std::pointer_traits

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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)

(C++26)

enable_shared_from_this

(C++11)

(C++11)

(C++11)

(C++23)

(C++23)

pointer_traits

(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)

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std::pointer_traits

Member functions
pointer_traits::pointer_to (C++11)
pointer_traits::to_address (C++20)(optional)

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Defined in header`<memory>`
template<class Ptr> struct pointer_traits;	(1)	(since C++11)
template<class T> struct pointer_traits<T*>;	(2)	(since C++11)

Thepointer_traits class template provides the standardized way to access certain properties of pointer-like types (fancy pointers, such asboost::interprocess::offset_ptr). The standard templatestd::allocator_traits relies onpointer_traits to determine the defaults for various typedefs required byAllocator.

1) The non-specializedpointer_traits conditionally declares the following members:

Let/*element-type-of*/<Ptr> be

Ptr::element_type if present;
otherwise,T ifPtr is a class template specializationTemplate<T, Args...>, whereArgs... is zero or more type arguments;
otherwise, not defined.

If/*element-type-of*/<Ptr> is not defined, the primary template has no members specified in this page.

[edit]Member types

Type	Definition
`pointer`	Ptr
`element_type`	/element-type-of/<Ptr>
`difference_type`	Ptr::difference_type if present, otherwisestd::ptrdiff_t

[edit]Member alias templates

Template	Definition
template<class U>using rebind	Ptr::rebind<U> if exists, otherwiseTemplate<U, Args...> if`Ptr` is a template specializationTemplate<T, Args...>

[edit]Member functions

pointer_to

[static]

obtains a dereferenceable pointer to its argument
(public static member function)[edit]

2) A specialization is provided for pointer types,T*, which declares the following members:

[edit]Member types

Type	Definition
`pointer`	T*
`element_type`	T
`difference_type`	std::ptrdiff_t

[edit]Member alias templates

Template	Definition
template<class U>using rebind	U*

[edit]Member functions

pointer_to

[static]

obtains a dereferenceable pointer to its argument
(public static member function)[edit]

[edit]Optional member functions of program-defined specializations

to_address

[static](C++20)(optional)

obtains a raw pointer from a fancy pointer (inverse ofpointer_to)
(public static member function)[edit]

[edit]Notes

The rebind member template alias makes it possible, given a pointer-like type that points toT, to obtain the same pointer-like type that points toU. For example,

using another_pointer= std::pointer_traits<std::shared_ptr<int>>::rebind<double>;static_assert(std::is_same<another_pointer,std::shared_ptr<double>>::value);

A specialization for user-defined fancy pointer types may provide an additional static member functionto_address to customize the behavior ofstd::to_address.

(since C++20)

Feature-test macro	Value	Std	Feature
`__cpp_lib_constexpr_memory`	`201811L`	(C++20)	constexpr in`std::pointer_traits`

[edit]Example

Run this code

#include <iostream>#include <memory> template<class Ptr>struct BlockList{// Predefine a memory blockstruct block; // Define a pointer to a memory block from the kind of pointer Ptr s// If Ptr is any kind of T*, block_ptr_t is block*// If Ptr is smart_ptr<T>, block_ptr_t is smart_ptr<block>using block_ptr_t=typename std::pointer_traits<Ptr>::template rebind<block>; struct block{std::size_t size{};        block_ptr_t next_block{};};     block_ptr_t free_blocks;}; int main(){[[maybe_unused]]    BlockList<int*> bl1;// The type of bl1.free_blocks is BlockList<int*>:: block*     BlockList<std::shared_ptr<char>> bl2;// The type of bl2.free_blocks is// std::shared_ptr<BlockList<std::shared_ptr<char>>::block>std::cout<< bl2.free_blocks.use_count()<<'\n';}

Output:

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3545	C++11	primary template caused hard error when`element_type` is invalid	made SFINAE-friendly

[edit]See also

allocator_traits (C++11)	provides information about allocator types (class template)[edit]
addressof (C++11)	obtains actual address of an object, even if the`&` operator is overloaded (function template)[edit]