P2447R5
std::span over an initializer list

1. Changelog

• R5 (post-LEWG 2023):

  • Reintroduce feature-test macro __cpp_lib_span_initializer_list.

  • Add Annex C entries to § 6 Proposed wording; LEWG points out that it’s easy for LWG to eliminate insertions they deem redundant.

• R4 (pre-Varna 2023):

  • Reorganize references to [P2752] and fix HTML goofs in proposed wording.

• R3:

  • Changed primary authorship from Federico Kircheis to Arthur O’Dwyer.

  • Removed R2’s feature-test macro __cpp_lib_span_init; it didn’t seem motivated.

• R2:

  • Discussed in LEWG telecon, 2022-07-26

2. Background

C++17 added string_view as a "view" over constant string data. Its main purpose is as a lightweight drop-in replacement for const string& function parameters.

int take(const std::string& s) {
    return s[0] + s.size();
}

int take(std::string_view sv) {
    return s[0] + s.size();
}

std::string abc = "abc";
take(abc);

std::string abc = "abc";
take(abc);

take("abc");

take("abc");

take(std::string("abc"));

take(std::string("abc"));
take(std::string_view("abc"));

C++20 added span<const T> as a "view" over constant contiguous data of type T (such as arrays and vectors). One of its main purposes (although not its only one) is as a lightweight drop-in replacement for const vector<T>& function parameters.

int take(const std::vector<int>& v) {
    return v[0] + v.size();
}

int take(std::span<const int> v) {
    return v[0] + v.size();
}

std::vector<int> abc = {1,2,3};
take(abc);

std::vector<int> abc = {1,2,3};
take(abc);

take({1,2,3});

take({}); // size=0
take({{1,2,3}});
take(std::vector{1,2,3});
take(std::initializer_list<int>{1,2,3});

take({}); // size=0
take({{1,2,3}});
take(std::vector{1,2,3});
take(std::initializer_list<int>{1,2,3});
take(std::span<const int>({1,2,3}));

This table has a conspicuous gap. The singly-braced initializer list {1,2,3} is implicitly convertible to std::vector<int>, but not to std::span<const int>.

3. Solution

We propose simply that std::span<const T> should be convertible from an appropriate braced-initializer-list. In practice this means adding a constructor from std::initializer_list.

3.1. Implementation experience

This proposal has been implemented in Arthur’s fork of libc++ since October 2021. See "span should have a converting constructor from initializer_list" (2021-10-03) and [Patch].

3.2. What about dangling?

span, like string_view, is specifically designed to bind to rvalues as well as lvalues. This is what lets us write useful code like:

int take(std::string_view s);
std::string give_string();
int x = take(give_string());

int take(std::span<const int> v);
std::vector<int> give_vector();
int x = take(give_vector());

Careless misuse of string_view and span outside a function parameter list can dangle:

std::string_view s = give_string(); // dangles
std::span<const int> v = give_vector(); // dangles

P2447 doesn’t propose to increase the risk in this area; dangling is already likely when span or string_view is carelessly misused. We simply propose to close the ergonomic syntax gap between span and string_view.

std::string_view      s = "abc";  // OK
std::string_view      s = "abc"s; // dangles
std::span<const char> v = "abc";  // OK
std::span<const char> v = "abc"s; // dangles

std::string_view      s = "abc";  // OK
std::string_view      s = "abc"s; // dangles
std::span<const char> v = "abc";  // OK
std::span<const char> v = "abc"s; // dangles

std::span<const int> v = std::vector{1,2,3}; // dangles
auto v = std::span<const int>({1,2,3});      // dangles
std::span<const int> v = {{1,2,3}};          // dangles

std::span<const int> v = std::vector{1,2,3}; // dangles
auto v = std::span<const int>({1,2,3});      // dangles
std::span<const int> v = {{1,2,3}};          // dangles
std::span<const int> v = {1,2,3};            // dangles

3.3. Why not just double the braces?

Since we can already write

std::span<const int> v = {{1,2,3}}; // dangles

then why not call that "good enough"? Why do we need to be able to use a single set of braces?

Well, a single set of braces is good enough for vector, and we want span to be a drop-in replacement for vector in function parameter lists, so we need to support the syntax vector does. There was a period right after C++11 where some people were writing

std::vector<int> v = {{1,2,3}};

but by C++14 we had settled firmly on "one set of braces" as the preferred style (matching the preferred style for C arrays, pairs, tuples, etc.)

