20 Memory management library [mem]

20.2 Memory [memory]

20.2.1 In general [memory.general]

Subclause [memory] describes the contents of the header <memory> and some of the contents of the header <cstdlib>.

20.2.2 Header <memory> synopsis [memory.syn]

The header <memory> defines several types and function templates that describe properties of pointers and pointer-like types, manage memory for containers and other template types, destroy objects, and construct objects in uninitialized memory buffers ([pointer.traits]–[specialized.addressof] and [specialized.algorithms]).
The header also defines the templates unique_ptr, shared_ptr, weak_ptr, out_ptr_t, inout_ptr_t, and various function templates that operate on objects of these types ([smartptr]).
Let POINTER_OF(T) denote a type that is
  • T​::​pointer if the qualified-id T​::​pointer is valid and denotes a type,
  • otherwise, T​::​element_type* if the qualified-id T​::​element_type is valid and denotes a type,
  • otherwise, pointer_traits<T>​::​element_type*.
Let POINTER_OF_OR(T, U) denote a type that is:
  • POINTER_OF(T) if POINTER_OF(T) is valid and denotes a type,
  • otherwise, U.
#include <compare> // see [compare.syn] namespace std { // [pointer.traits], pointer traits template<class Ptr> struct pointer_traits; // freestanding template<class T> struct pointer_traits<T*>; // freestanding // [pointer.conversion], pointer conversion template<class T> constexpr T* to_address(T* p) noexcept; // freestanding template<class Ptr> constexpr auto to_address(const Ptr& p) noexcept; // freestanding // [ptr.align], pointer alignment void* align(size_t alignment, size_t size, void*& ptr, size_t& space); // freestanding template<size_t N, class T> [[nodiscard]] constexpr T* assume_aligned(T* ptr); // freestanding // [obj.lifetime], explicit lifetime management template<class T> T* start_lifetime_as(void* p) noexcept; // freestanding template<class T> const T* start_lifetime_as(const void* p) noexcept; // freestanding template<class T> volatile T* start_lifetime_as(volatile void* p) noexcept; // freestanding template<class T> const volatile T* start_lifetime_as(const volatile void* p) noexcept; // freestanding template<class T> T* start_lifetime_as_array(void* p, size_t n) noexcept; // freestanding template<class T> const T* start_lifetime_as_array(const void* p, size_t n) noexcept; // freestanding template<class T> volatile T* start_lifetime_as_array(volatile void* p, size_t n) noexcept; // freestanding template<class T> const volatile T* start_lifetime_as_array(const volatile void* p, // freestanding size_t n) noexcept; // [allocator.tag], allocator argument tag struct allocator_arg_t { explicit allocator_arg_t() = default; }; // freestanding inline constexpr allocator_arg_t allocator_arg{}; // freestanding // [allocator.uses], uses_allocator template<class T, class Alloc> struct uses_allocator; // freestanding // [allocator.uses.trait], uses_allocator template<class T, class Alloc> constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value; // freestanding // [allocator.uses.construction], uses-allocator construction template<class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding Args&&... args) noexcept; template<class T, class Alloc, class Tuple1, class Tuple2> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept; template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept; // freestanding template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding U&& u, V&& v) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding pair<U, V>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding const pair<U, V>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding pair<U, V>&& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding const pair<U, V>&& pr) noexcept; template<class T, class Alloc, pair-like P> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding P&& p) noexcept; template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, // freestanding U&& u) noexcept; template<class T, class Alloc, class... Args> constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args); // freestanding template<class T, class Alloc, class... Args> constexpr T* uninitialized_construct_using_allocator(T* p, // freestanding const Alloc& alloc, Args&&... args); // [allocator.traits], allocator traits template<class Alloc> struct allocator_traits; // freestanding template<class Pointer, class SizeType = size_t> struct allocation_result { // freestanding Pointer ptr; SizeType count; }; // [default.allocator], the default allocator template<class T> class allocator; template<class T, class U> constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept; // [specialized.addressof], addressof template<class T> constexpr T* addressof(T& r) noexcept; // freestanding template<class T> const T* addressof(const T&&) = delete; // freestanding // [specialized.algorithms], specialized algorithms // [special.mem.concepts], special memory concepts template<class I> concept nothrow-input-iterator = see below; // exposition only template<class I> concept nothrow-forward-iterator = see below; // exposition only template<class S, class I> concept nothrow-sentinel-for = see below; // exposition only template<class R> concept nothrow-input-range = see below; // exposition only template<class R> concept nothrow-forward-range = see below; // exposition only template<class NoThrowForwardIterator> void uninitialized_default_construct(NoThrowForwardIterator first, // freestanding NoThrowForwardIterator last); template<class ExecutionPolicy, class NoThrowForwardIterator> void uninitialized_default_construct(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last); template<class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_default_construct_n(NoThrowForwardIterator first, Size n); // freestanding template<class ExecutionPolicy, class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_default_construct_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n); namespace ranges { template<nothrow-forward-iterator I, nothrow-sentinel-for<I> S> requires default_initializable<iter_value_t<I>> I uninitialized_default_construct(I first, S last); // freestanding template<nothrow-forward-range R> requires default_initializable<range_value_t<R>> borrowed_iterator_t<R> uninitialized_default_construct(R&& r); // freestanding template<nothrow-forward-iterator I> requires default_initializable<iter_value_t<I>> I uninitialized_default_construct_n(I first, iter_difference_t<I> n); // freestanding } template<class NoThrowForwardIterator> void uninitialized_value_construct(NoThrowForwardIterator first, // freestanding NoThrowForwardIterator last); template<class ExecutionPolicy, class NoThrowForwardIterator> void uninitialized_value_construct(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last); template<class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_value_construct_n(NoThrowForwardIterator first, Size n); // freestanding template<class ExecutionPolicy, class NoThrowForwardIterator, class Size> NoThrowForwardIterator uninitialized_value_construct_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n); namespace ranges { template<nothrow-forward-iterator I, nothrow-sentinel-for<I> S> requires default_initializable<iter_value_t<I>> I