { A a = 5; }; int arr[B().a.x];
Value-initialization of the object created by B() zero-initializes the object (9.4 [dcl.init] bullet 8.2), which should mean that the ++x in the mem-initilizer for A operates on a zero-initialized object, but current implementations reject this code as non-constant. It is not clear what in the current wording justifies this treatment.
Consider an example like the following:
struct A { constexpr A(int i) : val(i) { } constexpr operator int() const { return val; } constexpr operator float() const { return val; } private: int val; }; constexpr A a = 42; int ary[a];
According to 9.3.4.5 [dcl.array] paragraph 1, the array bound expression
shall be a converted constant expression of type std::size_t (7.7 [expr.const]).
The user-defined conversion to float would involve a floating-integral conversion (7.3.11 [conv.fpint]; however, such a conversion is not permitted by the list of acceptable conversions in 7.7 [expr.const] paragraph 10:
A converted constant expression of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only
user-defined conversions,
lvalue-to-rvalue conversions (7.3.2 [conv.lval]),
array-to-pointer conversions (7.3.3 [conv.array]),
function-to-pointer conversions (7.3.4 [conv.func]),
qualification conversions (7.3.6 [conv.qual]),
integral promotions (7.3.7 [conv.prom]),
integral conversions (7.3.9 [conv.integral]) other than narrowing conversions (9.4.5 [dcl.init.list]),
null pointer conversions (7.3.12 [conv.ptr]) from std::nullptr_t,
null member pointer conversions (7.3.13 [conv.mem]) from std::nullptr_t, and
function pointer conversions (7.3.14 [conv.fctptr]),
and where the reference binding (if any) binds directly.
It is not clear whether this list is intended to restrict the set of viable user-defined conversions, and there is implementation divergence on this point: clang accepts the example above, while g++ rejects it, presumably on the basis of an ambiguous conversion.
Notes from the August, 2020 teleconference:
No direction was established pending information about why the example is accepted by clang.
Additional note, December, 2020:
The clang behavior turns out to have been an oversight, corrected in the current version, so the example is now rejected by both compilers. However, it is unclear that this is desirable. In particular, given the example above, a can be used without error as a bit-field width, as an enumerator value, and as the operand of alignas. Presumably the difference between these integral constant expression contexts and an array bound is the fact that the target type is known to be size_t. However, both bit-field widths and alignas operands are also required to be non-negative. Furthermore, the definition of an “erroneous” array bound in 7.6.2.8 [expr.new] paragraph 9 goes to awkward lengths to check for negative values as the result of user-defined conversions, which might argue in favor of reconsidering the converted constant expression treatment of array bounds.
Notes from the February, 2021 teleconference:
CWG agreed with the considerations in the December, 2020 note, feeling that the difference in treatment between integral constant expressions and a converted constant expression to a specific integral type is somewhat gratuitous. However, it was felt that code like that of the example was unlikely to occur often in real-world code.
Consider:
struct A { int x = 1; int y; };
constinit A a; // static storage duration; #1
The treatment of this example changed with P1331R2 (Permitting trivial default initialization in constexpr contexts), adopted 2019-07. Prior to this paper, the default constructor of A was not constexpr because it left a data member uninitialized. With paper P1331, the restriction was shifted to reading uninitialized objects during constant evaluation, and the variable a now satisfies the requirements for "constant-initialized" in 7.7 [expr.const] paragraph 2:
A variable or temporary object o is constant-initialized if
- either it has an initializer or its default-initialization results in some initialization being performed, and
- the full-expression of its initialization is a constant expression when interpreted as a constant-expression, except that if o is an object, that full-expression may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types.
Zero-initialization is not performed prior to constant-initialization per 6.9.3.2 [basic.start.static] paragraph 2:
Constant initialization is performed if a variable or temporary object with static or thread storage duration is constant-initialized (7.7 [expr.const]). If constant initialization is not performed, a variable with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (6.7.5.3 [basic.stc.thread]) is zero-initialized (9.4 [dcl.init]). Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization.
Thus, #1 is valid and a is statically initialized, but a.y would remain uninitialized, which is surprising for an object with static storage duration.
Current implementations diagnose an error at #1, because the variable a is actually not considered to be constant-initialized.
This issue is closely related to issue 2558.
Suggested resolution [SUPERSEDED]:
Change in 7.7 [expr.const] paragraph 2:A variable or temporary object o is constant-initialized if
- either it has an initializer or its default-initialization results in some initialization being performed, and
- every non-variant non-static data member and base class subobject is initialized (11.9.3 [class.base.init]), and
- the full-expression of its initialization is a constant expression when interpreted as a constant-expression, except that if o is an object, that full-expression may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types.
