Difference between revisions of "cpp/memory/polymorphic allocator"
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The class template {{tt|std::pmr::polymorphic_allocator}} is an {{named req|Allocator}} which exhibits different allocation behavior depending upon the {{lc|std::pmr::memory_resource}} from which it is constructed. Since {{tt|memory_resource}} uses runtime polymorphism to manage allocations, different container instances with {{tt|polymorphic_allocator}} as their static allocator type are interoperable, but can behave as if they had different allocator types. | The class template {{tt|std::pmr::polymorphic_allocator}} is an {{named req|Allocator}} which exhibits different allocation behavior depending upon the {{lc|std::pmr::memory_resource}} from which it is constructed. Since {{tt|memory_resource}} uses runtime polymorphism to manage allocations, different container instances with {{tt|polymorphic_allocator}} as their static allocator type are interoperable, but can behave as if they had different allocator types. | ||
| − | All specializations of {{tt|polymorphic_allocator}} meet the {{lsd|cpp/named req/Allocator | + | All specializations of {{tt|polymorphic_allocator}} meet the {{lsd|cpp/named req/Allocator#Allocator completeness requirements}}.<!-- LWG 3037 --> |
| − | The {{tt|polymorphic_allocator::construct}} member function does {{lsd|cpp/memory/uses allocator | + | The {{tt|polymorphic_allocator::construct}} member function does {{lsd|cpp/memory/uses allocator#Uses-allocator construction}}, so that the elements of a container using a {{tt|polymorphic_allocator}} will use that same allocator for their own allocations. For example, a {{c|std::pmr::vector<std::pmr::string>}} will use the same {{tt|memory_resource}} for the {{tt|vector}}'s storage and each {{tt|string}}'s storage. |
For non-polymorphic allocators, similar propagation can be achieved with the help of {{lc|std::scoped_allocator_adaptor}}. | For non-polymorphic allocators, similar propagation can be achieved with the help of {{lc|std::scoped_allocator_adaptor}}. | ||
Latest revision as of 08:18, 25 October 2023
| Defined in header <memory_resource>
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template< class T > class polymorphic_allocator; |
(since C++17) (until C++20) |
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| 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
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) | |
[edit] Non-member functions
| (removed in C++20) |
compare two polymorphic_allocators (function) |
[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
| (C++17) |
an abstract interface for classes that encapsulate memory resources (class) |
