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.
| Member type | definition |
|---|---|
value_type | T |
constructs a polymorphic_allocator (public member function) |
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| (destructor)
(implicitly declared) | implicitly declared destructor (public member function) |
| operator=
[deleted] | copy assignment operator is deleted (public member function) |
Public member functions |
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| allocate memory (public member function) |
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| deallocate memory (public member function) |
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| constructs an object in allocated storage (public member function) |
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(deprecated in C++20) | destroys an object in allocated storage (public member function) |
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(C++20) | allocate raw aligned memory from the underlying resource (public member function) |
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(C++20) | free raw memory obtained from allocate_bytes (public member function) |
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(C++20) | allocates raw memory suitable for an object or an array (public member function) |
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(C++20) | frees raw memory obtained by allocate_object (public member function) |
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(C++20) | allocates and constructs an object (public member function) |
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(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) |
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| returns a pointer to the underlying memory resource (public member function) |
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(removed in C++20) | compare two polymorphic_allocators (function) |
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 |
|---|
__cpp_lib_polymorphic_allocator |
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(C++17) | an abstract interface for classes that encapsulate memory resources (class) |
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