Alternative suggested resolution (March, 2022) [SUPERSEDED]:
Change in 7.7 [expr.const] paragraph 11 as follows:
A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints:
- if the value is an object of class type, each non-static data member of reference type refers to an entity that is a permitted result of a constant expression,
- if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object (7.6.6 [expr.add]), the address of a non-immediate function, or a null pointer value,
- if the value is of pointer-to-member-function type, it does not designate an immediate function, and
- if the value is an object of class or array type, each subobject is initialized (11.9.3 [class.base.init]) and satisfies these constraints for the value.
Notes from the November, 2022 meeting
CWG preferred to zero-initialize a.y in the example, and keep #1 well-formed.
Possible resolution:
Change in 6.9.3.2 [basic.start.static] paragraph 2 as follows:
Constant initialization is performed if a variable or temporary object with static or thread storage duration is constant-initialized (7.7 [expr.const]).If constant initialization is not performed, aA variable with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (6.7.5.3 [basic.stc.thread]) or a subobject thereof is zero-initialized (9.4 [dcl.init]) if constant initialization is not performed or if it does not initialize that subobject. Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization. All static initialization strongly happens before (6.9.2.2 [intro.races]) any dynamic initialization.
CWG 2022-12-02
The resolution shown above would leave padding bits uninitialized. In contrast, zero-initialization does set padding bits to 0 to possibly facilitate memcmp. Additional example:
struct C { int a; int b; C() : b(a) {} // #1 }; constinit C x; // OK when zero-initializing first, because #1 reads zero-initialized a?
2022-12-03
Forwarded to EWG with cplusplus/papers#1380.
Additional notes (January, 2023)
The standard does not guarantee stable results when reading padding bits, i.e. bits that are not part of the value representation of some in-lifetime object. In C, explicit rules keep padding bits stable; they are allowed to change only if a store to any class member occurs.
7.7 [expr.const] paragraph 6 specifies that std::construct_at can be used during constant evaluation:
Similarly, the evaluation of a call to std::construct_at or std::ranges::construct_at does not disqualify E from being a core constant expression unless the first argument, of type T*, does not point to storage allocated with std::allocator<T> or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying constructor call disqualifies E from being a core constant expression.
Apparently, implementations are required to track whether an object is transparently replaceable (6.7.3 [basic.life] paragraph 8) during constant evaluation to satisfy 7.7 [expr.const] bullet 5.8, which requires that undefined behavior be detected and rejected during constant evaluation:
- ...
- an operation that would have undefined behavior as specified in Clause 4 through Clause 15;
- ...
For example,
struct A { int x, y; }; struct B { float a; int b; }; union C { A a; B b; }; constexpr int f() { C c = {}; std::construct_at(&c.b.b, 5); // Does this return 5 if c.a.y and c.b.b are laid out at the same address? return c.a.y; } static_assert(f()); // not a constant-expression
No known implementation diagnoses the violation of the rules for transparently replaceable in the following example, but there is implementation divergence for the results of f():
#include <memory> struct A { virtual constexpr char f() { return 'A'; } }; struct B : A { constexpr char f() override { return 'B'; } }; constexpr char f() { B b; A *p = &b; std::construct_at(p); return p->f(); // #1; alternative: return b.f() } static_assert(f() == 'A'); // error: #1 accesses out-of-lifetime object
See also issue 2866 for concerns about observing the effects of an attribute if the potentially-overlapping subobject was introduced via [[no_unique_address]].
Consider:
template <typename Ty> struct S { Ty i; consteval S() = default; }; template <typename Ty> struct T { Ty i; consteval T() {} }; S<int> one; // only Clang rejects T<int> two; // Clang, GCC, ICC, MSVC reject void locals() { S<int> three; // only Clang rejects T<int> four; // Clang, GCC, ICC, MSVC reject }
A consteval function should always be evaluated at compile time and never fall back to runtime, thus all four cases should be rejected. Issue 2558 is related.
Consider the example in 7.7 [expr.const] paragraph 7:
extern Swim dc;
constexpr auto& sandeno = typeid(dc); // OK, can only be typeid(Swim)
The comment in the example seems not to be backed by normative text. In particular, the dynamic type of dc is constexpr-unknown per 7.7 [expr.const] paragraph 7:
During the evaluation of an expression E as a core constant expression, all id-expressions and uses of *this that refer to an object or reference whose lifetime did not begin with the evaluation of E are treated as referring to a specific instance of that object or reference whose lifetime and that of all subobjects (including all union members) includes the entire constant evaluation. For such an object that is not usable in constant expressions, the dynamic type of the object is constexpr-unknown. ...
