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

Contents

[edit] Helper variable template

template< class T, class Alloc >
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)

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operator bool

converts the object to bool, returns value
(public member function)

operator()

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returns value
(public member function)

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Type Definition

value_type bool

type std::integral_constant <bool, value>

[edit] 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)

[edit] 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) [edit]

std::uses_allocator<std::queue>

(C++11)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::priority_queue>

(C++11)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::stack>

(C++11)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::flat_map>

(C++23)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::flat_set>

(C++23)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::flat_multimap>

(C++23)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::flat_multiset>

(C++23)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::function>

(C++11) (until C++17)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::promise>

(C++11)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

std::uses_allocator<std::packaged_task>

(C++11) (until C++17)

specializes the std::uses_allocator type trait
(class template specialization) [edit]

[edit] 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.

[edit] See also

allocator_argallocator_arg_t

(C++11)

a tag used to select allocator-aware constructors
(tag)[edit]

uses_allocator_construction_args

(C++20)

prepares the argument list matching the flavor of uses-allocator construction required by the given type
(function template) [edit]

make_obj_using_allocator

(C++20)

creates an object of the given type by means of uses-allocator construction
(function template) [edit]

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) [edit]

scoped_allocator_adaptor

(C++11)

implements multi-level allocator for multi-level containers
(class template) [edit]

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