std::pmr::polymorphic_allocator

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pmr::polymorphic_allocator
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std::pmr::polymorphic_allocator
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 template std::pmr::polymorphic_allocator is an Allocator which exhibits different allocation behavior depending upon the std::pmr::memory_resource from which it is constructed. Since memory_resource uses runtime polymorphism to manage allocations, different container instances with polymorphic_allocator

All specializations of polymorphic_allocator meet the allocator completeness requirements.

The polymorphic_allocator::construct member function does uses-allocator construction, so that the elements of a container using a polymorphic_allocator will use that same allocator for their own allocations. For example, a std::pmr::vector < std::pmr::string > will use the same memory_resource for the vector's storage and each string

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

Member types

Member type definition
value_type T

Member functions

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

Non-member functions

(removed in C++20)
 compare two polymorphic_allocators
(function)

Notes

polymorphic_allocator does not propagate on container copy assignment, move assignment, or swap. As a result, move assignment of a polymorphic_allocator-using container can throw, and swapping two polymorphic_allocator

Feature-test macro Value Std Feature
__cpp_lib_polymorphic_allocator 201902L (C++20) std::pmr::polymorphic_allocator<> as a vocabulary type

See also

(C++17)
 an abstract interface for classes that encapsulate memory resources
(class)
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