uninitialized_value_construct(I first, S last); // freestanding template<nothrow-forward-range R> requires default_initializable<range_value_t<R>> borrowed_iterator_t<R> uninitialized_value_construct(R&& r); // freestanding template<nothrow-forward-iterator I> requires default_initializable<iter_value_t<I>> I uninitialized_value_construct_n(I first, iter_difference_t<I> n); // freestanding } template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy(InputIterator first, // freestanding InputIterator last, NoThrowForwardIterator result); template<class ExecutionPolicy, class ForwardIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] ForwardIterator first, ForwardIterator last, NoThrowForwardIterator result); template<class InputIterator, class Size, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy_n(InputIterator first, Size n, // freestanding NoThrowForwardIterator result); template<class ExecutionPolicy, class ForwardIterator, class Size, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_copy_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] ForwardIterator first, Size n, NoThrowForwardIterator result); namespace ranges { template<class I, class O> using uninitialized_copy_result = in_out_result<I, O>; // freestanding template<input_iterator I, sentinel_for<I> S1, nothrow-forward-iterator O, nothrow-sentinel-for<O> S2> requires constructible_from<iter_value_t<O>, iter_reference_t<I>> uninitialized_copy_result<I, O> uninitialized_copy(I ifirst, S1 ilast, O ofirst, S2 olast); // freestanding template<input_range IR, nothrow-forward-range OR> requires constructible_from<range_value_t<OR>, range_reference_t<IR>> uninitialized_copy_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>> uninitialized_copy(IR&& in_range, OR&& out_range); // freestanding template<class I, class O> using uninitialized_copy_n_result = in_out_result<I, O>; // freestanding template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S> requires constructible_from<iter_value_t<O>, iter_reference_t<I>> uninitialized_copy_n_result<I, O> uninitialized_copy_n(I ifirst, iter_difference_t<I> n, // freestanding O ofirst, S olast); } template<class InputIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(InputIterator first, // freestanding InputIterator last, NoThrowForwardIterator result); template<class ExecutionPolicy, class ForwardIterator, class NoThrowForwardIterator> NoThrowForwardIterator uninitialized_move(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] ForwardIterator first, ForwardIterator last, NoThrowForwardIterator result); template<class InputIterator, class Size, class NoThrowForwardIterator> pair<InputIterator, NoThrowForwardIterator> uninitialized_move_n(InputIterator first, Size n, // freestanding NoThrowForwardIterator result); template<class ExecutionPolicy, class ForwardIterator, class Size, class NoThrowForwardIterator> pair<ForwardIterator, NoThrowForwardIterator> uninitialized_move_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] ForwardIterator first, Size n, NoThrowForwardIterator result); namespace ranges { template<class I, class O> using uninitialized_move_result = in_out_result<I, O>; // freestanding template<input_iterator I, sentinel_for<I> S1, nothrow-forward-iterator O, nothrow-sentinel-for<O> S2> requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>> uninitialized_move_result<I, O> uninitialized_move(I ifirst, S1 ilast, O ofirst, S2 olast); // freestanding template<input_range IR, nothrow-forward-range OR> requires constructible_from<range_value_t<OR>, range_rvalue_reference_t<IR>> uninitialized_move_result<borrowed_iterator_t<IR>, borrowed_iterator_t<OR>> uninitialized_move(IR&& in_range, OR&& out_range); // freestanding template<class I, class O> using uninitialized_move_n_result = in_out_result<I, O>; // freestanding template<input_iterator I, nothrow-forward-iterator O, nothrow-sentinel-for<O> S> requires constructible_from<iter_value_t<O>, iter_rvalue_reference_t<I>> uninitialized_move_n_result<I, O> uninitialized_move_n(I ifirst, iter_difference_t<I> n, // freestanding O ofirst, S olast); } template<class NoThrowForwardIterator, class T> void uninitialized_fill(NoThrowForwardIterator first, // freestanding NoThrowForwardIterator last, const T& x); template<class ExecutionPolicy, class NoThrowForwardIterator, class T> void uninitialized_fill(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last, const T& x); template<class NoThrowForwardIterator, class Size, class T> NoThrowForwardIterator uninitialized_fill_n(NoThrowForwardIterator first, Size n, const T& x); // freestanding template<class ExecutionPolicy, class NoThrowForwardIterator, class Size, class T> NoThrowForwardIterator uninitialized_fill_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n, const T& x); namespace ranges { template<nothrow-forward-iterator I, nothrow-sentinel-for<I> S, class T> requires constructible_from<iter_value_t<I>, const T&> I uninitialized_fill(I first, S last, const T& x); // freestanding template<nothrow-forward-range R, class T> requires constructible_from<range_value_t<R>, const T&> borrowed_iterator_t<R> uninitialized_fill(R&& r, const T& x); // freestanding template<nothrow-forward-iterator I, class T> requires constructible_from<iter_value_t<I>, const T&> I uninitialized_fill_n(I first, iter_difference_t<I> n, const T& x); // freestanding } // [specialized.construct], construct_at template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); // freestanding namespace ranges { template<class T, class... Args> constexpr T* construct_at(T* location, Args&&... args); // freestanding } // [specialized.destroy], destroy template<class T> constexpr void destroy_at(T* location); // freestanding template<class NoThrowForwardIterator> constexpr void destroy(NoThrowForwardIterator first, // freestanding NoThrowForwardIterator last); template<class ExecutionPolicy, class NoThrowForwardIterator> void destroy(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, NoThrowForwardIterator last); template<class NoThrowForwardIterator, class Size> constexpr NoThrowForwardIterator destroy_n(NoThrowForwardIterator first, // freestanding Size n); template<class ExecutionPolicy, class NoThrowForwardIterator, class Size> NoThrowForwardIterator destroy_n(ExecutionPolicy&& exec, // see [algorithms.parallel.overloads] NoThrowForwardIterator first, Size n); namespace ranges { template<destructible T> constexpr void destroy_at(T* location) noexcept; // freestanding template<nothrow-input-iterator I, nothrow-sentinel-for<I> S> requires destructible<iter_value_t<I>> constexpr I destroy(I first, S last) noexcept; // freestanding template<nothrow-input-range R> requires destructible<range_value_t<R>> constexpr borrowed_iterator_t<R> destroy(R&& r) noexcept; // freestanding template<nothrow-input-iterator I> requires destructible<iter_value_t<I>> constexpr I destroy_n(I first, iter_difference_t<I> n) noexcept; // freestanding } // [unique.ptr], class template unique_ptr template<class T> struct default_delete; // freestanding template<class T> struct default_delete<T[]>; // freestanding template<class T, class D = default_delete<T>> class unique_ptr; // freestanding template<class T, class D> class unique_ptr<T[], D>; // freestanding template<class T, class... Args> constexpr