Issue 1325: bitset

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1325. `bitset`

Section: 22.9.2.2 [bitset.cons] Status: C++11 Submitter: Christopher Jefferson Opened: 2010-03-07 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [bitset.cons].

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Discussion:

As mentioned on the boost mailing list:

The following code, valid in C++03, is broken in C++0x due to ambiguity between the "unsigned long long" and "char*" constructors.

#include <bitset>
std::bitset<10> b(0);

[ The proposed resolution has been reviewed by Stephan T. Lavavej. ]

[ Post-Rapperswil ]

The proposed resolution has two problems:

it fails to provide support for non-terminated strings, which could be easily added and constitutes an important use-case. For example, the following code would invoke UB with the current P/R:
```
char s[4] = { '0', '1', '0', '1' }; // notice: not null-terminated!
bitset<4> b(s, 0, 4);
```
because it requires the evaluation (under the as-if rule, to be fair, but it doesn't matter) of basic_string<char>(s)
it promotes a consistency between the two bitset constructors that take a const std::string& and a const char*, respectively, while practice established by std::basic_string would recommend a different set of parameters. In particular, the constructor of std::basic_string that takes a const char* does not have a pos parameter

Moved to Tentatively Ready with revised wording provided by Alberto Ganesh Babati after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

In the synopsis of header <bitset> in 22.9.2 [template.bitset]/1, replace the fourth bitset constructor:

explicit bitset(const char *str);
template <class charT>
  explicit bitset(
    const charT *str,
    typename basic_string<charT>::size_type n = basic_string<charT>::npos,
    charT zero = charT('0'), charT one = charT('1'));

In 22.9.2.2 [bitset.cons]/8:

explicit bitset(const char *str);
template <class charT>
explicit
bitset(const charT *str,
       typename basic_string<charT>::size_type n = basic_string<charT>::npos,
       charT zero = charT('0'), charT one = charT('1'));

Effects: Constructs an object of class bitset<N> as if by ~~bitset(string(str)).~~


bitset(
  n == basic_string<charT>::npos
    ? basic_string<charT>(str)
    : basic_string<charT>(str, n),
  0, n, zero, one)

1325. bitset

1325. `bitset`