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1325.bitset

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

Priority:Not Prioritized

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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 ambiguitybetween 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, whichcould be easily added and constitutes an important use-case. Forexample, the following code would invoke UB with the currentP/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) ofbasic_string<char>(s)

• it promotes a consistency between the twobitsetconstructors that take aconst std::string& and aconst char*, respectively, while practice established bystd::basic_string would recommend a different set ofparameters. In particular, the constructor ofstd::basic_string that takes aconst char* doesnot have apos 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> in22.9.2[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>explicitbitset(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 classbitset<N> as if bybitset(string(str)).

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