std::pmr::polymorphic_allocator - cppreference.com (original) (raw)
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 as their static allocator type are interoperable, but can behave as if they had different allocator types.
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's storage.
For non-polymorphic allocators, similar propagation can be achieved with the help of std::scoped_allocator_adaptor.
Contents
[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
[edit] 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-using containers whose allocators do not compare equal results in undefined behavior.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
| __cpp_lib_polymorphic_allocator | 201902L | (C++20) | std::pmr::polymorphic_allocator<> as a vocabulary type |
[edit] See also
| | an abstract interface for classes that encapsulate memory resources (class) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |