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4259. P1148R0 changed the return values of searching functions of std::basic_string on some platforms

Section: 27.4.3.8.2 [string.find] Status: C++26 Submitter: Jiang An Opened: 2025-05-05 Last modified: 2026-06-09

Priority: 3

View all other issues in [string.find].

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Discussion:

P1148R0 respecified the searching functions of std::basic_string to return corresponding string view type's npos member constant (equal to std::size_t(-1)), converted to the string type S's member S::size_type, when the search fails. Before the change, S::npos (equal to S::size_type(-1)) was returned on failure.

On platforms where std::size_t isn't the widest unsigned integer type (e.g. on usual 32-bit platforms), the return value can change. Because there can be an allocator with a wider size_type, and when the basic_string type S uses such an allocator, S::size_type is specified to be that type, which in turn makes S::size_type(std::size_t(-1)) not equal to S::size_type(-1).

Do we want to restore the old return values?

Previous resolution [SUPERSEDED]

This wording is relative to N5008.

  1. Modify 27.4.3.8.2 [string.find] as indicated:

    template<class T>
  constexpr size_type find(const T& t, size_type pos = 0) const noexcept(see below);
[…]
template<class T>
  constexpr size_type find_last_not_of(const T& t, size_type pos = npos) const noexcept(see below);

    -2- Constraints: […]

    -3- Effects: Let G be the name of the function. Equivalent to: 

    basic_string_view<charT, traits> s = *this, sv = t;
return s.G(sv, pos);
if (auto result = s.G(sv, pos); result == size_t(-1))
  return npos;
else
  return result;

[2025-06-10, reflector discussion]

During reflector discussion of this issue there was a preference to adjust the proposed wording to use s.npos instead of size_t(-1).

[2025-10-21; Reflector poll.]

Set priority to 3 after reflector poll.

[2025-12-04; Reflector poll.]

Set status to Tentatively Ready after five votes in favour during reflector poll.

[Croydon 2026-03-28; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

  1. Modify 27.4.3.8.2 [string.find] as indicated:

    template<class T>
  constexpr size_type find(const T& t, size_type pos = 0) const noexcept(see below);
[…]
template<class T>
  constexpr size_type find_last_not_of(const T& t, size_type pos = npos) const noexcept(see below);

    -2- Constraints: […]

    -3- Effects: Let G be the name of the function. Equivalent to: 

    basic_string_view<charT, traits> s = *this, sv = t;
return s.G(sv, pos);
if (auto result = s.G(sv, pos); result == s.npos)
  return npos;
else
  return result;