So I prefer to turn the question around and say: Since we can already implicitly treat {{1,2,3}} as a span, how could there be any additional harm in treating {1,2,3} as a span?

3.3.1. Better performance via synergy with P2752

[P2752], adopted as a DR at Varna 2023, allows a constant initializer_list like {1,2,3} to refer to a backing array in static storage, rather than forcing all backing arrays onto the stack. This doesn’t change anything dangling-wise: referring to the backing array of an initializer_list outside that initializer_list’s lifetime remains undefined behavior.

std::string_view s = "abc";            // OK, no dangling
std::span<const int> v1 = {1,2,3};     // dangles, even after P2752

Today, {{1,2,3}} converts to span by materializing a temporary const int[3] on the stack. Tomorrow, if P2447 is adopted, {1,2,3} will convert to span via an initializer_list that refers to a backing array in static storage. In other words, the initializer_list constructor we propose here in P2447 is "more optimizer-friendly" than today’s array-temporary constructor.

This example (Godbolt) shows how P2447 lets us benefit from P2752’s optimization:

int perf(std::span<const int>);

int test() {
    return perf({{1,2,3}});
}

In each row, there’s no performance difference between the single-braced or double-braced form. But the only way to reach the bottom row (tail-call, no stack usage) in either column is to adopt P2447, which by a happy coincidence also permits the single-braced form.

4. Breaking changes

This change will, of course, break some code (most of it pathological). We propose adding three new examples to Annex C. But any change to overload sets can break code, and sometimes LWG doesn’t bother with an Annex C entry. For example, C++23 adopted [P1425] "Iterator-pair constructors for stack and queue" with no change to Annex C, despite its breaking code like this:

void zero(queue<int>);
void zero(pair<int*,int*>);
int a[10];
void test() { zero({a, a+10}); }

Before: Calls zero(pair<int, int>).
After P1425: Ambiguous.
To fix: Eliminate the ambiguous overloading, or cast the argument to pair.

Therefore, we’re happy for LWG to eliminate any or all of our proposed Annex C entries if they’re going too far into the weeds.

For explanation and suggested fixits for each of the Annex C examples included in § 6 Proposed wording, see P2447R4 §4.

5. Straw polls

P2447R4 was presented to LEWG on 2023-09-12. The following polls were taken. The first was classified as "no consensus," the second as "weak consensus."

6. Proposed wording

Modify [span.syn] as follows:

#include <initializer_list>     // see [initializer.list.syn]

Modify [span.overview] as follows:

  template<size_t N>
    constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
  template<class T, size_t N>
    constexpr span(array<T, N>& arr) noexcept;
  template<class T, size_t N>
    constexpr span(const array<T, N>& arr) noexcept;
  template<class R>
    constexpr explicit(extent != dynamic_extent) span(R&& r);

  constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il) noexcept;
  constexpr span(const span& other) noexcept = default;
  template<class OtherElementType, size_t OtherExtent>
    constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;

Modify [span.cons] as follows:

constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il) noexcept;

Constraints: is_const_v<element_type> is true.

Preconditions: If extent is not equal to dynamic_extent, then il.size() is equal to extent.

Effects: Initializes data_ with il.begin() and size_ with il.size().

Modify [diff.cpp26] as follows:

Note: For explanation and suggested fixits for each of these examples, see P2447R4 §4. My understanding is that Annex C wording shouldn’t contain that extra material.

[containers]: containers library

1․ Affected subclause: [span.overview]
Change: span<const T> is constructible from initializer_list<T>.
Rationale: Permit passing a braced initializer list to a function taking span.
Effect on original feature: Valid C++ 2023 code that relies on the lack of this constructor may refuse to compile, or change behavior. For example:

void one(pair<int, int>);    // #1
void one(span<const int>);   // #2
void t1() { one({1,2}); }  // ambiguous between #1 and #2; previously called #1

void two(span<const int, 2>);
void t2() { two({{1,2}}); }  // ill-formed; previously well-formed

void *a[10];
int x = span<void* const>{a, 0}.size(); // x is 2; previously 0
any b[10];
int y = span<const any>{b, b+10}.size(); // y is 2; previously 10

Add a feature-test macro to [version.syn]/2 as follows:

#define __cpp_lib_span                    202002L // also in <span>
#define __cpp_lib_span_initializer_list   XXYYZZL // also in <span>
#define __cpp_lib_spanstream              202106L // also in <spanstream>

7. Acknowledgments

• Thanks to Federico Kircheis for writing the first drafts of this paper.

• Thanks to Jarrad Waterloo for his support.