unique_ptr<T> make_unique(Args&&... args); // T is not array template<class T> constexpr unique_ptr<T> make_unique(size_t n); // T is U[] template<class T, class... Args> unspecified make_unique(Args&&...) = delete; // T is U[N] template<class T> constexpr unique_ptr<T> make_unique_for_overwrite(); // T is not array template<class T> constexpr unique_ptr<T> make_unique_for_overwrite(size_t n); // T is U[] template<class T, class... Args> unspecified make_unique_for_overwrite(Args&&...) = delete; // T is U[N] template<class T, class D> constexpr void swap(unique_ptr<T, D>& x, unique_ptr<T, D>& y) noexcept; // freestanding template<class T1, class D1, class T2, class D2> constexpr bool operator==(const unique_ptr<T1, D1>& x, // freestanding const unique_ptr<T2, D2>& y); template<class T1, class D1, class T2, class D2> bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); // freestanding template<class T1, class D1, class T2, class D2> bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); // freestanding template<class T1, class D1, class T2, class D2> bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); // freestanding template<class T1, class D1, class T2, class D2> bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); // freestanding template<class T1, class D1, class T2, class D2> requires three_way_comparable_with<typename unique_ptr<T1, D1>::pointer, typename unique_ptr<T2, D2>::pointer> compare_three_way_result_t<typename unique_ptr<T1, D1>::pointer, typename unique_ptr<T2, D2>::pointer> operator<=>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y); // freestanding template<class T, class D> constexpr bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept; // freestanding template<class T, class D> constexpr bool operator<(const unique_ptr<T, D>& x, nullptr_t); // freestanding template<class T, class D> constexpr bool operator<(nullptr_t, const unique_ptr<T, D>& y); // freestanding template<class T, class D> constexpr bool operator>(const unique_ptr<T, D>& x, nullptr_t); // freestanding template<class T, class D> constexpr bool operator>(nullptr_t, const unique_ptr<T, D>& y); // freestanding template<class T, class D> constexpr bool operator<=(const unique_ptr<T, D>& x, nullptr_t); // freestanding template<class T, class D> constexpr bool operator<=(nullptr_t, const unique_ptr<T, D>& y); // freestanding template<class T, class D> constexpr bool operator>=(const unique_ptr<T, D>& x, nullptr_t); // freestanding template<class T, class D> constexpr bool operator>=(nullptr_t, const unique_ptr<T, D>& y); // freestanding template<class T, class D> requires three_way_comparable<typename unique_ptr<T, D>::pointer> constexpr compare_three_way_result_t<typename unique_ptr<T, D>::pointer> operator<=>(const unique_ptr<T, D>& x, nullptr_t); // freestanding template<class E, class T, class Y, class D> basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const unique_ptr<Y, D>& p); // [util.smartptr.weak.bad], class bad_weak_ptr class bad_weak_ptr; // [util.smartptr.shared], class template shared_ptr template<class T> class shared_ptr; // [util.smartptr.shared.create], shared_ptr creation template<class T, class... Args> shared_ptr<T> make_shared(Args&&... args); // T is not array template<class T, class A, class... Args> shared_ptr<T> allocate_shared(const A& a, Args&&... args); // T is not array template<class T> shared_ptr<T> make_shared(size_t N); // T is U[] template<class T, class A> shared_ptr<T> allocate_shared(const A& a, size_t N); // T is U[] template<class T> shared_ptr<T> make_shared(); // T is U[N] template<class T, class A> shared_ptr<T> allocate_shared(const A& a); // T is U[N] template<class T> shared_ptr<T> make_shared(size_t N, const remove_extent_t<T>& u); // T is U[] template<class T, class A> shared_ptr<T> allocate_shared(const A& a, size_t N, const remove_extent_t<T>& u); // T is U[] template<class T> shared_ptr<T> make_shared(const remove_extent_t<T>& u); // T is U[N] template<class T, class A> shared_ptr<T> allocate_shared(const A& a, const remove_extent_t<T>& u); // T is U[N] template<class T> shared_ptr<T> make_shared_for_overwrite(); // T is not U[] template<class T, class A> shared_ptr<T> allocate_shared_for_overwrite(const A& a); // T is not U[] template<class T> shared_ptr<T> make_shared_for_overwrite(size_t N); // T is U[] template<class T, class A> shared_ptr<T> allocate_shared_for_overwrite(const A& a, size_t N); // T is U[] // [util.smartptr.shared.cmp], shared_ptr comparisons template<class T, class U> bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T, class U> strong_ordering operator<=>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept; template<class T> bool operator==(const shared_ptr<T>& x, nullptr_t) noexcept; template<class T> strong_ordering operator<=>(const shared_ptr<T>& x, nullptr_t) noexcept; // [util.smartptr.shared.spec], shared_ptr specialized algorithms template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept; // [util.smartptr.shared.cast], shared_ptr casts template<class T, class U> shared_ptr<T> static_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U>&& r) noexcept; template<class T, class U> shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U>&& r) noexcept; template<class T, class U> shared_ptr<T> const_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U>&& r) noexcept; template<class T, class U> shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U>& r) noexcept; template<class T, class U> shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U>&& r) noexcept; // [util.smartptr.getdeleter], shared_ptr get_deleter template<class D, class T> D* get_deleter(const shared_ptr<T>& p) noexcept; // [util.smartptr.shared.io], shared_ptr I/O template<class E, class T, class Y> basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os, const shared_ptr<Y>& p); // [util.smartptr.weak], class template weak_ptr template<class T> class weak_ptr; // [util.smartptr.weak.spec], weak_ptr specialized algorithms template<class T> void swap(weak_ptr<T>& a, weak_ptr<T>& b) noexcept; // [util.smartptr.ownerless], class template owner_less template<class T = void> struct owner_less; // [util.smartptr.owner.hash], struct owner_hash struct owner_hash; // [util.smartptr.owner.equal], struct owner_equal struct owner_equal; // [util.smartptr.enab], class template enable_shared_from_this template<class T> class enable_shared_from_this; // [util.smartptr.hash], hash support template<class T> struct hash; // freestanding template<class T, class D> struct hash<unique_ptr<T, D>>; // freestanding template<class T> struct hash<shared_ptr<T>>; // [util.smartptr.atomic], atomic smart pointers template<class T> struct atomic; // freestanding template<class T> struct atomic<shared_ptr<T>>; template<class T> struct atomic<weak_ptr<T>>; // [out.ptr.t], class template out_ptr_t template<class Smart, class Pointer, class... Args> class out_ptr_t; // [out.ptr], function template out_ptr template<class Pointer = void, class Smart, class... Args> auto out_ptr(Smart& s, Args&&... args); // [inout.ptr.t], class template inout_ptr_t template<class Smart, class Pointer, class... Args> class inout_ptr_t; // [inout.ptr], function template inout_ptr template<class Pointer = void, class Smart, class... Args> auto inout_ptr(Smart& s, Args&&... args); }

