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: Removal of atomic access API for shared_ptr objects.

Rationale: The old behavior was brittle.

shared_ptr objects using the old API were not protected by the type system, and certain interactions with code not using this API would, in some cases, silently produce undefined behavior.

A complete type-safe replacement is provided in the form of atomic<shared_ptr<T>>.

Effect on original feature: A valid C++ 2023 program that relies on the presence of the removed functions may fail to compile.

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:

• (4.1)
  codecvt_mode,
• (4.2)
  codecvt_utf16,
• (4.3)
  codecvt_utf8,
• (4.4)
  codecvt_utf8_utf16,
• (4.5)
  consume_header,
• (4.6)
  generate_header, and
• (4.7)
  little_endian.

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

Rationale: The header has been deprecated since the original C++ standard; the <spanstream> header provides an updated, safer facility.

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 become ill-formed.

Code that uses any of the following classes by importing one of the standard library modules may become ill-formed:

• (5.1)
  istrstream
• (5.2)
  ostrstream
• (5.3)
  strstream
• (5.4)
  strstreambuf

Change: Remove convenience interfaces wstring_convert and wbuffer_convert.

Rationale: These features were underspecified with no clear error reporting mechanism and were deprecated for the last three editions of this standard.

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

Effect on original feature: A valid C++ 2023 program using these interfaces may become ill-formed.