Difference between revisions of "cpp/memory/polymorphic allocator"
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{{dcl header|memory_resource}} | {{dcl header|memory_resource}} | ||
{{dcl rev multi | {{dcl rev multi | ||
| − | | since1=c++17 | dcl1= | + | |since1=c++17|dcl1= |
template< class T > | template< class T > | ||
class polymorphic_allocator; | class polymorphic_allocator; | ||
| − | | since2=c++20 | dcl2= | + | |since2=c++20|dcl2= |
template< class T = std::byte > | template< class T = std::byte > | ||
class polymorphic_allocator; | class 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 | + | All specializations of {{tt|polymorphic_allocator}} meet the {{lsd|cpp/named req/Allocator#Allocator completeness requirements}}.<!-- LWG 3037 --> |
| − | === Member types === | + | 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}}. | ||
| + | |||
| + | ===Member types=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc hitem| Member type | definition}} | + | {{dsc hitem|Member type|definition}} |
| − | {{dsc |{{tt|value_type}} | {{tt|T}}}} | + | {{dsc|{{tt|value_type}}|{{tt|T}}}} |
{{dsc end}} | {{dsc end}} | ||
===Member functions=== | ===Member functions=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc constructor}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc constructor}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc destructor}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc destructor}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc operator{{=}}}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc operator{{=}}}} |
| − | {{dsc h2 | Public member functions}} | + | {{dsc h2|Public member functions}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc allocate}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc allocate}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc deallocate}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc deallocate}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc construct}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc construct}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc destroy}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc destroy}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc allocate_bytes}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc allocate_bytes}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc deallocate_bytes}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc deallocate_bytes}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc allocate_object}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc allocate_object}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc deallocate_object}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc deallocate_object}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc new_object}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc new_object}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc delete_object}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc delete_object}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc select_on_container_copy_construction}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc select_on_container_copy_construction}} |
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc resource}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc resource}} |
{{dsc end}} | {{dsc end}} | ||
| − | === Non-member functions === | + | ===Non-member functions=== |
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/memory/polymorphic_allocator/dsc operator eq}} | + | {{dsc inc|cpp/memory/polymorphic_allocator/dsc operator eq}} |
{{dsc end}} | {{dsc end}} | ||
| − | === Notes === | + | ===Notes=== |
{{tt|polymorphic_allocator}} does not propagate on container copy assignment, move assignment, or swap. As a result, move assignment of a {{tt|polymorphic_allocator}}-using container can throw, and swapping two {{tt|polymorphic_allocator}}-using containers whose allocators do not compare equal results in undefined behavior. | {{tt|polymorphic_allocator}} does not propagate on container copy assignment, move assignment, or swap. As a result, move assignment of a {{tt|polymorphic_allocator}}-using container can throw, and swapping two {{tt|polymorphic_allocator}}-using containers whose allocators do not compare equal results in undefined behavior. | ||
| + | |||
| + | {{feature test macro|__cpp_lib_polymorphic_allocator|{{tt|std::pmr::polymorphic_allocator<>}} as a vocabulary type|value=201902L|std=C++20}} | ||
===See also=== | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/memory/dsc memory_resource}} | + | {{dsc inc|cpp/memory/dsc memory_resource}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|es|ja|ru|zh}} | {{langlinks|es|ja|ru|zh}} | ||
Latest revision as of 08:18, 25 October 2023
| 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
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) |
