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].

View all issues with C++11 status.

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:

1. 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'));
   

2. 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)