20.2.3 Pointer traits [pointer.traits]

20.2.3.1 General [pointer.traits.general]

The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.
namespace std { template<class Ptr> struct pointer_traits { see below; }; template<class T> struct pointer_traits<T*> { using pointer = T*; using element_type = T; using difference_type = ptrdiff_t; template<class U> using rebind = U*; static constexpr pointer pointer_to(see below r) noexcept; }; }

20.2.3.2 Member types [pointer.traits.types]

The definitions in this subclause make use of the following exposition-only class template and concept: template<class T> struct ptr-traits-elem // exposition only { }; template<class T> requires requires { typename T::element_type; } struct ptr-traits-elem<T> { using type = typename T::element_type; }; template<template<class...> class SomePointer, class T, class... Args> requires (!requires { typename SomePointer<T, Args...>::element_type; }) struct ptr-traits-elem<SomePointer<T, Args...>> { using type = T; }; template<class Ptr> concept has-elem-type = // exposition only requires { typename ptr-traits-elem<Ptr>::type; }
If Ptr satisfies has-elem-type, a specialization pointer_traits<Ptr> generated from the pointer_traits primary template has the following members as well as those described in [pointer.traits.functions]; otherwise, such a specialization has no members by any of those names.
using pointer = see below;
Type: Ptr.
using element_type = see below;
Type: typename ptr-traits-elem<Ptr>​::​type.
using difference_type = see below;
Type: Ptr​::​difference_type if the qualified-id Ptr​::​difference_type is valid and denotes a type ([temp.deduct]); otherwise, ptrdiff_t.
template<class U> using rebind = see below;
Alias template: Ptr​::​rebind<U> if the qualified-id Ptr​::​rebind<U> is valid and denotes a type ([temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.

20.2.3.3 Member functions [pointer.traits.functions]

static pointer pointer_traits::pointer_to(see below r); static constexpr pointer pointer_traits<T*>::pointer_to(see below r) noexcept;
Mandates: For the first member function, Ptr​::​pointer_to(r) is well-formed.
Preconditions: For the first member function, Ptr​::​pointer_to(r) returns a pointer to r through which indirection is valid.
Returns: The first member function returns Ptr​::​pointer_to(r).
The second member function returns addressof(r).
Remarks: If element_type is cv void, the type of r is unspecified; otherwise, it is element_type&.

