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Showing content from https://cplusplus.github.io/LWG/issue4294 below:

bitset(const CharT*) constructor needs to be constrained

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of New status.

Section: 22.9.2.2 [bitset.cons] Status: New Submitter: Jonathan Wakely Opened: 2025-07-12 Last modified: 2025-07-15

Priority: Not Prioritized

View all other issues in [bitset.cons].

View all issues with New status.

Discussion:

This code might be ill-formed, with an error outside the immediate context that users cannot prevent:

#include <bitset>
struct NonTrivial { ~NonTrivial() { } };
static_assert( ! std::is_constructible_v<std::bitset<1>, NonTrivial*> );

The problem is that the bitset(const CharT*) constructor tries to instantiate basic_string_view<NonTrivial> to find its size_type, and that instantiation might ill-formed, e.g. if std::basic_string_view or std::char_traits has a static assert enforcing the requirement for their character type to be sufficiently char-like. 27.1 [strings.general] defines a char-like type as "any non-array trivially copyable standard-layout (6.9.1 [basic.types.general]) type T where is_trivially_default_constructible_v<T> is true."

Proposed resolution:

This wording is relative to N5008.

1. Modify 22.9.2.2 [bitset.cons] as indicated:

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

   -?- Constraints:

   • is_array_v<charT> is false,
   • is_trivially_copyable_v<charT> is true,
   • is_standard_layout_v<charT> is true, and
   • is_trivially_default_constructible_v<charT> is true.

   -8- Effects: As if by:

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

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