Thus, typeid(dc) is not a core constant expression per 7.7 [expr.const] bullet 5.26:
- a dynamic_cast (7.6.1.7 [expr.dynamic.cast]) or typeid (7.6.1.8 [expr.typeid]) expression on a glvalue that refers to an object whose dynamic type is constexpr-unknown or that would throw an exception;
Issue 2529 says that a constexpr reference bound to a lifetime-extended temporary is required to have constant destruction. However, that rule overlooks constexpr objects with a reference member bound to a lifetime-extended temporary, recursively, and the case for std::initializer_list lifetime extension.
Consider:
consteval int a(auto); constexpr int g(auto e); constexpr int f(auto e); constexpr int f(auto e) { return a(e); } auto ff = f<int>; // error: f is an immediate function auto gg = g<int>; // ok? constexpr int g(auto e) { // g is not an immediate function? return a(e); }
The standard is silent as to what happen if no function body is available at the point where we need to determine whether a constexpr function template is an immediate function template.
Furthermore, there is no wording that taking the address of a constexpr function template specialization instantiates its body to determine whether it is an immediate function, unlike the handling of auto return types.
Subclause 13.9.2 [temp.inst] paragraph 5 specifies:
Unless a function template specialization is a declared specialization, the function template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist or if the existence of the definition affects the semantics of the program. ...
See also issue 2497.
Consider:
struct A { A(); virtual constexpr int f() { return 1; } }; struct B : A { virtual constexpr int f() { return 2; } }; B b; // not constexpr auto &id = typeid(b); // error, not constant
The example had been valid, but became invalid under P2280. The same happened for dynamic_cast. Furthermore, P2280 seems to be missing a change for static_cast from base to derived and for pointer-to-member access expressions, which both depend on the dynamic type and have historically been required to work.
Maybe a better rule would be that objects (not references) whose lifetime did not begin within E are treated as having a dynamic type that is their declared type.
(From submission #215.)
Consider:
void f() { std::nullptr_t np; // uninitialized, thus np contains an erroneous value constexpr void *p1 = np; // error: converted initializer is not a constant expression }
The lvalue-to-rvalue conversion on np does not actually read the value of np (7.3.2 [conv.lval] bullet 3.1), yet the situation is made ill-formed by 7.7 [expr.const] bullet 5.9.
Possible resolution:
Change in 7.7 [expr.const] bullet 5.9 as follows:
An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine (6.9.1 [intro.execution]), would evaluate one of the following:
- ...
- an lvalue-to-rvalue conversion (7.3.2 [conv.lval]) unless it is applied to
- a glvalue of type cv std::nullptr_t,
- a non-volatile glvalue that refers to an object that is usable in constant expressions, or
- a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
- ...
Subclause 7.7 [expr.const] paragraph 2 defines "constant-initialized" using the following rule:
A variable or temporary object o is constant-initialized if
- either it has an initializer or its default-initialization results in some initialization being performed, and
- ...
However, the rules for constexpr are slightly different, per 9.2.6 [dcl.constexpr] paragraph 6:
A constexpr specifier used in an object declaration declares the object as const. Such an object shall have literal type and shall be initialized. ...
The difference manifests for an example such as:
struct S {}; int main() { constexpr S s; // OK constexpr S s2 = s; // error: s is not constant-initialized }
Is the difference intentional?
According to 8.7.4 [stmt.return] paragraph 1,
A return statement with any other operand shall be used only in a function whose return type is not cv void; the return statement initializes the glvalue result or prvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand.
It is not clear what a “glvalue result” is or what it means to initialize it.
Suggested resolution:
A return statement with any other operand shall be used only in a function whose return type is not cv void
;. If the function call is a prvalue, the return statement initializes theglvalue result orprvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand. Otherwise, the return statement is equivalent to the following hypothetical declarationT t = e;
If the operand of the return statement, X, is a comma expression without parentheses, e is (X), otherwise e is X. T is the return type of the function call; the invented variable t is the result of the function call.
Notes from the August, 2021 teleconference:
A simpler approach would be simply to use a phrase like “returned object or reference” in place of the current wording referring to glvalues and prvalues. This change was regarded as editorial. The issue will remain in "review" status until CWG can look over the wording change.