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C++

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

[edit]

Memory management library

voidify

(exposition only*)

Allocators

allocator
allocator_traits (C++11)
allocation_result (C++23)
scoped_allocator_adaptor (C++11)
pmr::polymorphic_allocator (C++17)

allocator_arg (C++11)
uses_allocator (C++11)
uses_allocator_construction_args (C++20)
make_obj_using_allocator (C++20)
uninitialized_construct_using_allocator (C++20)

Uninitialized memory algorithms

uninitialized_copy
uninitialized_fill
uninitialized_move (C++17)
uninitialized_copy_n (C++11)
uninitialized_fill_n
uninitialized_move_n (C++17)
construct_at (C++20)

uninitialized_default_construct (C++17)
uninitialized_value_construct (C++17)
destroy (C++17)
uninitialized_default_construct_n (C++17)
uninitialized_value_construct_n (C++17)
destroy_n (C++17)
destroy_at (C++17)

Constrained uninitialized memory algorithms

ranges::uninitialized_copy (C++20)
ranges::uninitialized_fill (C++20)
ranges::uninitialized_move (C++20)
ranges::uninitialized_copy_n (C++20)
ranges::uninitialized_fill_n (C++20)
ranges::uninitialized_move_n (C++20)
ranges::construct_at (C++20)

ranges::uninitialized_default_construct (C++20)
ranges::uninitialized_value_construct (C++20)
ranges::destroy (C++20)
ranges::uninitialized_default_construct_n (C++20)
ranges::uninitialized_value_construct_n (C++20)
ranges::destroy_n (C++20)
ranges::destroy_at (C++20)

Memory resources

pmr::memory_resource (C++17)
pmr::get_default_resource (C++17)
pmr::set_default_resource (C++17)
pmr::new_delete_resource (C++17)
pmr::pool_options (C++17)

pmr::null_memory_resource (C++17)
pmr::synchronized_pool_resource (C++17)
pmr::unsynchronized_pool_resource (C++17)
pmr::monotonic_buffer_resource (C++17)

Explicit lifetime management
start_lifetime_as (C++23)
start_lifetime_as_array (C++23)

Types for composite class design
indirect (C++26)
polymorphic (C++26)

Uninitialized storage(until C++20)

raw_storage_iterator (until C++20*)
get_temporary_buffer (until C++20*)

return_temporary_buffer (until C++20*)

Garbage collector support(until C++23)

declare_reachable (C++11)(until C++23)
declare_no_pointers (C++11)(until C++23)
pointer_safety (C++11)(until C++23)

undeclare_reachable (C++11)(until C++23)
undeclare_no_pointers (C++11)(until C++23)
get_pointer_safety (C++11)(until C++23)

Low level memory
management

operator newoperator new[]

operator deleteoperator delete[]

(C++11)

(C++11)

(C++17)

(C++20)

(C++17)

(C++11)

(C++11)

(C++11)

(until C++17*)

(C++11)

(C++17)

(C++26)

owner_equal

(C++26)

enable_shared_from_this

(C++11)

(C++11)

(C++11)

(C++23)

(C++23)

(C++11)

(C++20)

(C++11)

(C++11)

(C++20)

is_sufficiently_aligned

(C++26)

C Library

malloc
calloc
realloc

free
aligned_alloc (C++17)

[edit]

Defined in header`<memory>`
struct owner_equal;		(since C++26)

This function object provides owner-based (as opposed to value-based) mixed-type equal comparison of bothstd::weak_ptr andstd::shared_ptr. The comparison is such that two smart pointers compare equivalent only if they are both empty or if they share ownership, even if the values of the raw pointers obtained byget() are different (e.g. because they point at different subobjects within the same object).

1) Owner-based mixed-type equal comparison is not provided for types other thanstd::shared_ptr andstd::weak_ptr.

2) The owner-based mixed-type equal comparison ofstd::shared_ptr andstd::weak_ptr.

It is the preferred comparison predicate when building unordered associative containers withstd::shared_ptr andstd::weak_ptr as keys together withstd::owner_hash, that is,std::unordered_map<std::shared_ptr<T>, U, std::owner_hash, std::owner_equal> orstd::unordered_map<std::weak_ptr<T>, U, std::owner_hash, std::owner_equal>.

3)std::owner_equal deduces the parameter types from the arguments.

[edit]Nested types

Nested type	Definition
`is_transparent`	unspecified

[edit]Member functions

operator()

compares its arguments using owner-based semantics
(function)

std::owner_equal::operator()

template<class T,class U> bool operator()(conststd::shared_ptr<T>& lhs, conststd::shared_ptr<U>& rhs)constnoexcept;		(since C++26)
template<class T,class U> bool operator()(conststd::shared_ptr<T>& lhs, conststd::weak_ptr<U>& rhs)constnoexcept;		(since C++26)
template<class T,class U> bool operator()(conststd::weak_ptr<T>& lhs, conststd::shared_ptr<U>& rhs)constnoexcept;		(since C++26)
template<class T,class U> bool operator()(conststd::weak_ptr<T>& lhs, conststd::weak_ptr<U>& rhs)constnoexcept;		(since C++26)

Compareslhs andrhs using owner-based semantics. Effectively callslhs.owner_equal(rhs).

The equal comparison is an equivalence relation.

lhs andrhs are equivalent only if they are both empty or share ownership.

Parameters

lhs, rhs

shared-ownership pointers to compare

Return value

true iflhs andrhs are both empty or share ownership as determined by the owner-based equal comparison,false otherwise.

[edit]Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_smart_ptr_owner_equality`	`202306L`	(C++26)	Enabling the use of`std::shared_ptr` and`std::weak_ptr` as keys inunordered associative containers

[edit]See also

owner_equal (C++26)	provides owner-based equal comparison of shared pointers (public member function of`std::shared_ptr<T>`)[edit]
owner_equal (C++26)	provides owner-based equal comparison of weak pointers (public member function of`std::weak_ptr<T>`)[edit]