Annex C (informative) Compatibility [diff]

Change: Remove the type alias allocator<T>​::​is_always_equal.

Rationale: Non-empty allocator classes derived from allocator needed to explicitly define an is_always_equal member type so that allocator_traits would not use the one from the allocator base class.

Effect on original feature: It is simpler to correctly define an allocator class with an allocator base class.

For example: template <class T> struct MyAlloc : allocator<T> { int tag; }; static_assert(!allocator_traits<MyAlloc<int>>::is_always_equal); // Error in C++ 2023, // OK in C++ 2026

Change: Remove the basic_string​::​reserve() overload with no parameters.

Rationale: The overload of reserve with no parameters is redundant.

The shrink_to_fit member function can be used instead.

Effect on original feature: A valid C++ 2023 program that calls reserve() on a basic_string object may fail to compile.

The old functionality can be achieved by calling shrink_to_fit() instead, or the function call can be safely eliminated with no side effects.

Change: Remove header <codecvt> and all its contents.

Rationale: The header has been deprecated for the previous three editions of this standard and no longer implements the current Unicode standard, supporting only the obsolete UCS-2 encoding.

Ongoing support is at implementer's discretion, exercising freedoms granted by [zombie.names].

Effect on original feature: A valid C++ 2023 program #include-ing the header or importing the header unit may fail to compile.

Code that uses any of the following names by importing the standard library modules may fail to compile:

• (3.1)
  codecvt_mode,
• (3.2)
  codecvt_utf16,
• (3.3)
  codecvt_utf8,
• (3.4)
  codecvt_utf8_utf16,
• (3.5)
  consume_header,
• (3.6)
  generate_header, and
• (3.7)
  little_endian.