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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 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.

Contents

[edit] 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>

[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::polymoprhic_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

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:

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

an object of type std::allocator_arg_t used to select allocator-aware constructors
(constant) [edit]
tag type used to select allocator-aware constructor overloads
(class) [edit]
prepares the argument list matching the flavor of uses-allocator construction required by the given type
(function template) [edit]
creates an object of the given type by means of uses-allocator construction
(function template) [edit]
creates an object of the given type at specified memory location by means of uses-allocator construction
(function template) [edit]
implements multi-level allocator for multi-level containers
(class template) [edit]