20.2.3.4 Optional members [pointer.traits.optmem]

Specializations of pointer_traits may define the member declared in this subclause to customize the behavior of the standard library.
A specialization generated from the pointer_traits primary template has no member by this name.
static element_type* to_address(pointer p) noexcept;
Returns: A pointer of type element_type* that references the same location as the argument p.
[Note 1: 
This function is intended to be the inverse of pointer_to.
If defined, it customizes the behavior of the non-member function to_address ([pointer.conversion]).
β€” end note]

20.2.4 Pointer conversion [pointer.conversion]

template<class T> constexpr T* to_address(T* p) noexcept;
Mandates: T is not a function type.
Returns: p.
template<class Ptr> constexpr auto to_address(const Ptr& p) noexcept;
Returns: pointer_traits<Ptr>​::​to_address(p) if that expression is well-formed (see [pointer.traits.optmem]), otherwise to_address(p.operator->()).

20.2.5 Pointer alignment [ptr.align]

void* align(size_t alignment, size_t size, void*& ptr, size_t& space);
Preconditions:
  • alignment is a power of two
  • ptr represents the address of contiguous storage of at least space bytes
Effects: If it is possible to fit size bytes of storage aligned by alignment into the buffer pointed to by ptr with length space, the function updates ptr to represent the first possible address of such storage and decreases space by the number of bytes used for alignment.
Otherwise, the function does nothing.
Returns: A null pointer if the requested aligned buffer would not fit into the available space, otherwise the adjusted value of ptr.
[Note 1: 
The function updates its ptr and space arguments so that it can be called repeatedly with possibly different alignment and size arguments for the same buffer.
β€” end note]
template<size_t N, class T> [[nodiscard]] constexpr T* assume_aligned(T* ptr);
Mandates: N is a power of two.
Preconditions: ptr points to an object X of a type similar ([conv.qual]) to T, where X has alignment N ([basic.align]).
Returns: ptr.
Throws: Nothing.
[Note 2: 
The alignment assumption on an object X expressed by a call to assume_aligned might result in generation of more efficient code.
It is up to the program to ensure that the assumption actually holds.
The call does not cause the implementation to verify or enforce this.
An implementation might only make the assumption for those operations on X that access X through the pointer returned by assume_aligned.
β€” end note]

20.2.6 Explicit lifetime management [obj.lifetime]

template<class T> T* start_lifetime_as(void* p) noexcept; template<class T> const T* start_lifetime_as(const void* p) noexcept; template<class T> volatile T* start_lifetime_as(volatile void* p) noexcept; template<class T> const volatile T* start_lifetime_as(const volatile void* p) noexcept;
Mandates: T is an implicit-lifetime type ([basic.types.general]) and not an incomplete type ([basic.types.general]).
Preconditions: [p, (char*)p + sizeof(T)) denotes a region of allocated storage that is a subset of the region of storage reachable through ([basic.compound]) p and suitably aligned for the type T.
Effects: Implicitly creates objects ([intro.object]) within the denoted region consisting of an object a of type T whose address is p, and objects nested within a, as follows: The object representation of a is the contents of the storage prior to the call to start_lifetime_as.
The value of each created object o of trivially copyable type ([basic.types.general]) U is determined in the same manner as for a call to bit_cast<U>(E) ([bit.cast]), where E is an lvalue of type U denoting o, except that the storage is not accessed.
The value of any other created object is unspecified.
[Note 1: 
The unspecified value can be indeterminate.
β€” end note]
Returns: A pointer to the a defined in the Effects paragraph.
template<class T> T* start_lifetime_as_array(void* p, size_t n) noexcept; template<class T> const T* start_lifetime_as_array(const void* p, size_t n) noexcept; template<class T> volatile T* start_lifetime_as_array(volatile void* p, size_t n) noexcept; template<class T> const volatile T* start_lifetime_as_array(const volatile void* p, size_t n) noexcept;
Mandates: T is a complete type.
Preconditions: p is suitably aligned for an array of T or is null.
n <= size_t(-1) / sizeof(T) is true.
If n > 0 is true, [(char*)p, (char*)p + (n * sizeof(T))) denotes a region of allocated storage that is a subset of the region of storage reachable through ([basic.compound]) p.
Effects: If n > 0 is true, equivalent to start_lifetime_as<U>(p) where U is the type β€œarray of n T”.
Otherwise, there are no effects.
Returns: A pointer to the first element of the created array, if any; otherwise, a pointer that compares equal to p ([expr.eq]).

20.2.7 Allocator argument tag [allocator.tag]

namespace std { struct allocator_arg_t { explicit allocator_arg_t() = default; }; inline constexpr allocator_arg_t allocator_arg{}; }
The allocator_arg_t struct is an empty class type used as a unique type to disambiguate constructor and function overloading.
Specifically, several types (see tuple [tuple]) have constructors with allocator_arg_t as the first argument, immediately followed by an argument of a type that meets the Cpp17Allocator requirements ([allocator.requirements.general]).

20.2.8 uses_allocator [allocator.uses]

20.2.8.1 uses_allocator trait [allocator.uses.trait]

template<class T, class Alloc> struct uses_allocator;
Remarks: Automatically detects whether T has a nested allocator_type that is convertible from Alloc.
Meets the Cpp17BinaryTypeTrait requirements ([meta.rqmts]).
The implementation shall provide a definition that is derived from true_type if the qualified-id T​::​allocator_type is valid and denotes a type ([temp.deduct]) and is_convertible_v<Alloc, T​::​allocator_type> != false, otherwise it shall be derived from false_type.
A program may specialize this template to derive from true_type for a program-defined type T that does not have a nested allocator_type but nonetheless can be constructed with an allocator where either:
  • the first argument of a constructor has type allocator_arg_t and the second argument has type Alloc or
  • the last argument of a constructor has type Alloc.

