std::uses_allocator

< cpp‎ | memory

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>

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

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::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.

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)
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)