std:: uses_allocator

Defined in header <memory>
template < class T, class Alloc > struct uses_allocator ;		(since C++11)

If T has a nested type allocator_type which is convertible from Alloc , the member constant value is true . Otherwise value is false .

Helper variable template

template < class T, class Alloc > constexpr bool uses_allocator_v = uses_allocator < T, Alloc > :: value ;		(since C++17)

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).

• 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

Given a program-defined type T that does not have a nested allocator_type , a program can specialize std::uses_allocator to derive from std::true_type for T if any of the following requirements is satisfied:

• 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:

std::uses_allocator <std::tuple> (C++11)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::queue> (C++11)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::priority_queue> (C++11)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::stack> (C++11)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::flat_map> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::flat_set> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::flat_multimap> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::flat_multiset> (C++23)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::function> (C++11) (until C++17)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::promise> (C++11)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator <std::packaged_task> (C++11) (until C++17)	specializes the std::uses_allocator type trait (class template specialization)

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.

See also

allocator_arg allocator_arg_t (C++11)	a tag used to select allocator-aware constructors (tag)
uses_allocator_construction_args (C++20)	prepares the argument list matching the flavor of uses-allocator construction required by the given type (function template)
make_obj_using_allocator (C++20)	creates an object of the given type by means of uses-allocator construction (function template)
uninitialized_construct_using_allocator (C++20)	creates an object of the given type at specified memory location by means of uses-allocator construction (function template)
scoped_allocator_adaptor (C++11)	implements multi-level allocator for multi-level containers (class template)