20.2.8.2 Uses-allocator construction [allocator.uses.construction]

Uses-allocator construction with allocator alloc and constructor arguments args... refers to the construction of an object of type T such that alloc is passed to the constructor of T if T uses an allocator type compatible with alloc.
When applied to the construction of an object of type T, it is equivalent to initializing it with the value of the expression make_obj_using_allocator<T>(alloc, args...), described below.
The following utility functions support three conventions for passing alloc to a constructor:
  • If T does not use an allocator compatible with alloc, then alloc is ignored.
  • Otherwise, if T has a constructor invocable as T(allocator_arg, alloc, args...) (leading-allocator convention), then uses-allocator construction chooses this constructor form.
  • Otherwise, if T has a constructor invocable as T(args..., alloc) (trailing-allocator convention), then uses-allocator construction chooses this constructor form.
The uses_allocator_construction_args function template takes an allocator and argument list and produces (as a tuple) a new argument list matching one of the above conventions.
Additionally, overloads are provided that treat specializations of pair such that uses-allocator construction is applied individually to the first and second data members.
The make_obj_using_allocator and uninitialized_construct_using_allocator function templates apply the modified constructor arguments to construct an object of type T as a return value or in-place, respectively.
[Note 1: 
For uses_allocator_construction_args and make_obj_using_allocator, type T is not deduced and must therefore be specified explicitly by the caller.
β€” end note]
template<class T, class Alloc, class... Args> constexpr auto uses_allocator_construction_args(const Alloc& alloc, Args&&... args) noexcept;
Constraints: remove_cv_t<T> is not a specialization of pair.
Returns: A tuple value determined as follows:
  • If uses_allocator_v<remove_cv_t<T>, Alloc> is false and is_constructible_v<T,
    Args...>
    is true, return forward_as_tuple(std​::​forward<Args>(args)...).
  • Otherwise, if uses_allocator_v<remove_cv_t<T>, Alloc> is true and is_constructible_v<T, allocator_arg_t, const Alloc&, Args...> is true, return tuple<allocator_arg_t, const Alloc&, Args&&...>( allocator_arg, alloc, std::forward<Args>(args)...)
  • Otherwise, if uses_allocator_v<remove_cv_t<T>, Alloc> is true and is_constructible_v<T, Args..., const Alloc&> is true, return forward_as_tuple(std​::​forward<Args>(args)..., alloc).
  • Otherwise, the program is ill-formed.
[Note 2: 
This definition prevents a silent failure to pass the allocator to a constructor of a type for which uses_allocator_v<T, Alloc> is true.
β€” end note]
template<class T, class Alloc, class Tuple1, class Tuple2> constexpr auto uses_allocator_construction_args(const Alloc& alloc, piecewise_construct_t, Tuple1&& x, Tuple2&& y) noexcept;
Let T1 be T​::​first_type.
Let T2 be T​::​second_type.
Constraints: remove_cv_t<T> is a specialization of pair.
Effects: Equivalent to: return make_tuple( piecewise_construct, apply([&alloc](auto&&... args1) { return uses_allocator_construction_args<T1>( alloc, std::forward<decltype(args1)>(args1)...); }, std::forward<Tuple1>(x)), apply([&alloc](auto&&... args2) { return uses_allocator_construction_args<T2>( alloc, std::forward<decltype(args2)>(args2)...); }, std::forward<Tuple2>(y)));
template<class T, class Alloc> constexpr auto uses_allocator_construction_args(const Alloc& alloc) noexcept;
Constraints: remove_cv_t<T> is a specialization of pair.
Effects: Equivalent to: return uses_allocator_construction_args<T>(alloc, piecewise_construct, tuple<>{}, tuple<>{});
template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u, V&& v) noexcept;
Constraints: remove_cv_t<T> is a specialization of pair.
Effects: Equivalent to: return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(std::forward<U>(u)), forward_as_tuple(std::forward<V>(v)));
template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>& pr) noexcept;
Constraints: remove_cv_t<T> is a specialization of pair.
Effects: Equivalent to: return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(pr.first), forward_as_tuple(pr.second));
template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, pair<U, V>&& pr) noexcept; template<class T, class Alloc, class U, class V> constexpr auto uses_allocator_construction_args(const Alloc& alloc, const pair<U, V>&& pr) noexcept;
Constraints: remove_cv_t<T> is a specialization of pair.
Effects: Equivalent to: return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(get<0>(std::move(pr))), forward_as_tuple(get<1>(std::move(pr))));
template<class T, class Alloc, pair-like P> constexpr auto uses_allocator_construction_args(const Alloc& alloc, P&& p) noexcept;
Constraints: remove_cv_t<T> is a specialization of pair and remove_cvref_t<P> is not a specialization of ranges​::​subrange.
Effects: Equivalent to: return uses_allocator_construction_args<T>(alloc, piecewise_construct, forward_as_tuple(get<0>(std::forward<P>(p))), forward_as_tuple(get<1>(std::forward<P>(p))));
template<class T, class Alloc, class U> constexpr auto uses_allocator_construction_args(const Alloc& alloc, U&& u) noexcept;
Let FUN be the function template: template<class A, class B> void FUN(const pair<A, B>&);
Constraints: remove_cv_t<T> is a specialization of pair, and either:
  • remove_cvref_t<U> is a specialization of ranges​::​subrange, or
  • U does not satisfy pair-like and the expression FUN(u) is not well-formed when considered as an unevaluated operand.
Let pair-constructor be an exposition-only class defined as follows: class pair-constructor { using pair-type = remove_cv_t<T>; // exposition only constexpr auto do-construct(const pair-type& p) const { // exposition only return make_obj_using_allocator<pair-type>(alloc_, p); } constexpr auto do-construct(pair-type&& p) const { // exposition only return make_obj_using_allocator<pair-type>(alloc_, std::move(p)); } const Alloc& alloc_; // exposition only U& u_; // exposition only public: constexpr operator pair-type() const { return do-construct(std::forward<U>(u_)); } };
Returns: make_tuple(pc), where pc is a pair-constructor object whose alloc_ member is initialized with alloc and whose u_ member is initialized with u.
template<class T, class Alloc, class... Args> constexpr T make_obj_using_allocator(const Alloc& alloc, Args&&... args);
Effects: Equivalent to: return make_from_tuple<T>(uses_allocator_construction_args<T>( alloc, std::forward<Args>(args)...));
template<class T, class Alloc, class... Args> constexpr T* uninitialized_construct_using_allocator(T* p, const Alloc& alloc, Args&&... args);
Effects: Equivalent to: return apply([&]<class... U>(U&&... xs) { return construct_at(p, std::forward<U>(xs)...); }, uses_allocator_construction_args<T>(alloc, std::forward<Args>(args)...));

