[diff.cpp23.depr]

# Annex C (informative) Compatibility [[diff]](./#diff)

## C.1 C++ and ISO C++ 2023 [[diff.cpp23]](diff.cpp23#depr)

### C.1.11 [[depr]](depr "Annex D (normative) Compatibility features"): compatibility features [diff.cpp23.depr]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L358)

**Change:** Remove the type alias allocator<T>​::​is_always_equal[.](#1.sentence-1)

  


**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[.](#1.sentence-2)

  


**Effect on original feature:** It is simpler to correctly define an allocator class with an allocator base
class[.](#1.sentence-3)

[*Example [1](#example-1)*: 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 — *end example*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L380)

**Change:** Removal of atomic access API for shared_ptr objects[.](#2.sentence-1)

  


**Rationale:** The old behavior was brittle[.](#2.sentence-2)

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[.](#2.sentence-3)

A complete
type-safe replacement is provided in the form of atomic<shared_ptr<T>>[.](#2.sentence-4)

  


**Effect on original feature:** A valid C++ 2023 program that relies on the presence of the removed functions
may fail to compile[.](#2.sentence-5)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L392)

**Change:** Remove the basic_string​::​reserve() overload with no parameters[.](#3.sentence-1)

  


**Rationale:** The overload of reserve with no parameters is redundant[.](#3.sentence-2)

The shrink_to_fit member function can be used instead[.](#3.sentence-3)

  


**Effect on original feature:** A valid C++ 2023 program that calls reserve() on a basic_string object may fail to compile[.](#3.sentence-4)

The old functionality can be achieved by calling shrink_to_fit() instead,
or the function call can be safely eliminated with no side effects[.](#3.sentence-5)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L404)

**Change:** Remove header <codecvt> and all its contents[.](#4.sentence-1)

  


**Rationale:** The header has been deprecated for the previous three editions of this document
and no longer implements the current Unicode standard, supporting only the
obsolete UCS-2 encoding[.](#4.sentence-2)

Ongoing support is at implementer's discretion,
exercising freedoms granted by [[zombie.names]](zombie.names "16.4.5.3.2 Zombie names")[.](#4.sentence-3)

  


**Effect on original feature:** A valid C++ 2023 program #include-ing the header or importing the
header unit may fail to compile[.](#4.sentence-4)

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

- [(4.1)](#4.1)

codecvt_mode,

- [(4.2)](#4.2)

codecvt_utf16,

- [(4.3)](#4.3)

codecvt_utf8,

- [(4.4)](#4.4)

codecvt_utf8_utf16,

- [(4.5)](#4.5)

consume_header,

- [(4.6)](#4.6)

generate_header, and

- [(4.7)](#4.7)

little_endian[.](#4.sentence-5)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L427)

**Change:** Remove header <strstream> and all its contents[.](#5.sentence-1)

  


**Rationale:** The header has been deprecated since the original C++ standard; the[<spanstream>](span.streams.overview#header:%3cspanstream%3e "31.9.1 Overview [span.streams.overview]") header provides an updated, safer facility[.](#5.sentence-2)

Ongoing support is at implementer's discretion,
exercising freedoms granted by [[zombie.names]](zombie.names "16.4.5.3.2 Zombie names")[.](#5.sentence-3)

  


**Effect on original feature:** A valid C++ 2023 program #include-ing the header or importing the
header unit may become ill-formed[.](#5.sentence-4)

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

- [(5.1)](#5.1)

istrstream

- [(5.2)](#5.2)

ostrstream

- [(5.3)](#5.3)

strstream

- [(5.4)](#5.4)

strstreambuf

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/compatibility.tex#L446)

**Change:** Remove convenience interfaces wstring_convert andwbuffer_convert[.](#6.sentence-1)

  


**Rationale:** These features were underspecified with no clear error reporting mechanism and
were deprecated for the last three editions of this document[.](#6.sentence-2)

Ongoing support is at implementer's discretion,
exercising freedoms granted by [[zombie.names]](zombie.names "16.4.5.3.2 Zombie names")[.](#6.sentence-3)

  


**Effect on original feature:** A valid C++ 2023 program using these interfaces may become ill-formed[.](#6.sentence-4)