std::uses_allocator
| Defined in | memory |
template< class T, class Alloc > struct uses_allocator;
If T has a member typedef allocator_type which is convertible from Alloc
***or is an alias of std::experimental::erased_type ***(library fundamentals TS),
the member constant value is true. Otherwise value is false.
Helper variable template
template< class T, class Alloc >
inline constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;
Inherited from std::integral_constant
Member constants
| pub | valuestatic | true if T uses allocator Alloc, false otherwise |
Member functions
| pub | operator bool | converts the object to bool, returns value |
| pub | operator()(C++14) | returns value |
Member types
| pub | Type | Definition |
| pub | value_type | bool |
| pub | type | std::integral_constant<bool, value> |
Uses-allocator construction
There are three conventions of passing an allocator alloc to a constructor of some type T:
- if T does not use a compatible allocator (
std::uses_allocator_v<T, Alloc>is false), then alloc is ignored. - otherwise,
std::uses_allocator_v<T, Alloc>is true, and- if T uses the leading-allocator convention (is invocable as
T(std::allocator_arg, alloc, args...)), then uses-allocator construction uses this form - if T uses the trailing-allocator convention (is invocable as
T(args..., alloc)), then uses-allocator construction uses this form - otherwise, the program is ill-formed (this means
std::uses_allocator_v<T, Alloc>is true, but the type does not follow either of the two allowed conventions)
- if T uses the leading-allocator convention (is invocable as
- As a special case, std::pair is treated as a uses-allocator type even though std::uses_allocator is false for pairs
(unlike e.g. std::tuple): see pair-specific overloads
of std::pmr::polymorphic_allocator::construct
and std::scoped_allocator_adaptor::construct (until C++20)
std::uses_allocator_construction_args (since C++20)
The utility functions std::make_obj_using_allocator, and std::uninitialized_construct_using_allocator may be used to explicitly create an object following the above protocol, and std::uses_allocator_construction_args can be used to prepare the argument list that matches the flavor of uses-allocator construction expected by the type. (since C++20)
Specializations
Custom specializations of the type trait std::uses_allocator are allowed for types that do not have the member typedef allocator_type but satisfy one of the following two requirements:
- T has a constructor which takes std::allocator_arg_t as the first argument, and Alloc as the second argument.
- T has a constructor which takes Alloc as the last argument.
In the above, Alloc is a type that satisfies Allocator
or is a pointer type convertible to std::experimental::pmr::memory_resource* (library fundamentals TS).
The following specializations are already provided by the standard library:
| pub | std::uses_allocator<std::tuple> (C++11) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::queue> (C++11) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::priority_queue> (C++11) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::stack> (C++11) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::flat_map> (C++23) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::flat_set> (C++23) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::flat_multimap> (C++23) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::flat_multiset> (C++23) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::function> (C++11) (until C++17) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::promise> (C++11) | specializes the std::uses_allocator type trait |
| pub | std::uses_allocator<std::packaged_task> (C++11) (until C++17) | specializes the std::uses_allocator type trait |
Notes
This type trait is used by std::tuple, std::scoped_allocator_adaptor, and std::pmr::polymorphic_allocator. It may also be used by custom allocators or wrapper types to determine whether the object or member being constructed is itself capable of using an allocator (e.g. is a container), in which case an allocator should be passed to its constructor.