20.2.9 Allocator traits [allocator.traits]

20.2.9.1 General [allocator.traits.general]

The class template allocator_traits supplies a uniform interface to all allocator types.
An allocator cannot be a non-class type, however, even if allocator_traits supplies the entire required interface.
[Note 1: 
Thus, it is always possible to create a derived class from an allocator.
β€” end note]
If a program declares an explicit or partial specialization of allocator_traits, the program is ill-formed, no diagnostic required.
namespace std { template<class Alloc> struct allocator_traits { using allocator_type = Alloc; using value_type = typename Alloc::value_type; using pointer = see below; using const_pointer = see below; using void_pointer = see below; using const_void_pointer = see below; using difference_type = see below; using size_type = see below; using propagate_on_container_copy_assignment = see below; using propagate_on_container_move_assignment = see below; using propagate_on_container_swap = see below; using is_always_equal = see below; template<class T> using rebind_alloc = see below; template<class T> using rebind_traits = allocator_traits<rebind_alloc<T>>; [[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n); [[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n, const_void_pointer hint); [[nodiscard]] static constexpr allocation_result<pointer, size_type> allocate_at_least(Alloc& a, size_type n); static constexpr void deallocate(Alloc& a, pointer p, size_type n); template<class T, class... Args> static constexpr void construct(Alloc& a, T* p, Args&&... args); template<class T> static constexpr void destroy(Alloc& a, T* p); static constexpr size_type max_size(const Alloc& a) noexcept; static constexpr Alloc select_on_container_copy_construction(const Alloc& rhs); }; }

20.2.9.2 Member types [allocator.traits.types]

using pointer = see below;
Type: Alloc​::​pointer if the qualified-id Alloc​::​pointer is valid and denotes a type ([temp.deduct]); otherwise, value_type*.
using const_pointer = see below;
Type: Alloc​::​const_pointer if the qualified-id Alloc​::​const_pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>​::​rebind<const value_type>.
using void_pointer = see below;
Type: Alloc​::​void_pointer if the qualified-id Alloc​::​void_pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>​::​rebind<void>.
using const_void_pointer = see below;
Type: Alloc​::​const_void_pointer if the qualified-id Alloc​::​const_void_pointer is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>​::​​rebind<const void>.
using difference_type = see below;
Type: Alloc​::​difference_type if the qualified-id Alloc​::​difference_type is valid and denotes a type ([temp.deduct]); otherwise, pointer_traits<pointer>​::​difference_type.
using size_type = see below;
Type: Alloc​::​size_type if the qualified-id Alloc​::​size_type is valid and denotes a type ([temp.deduct]); otherwise, make_unsigned_t<difference_type>.
using propagate_on_container_copy_assignment = see below;
Type: Alloc​::​propagate_on_container_copy_assignment if the qualified-id Alloc​::​propagate_on_container_copy_assignment is valid and denotes a type ([temp.deduct]); otherwise false_type.
using propagate_on_container_move_assignment = see below;
Type: Alloc​::​propagate_on_container_move_assignment if the qualified-id Alloc​::​propagate_on_container_move_assignment is valid and denotes a type ([temp.deduct]); otherwise false_type.
using propagate_on_container_swap = see below;
Type: Alloc​::​propagate_on_container_swap if the qualified-id Alloc​::​propagate_on_container_swap is valid and denotes a type ([temp.deduct]); otherwise false_type.
using is_always_equal = see below;
Type: Alloc​::​is_always_equal if the qualified-id Alloc​::​is_always_equal is valid and denotes a type ([temp.deduct]); otherwise is_empty<Alloc>​::​type.
template<class T> using rebind_alloc = see below;
Alias template: Alloc​::​rebind<T>​::​other if the qualified-id Alloc​::​rebind<T>​::​other is valid and denotes a type ([temp.deduct]); otherwise, Alloc<T, Args> if Alloc is a class template instantiation of the form Alloc<U, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind_alloc is ill-formed.

