std::pmr::polymorphic_allocator

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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

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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)

(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::pmr::polymorphic_allocator

Member functions
polymorphic_allocator::polymorphic_allocator
polymorphic_allocator::allocate
polymorphic_allocator::deallocate
polymorphic_allocator::construct
polymorphic_allocator::destroy (C++20/26*)
polymorphic_allocator::allocate_bytes (C++20)
polymorphic_allocator::deallocate_bytes (C++20)
polymorphic_allocator::allocate_object (C++20)
polymorphic_allocator::deallocate_object (C++20)
polymorphic_allocator::new_object (C++20)
polymorphic_allocator::delete_object (C++20)
polymorphic_allocator::select_on_container_copy_construction
polymorphic_allocator::resource
Non-member functions
operator==operator!= (until C++20)

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Defined in header`<memory_resource>`
template<class T> class polymorphic_allocator;		(since C++17) (until C++20)
template<class T=std::byte> class polymorphic_allocator;		(since C++20)

The class templatestd::pmr::polymorphic_allocator is anAllocator which exhibits different allocation behavior depending upon thestd::pmr::memory_resource from which it is constructed. Sincememory_resource uses runtime polymorphism to manage allocations, different container instances withpolymorphic_allocator as their static allocator type are interoperable, but can behave as if they had different allocator types.

All specializations ofpolymorphic_allocator meet theallocator completeness requirements.

Thepolymorphic_allocator::construct member function doesuses-allocator construction, so that the elements of a container using apolymorphic_allocator will use that same allocator for their own allocations. For example, astd::pmr::vector<std::pmr::string> will use the samememory_resource for thevector's storage and eachstring's storage.

For non-polymorphic allocators, similar propagation can be achieved with the help ofstd::scoped_allocator_adaptor.

[edit]Member types

Member type	definition
`value_type`	`T`

[edit]Member functions

(constructor)	constructs a`polymorphic_allocator` (public member function)[edit]
(destructor) (implicitly declared)	implicitly declared destructor (public member function)[edit]
operator= [deleted]	copy assignment operator is deleted (public member function)[edit]
Public member functions
allocate	allocate memory (public member function)[edit]
deallocate	deallocate memory (public member function)[edit]
construct	constructs an object in allocated storage (public member function)[edit]
destroy (deprecated in C++20)(undeprecated in C++26)	destroys an object in allocated storage (public member function)[edit]
allocate_bytes (C++20)	allocate raw aligned memory from the underlying resource (public member function)[edit]
deallocate_bytes (C++20)	free raw memory obtained from`allocate_bytes` (public member function)[edit]
allocate_object (C++20)	allocates raw memory suitable for an object or an array (public member function)[edit]
deallocate_object (C++20)	frees raw memory obtained by`allocate_object` (public member function)[edit]
new_object (C++20)	allocates and constructs an object (public member function)[edit]
delete_object (C++20)	destroys and deallocates an object (public member function)[edit]
select_on_container_copy_construction	create a new`polymorphic_allocator` for use by a container's copy constructor (public member function)[edit]
resource	returns a pointer to the underlying memory resource (public member function)[edit]

[edit]Non-member functions

operator==operator!=

(removed in C++20)

compare twopolymorphic_allocators
(function)[edit]

[edit]Notes

polymorphic_allocator does not propagate on container copy assignment, move assignment, or swap. As a result, move assignment of apolymorphic_allocator-using container can throw, and swapping twopolymorphic_allocator-using containers whose allocators do not compare equal results in undefined behavior.