std::uses_allocator

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;

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::polymoprhic_allocator::construct and std::scoped_allocator_adaptor::construct (until C++20)std::uses_allocator_construction_args (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:

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:

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