20.2.9.3 Static member functions [allocator.traits.members]

[[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n);
Returns: a.allocate(n).
[[nodiscard]] static constexpr pointer allocate(Alloc& a, size_type n, const_void_pointer hint);
Returns: a.allocate(n, hint) if that expression is well-formed; otherwise, a.allocate(n).
[[nodiscard]] static constexpr allocation_result<pointer, size_type> allocate_at_least(Alloc& a, size_type n);
Returns: a.allocate_at_least(n) if that expression is well-formed; otherwise, {a.allocate(n), n}.
static constexpr void deallocate(Alloc& a, pointer p, size_type n);
Effects: Calls a.deallocate(p, n).
Throws: Nothing.
template<class T, class... Args> static constexpr void construct(Alloc& a, T* p, Args&&... args);
Effects: Calls a.construct(p, std​::​forward<Args>(args)...) if that call is well-formed; otherwise, invokes construct_at(p, std​::​forward<Args>(args)...).
template<class T> static constexpr void destroy(Alloc& a, T* p);
Effects: Calls a.destroy(p) if that call is well-formed; otherwise, invokes destroy_at(p).
static constexpr size_type max_size(const Alloc& a) noexcept;
Returns: a.max_size() if that expression is well-formed; otherwise, numeric_limits<size_type>​::​​max()/sizeof(value_type).
static constexpr Alloc select_on_container_copy_construction(const Alloc& rhs);
Returns: rhs.select_on_container_copy_construction() if that expression is well-formed; otherwise, rhs.

20.2.9.4 Other [allocator.traits.other]

The class template allocation_result has the template parameters, data members, and special members specified above.
It has no base classes or members other than those specified.

20.2.10 The default allocator [default.allocator]

20.2.10.1 General [default.allocator.general]

All specializations of the default allocator meet the allocator completeness requirements ([allocator.requirements.completeness]).
namespace std { template<class T> class allocator { public: using value_type = T; using size_type = size_t; using difference_type = ptrdiff_t; using propagate_on_container_move_assignment = true_type; constexpr allocator() noexcept; constexpr allocator(const allocator&) noexcept; template<class U> constexpr allocator(const allocator<U>&) noexcept; constexpr ~allocator(); constexpr allocator& operator=(const allocator&) = default; [[nodiscard]] constexpr T* allocate(size_t n); [[nodiscard]] constexpr allocation_result<T*> allocate_at_least(size_t n); constexpr void deallocate(T* p, size_t n); }; }
allocator_traits<allocator<T>>​::​is_always_equal​::​value is true for any T.

20.2.10.2 Members [allocator.members]

Except for the destructor, member functions of the default allocator shall not introduce data races ([intro.multithread]) as a result of concurrent calls to those member functions from different threads.
Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.
[[nodiscard]] constexpr T* allocate(size_t n);
Mandates: T is not an incomplete type ([basic.types.general]).
Returns: A pointer to the initial element of an array of n T.
Throws: bad_array_new_length if numeric_limits<size_t>​::​max() / sizeof(T) < n, or bad_alloc if the storage cannot be obtained.
Remarks: The storage for the array is obtained by calling ​::​operator new ([new.delete]), but it is unspecified when or how often this function is called.
This function starts the lifetime of the array object, but not that of any of the array elements.
[[nodiscard]] constexpr allocation_result<T*> allocate_at_least(size_t n);
Mandates: T is not an incomplete type ([basic.types.general]).
Returns: allocation_result<T*>{ptr, count}, where ptr is a pointer to the initial element of an array of count T and count  β‰₯ n.
Throws: bad_array_new_length if numeric_limits<size_t>​::​max() / sizeof(T) < n, or bad_alloc if the storage cannot be obtained.
Remarks: The storage for the array is obtained by calling ​::​operator new, but it is unspecified when or how often this function is called.
This function starts the lifetime of the array object, but not that of any of the array elements.
constexpr void deallocate(T* p, size_t n);
Preconditions:
  • If p is memory that was obtained by a call to allocate_at_least, let ret be the value returned and req be the value passed as the first argument to that call.
    p is equal to ret.ptr and n is a value such that req  ≀ n  ≀ ret.count.
  • Otherwise, p is a pointer value obtained from allocate.
    n equals the value passed as the first argument to the invocation of allocate which returned p.
Effects: Deallocates the storage referenced by p.
Remarks: Uses ​::​operator delete ([new.delete]), but it is unspecified when this function is called.

20.2.10.3 Operators [allocator.globals]

template<class T, class U> constexpr bool operator==(const allocator<T>&, const allocator<U>&) noexcept;
Returns: true.

20.2.11 addressof [specialized.addressof]

template<class T> constexpr T* addressof(T& r) noexcept;
Returns: The actual address of the object or function referenced by r, even in the presence of an overloaded operator&.
Remarks: An expression addressof(E) is a constant subexpression ([defns.const.subexpr]) if E is an lvalue constant subexpression.

20.2.12 C library memory allocation [c.malloc]

[Note 1: 
The header <cstdlib> declares the functions described in this subclause.
β€” end note]
void* aligned_alloc(size_t alignment, size_t size); void* calloc(size_t nmemb, size_t size); void* malloc(size_t size); void* realloc(void* ptr, size_t size);
Effects: These functions have the semantics specified in the C standard library.
Remarks: These functions do not attempt to allocate storage by calling ​::​operator new() ([new.delete]).
These functions implicitly create objects ([intro.object]) in the returned region of storage and return a pointer to a suitable created object.
In the case of calloc and realloc, the objects are created before the storage is zeroed or copied, respectively.
void free(void* ptr);
Effects: This function has the semantics specified in the C standard library.
Remarks: This function does not attempt to deallocate storage by calling ​::​operator delete().
See also: ISO/IEC 9899:2018, 7.22.3