std::uses_allocator

 

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

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;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

true if T uses allocator Alloc, false otherwise (public static member constant)

Member functions

operator bool	converts the object to bool, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
value_type	bool
type	std::integral_constant<bool, value>

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:

1) T has a constructor which takes std::allocator_arg_t as the first argument, and Alloc as the second argument.

2) 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::variant> (C++17)	specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator<std::queue> (C++11)	specializes the std::uses_allocator type trait (function template)
std::uses_allocator<std::priority_queue> (C++11)	specializes the std::uses_allocator type trait (function template)
std::uses_allocator<std::stack> (C++11)	specializes the std::uses_allocator type trait (function template)
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::scoped_allocator_adaptor, std::tuple, std::variant, 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 (C++11)	an object of type std::allocator_arg_t used to select allocator-aware constructors (constant)
allocator_arg_t (C++11)	tag type used to select allocator-aware constructor overloads (class)
scoped_allocator_adaptor (C++11)	implements multi-level allocator for multi-level containers (class template)