std::uses_allocator (3) - Linux Manuals

std::uses_allocator: std::uses_allocator

NAME

std::uses_allocator - std::uses_allocator

Synopsis

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 > (since C++17)
inline constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;

Inherited from std::integral_constant

Member constants

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

Member functions

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

operator() returns value
              (public member function)
(C++14)

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::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)

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> specializes the std::uses_allocator type trait
                                         (class template specialization)
(C++11)

std::uses_allocator<std::queue> specializes the std::uses_allocator type trait
                                         (function template)
(C++11)

std::uses_allocator<std::priority_queue> specializes the std::uses_allocator type trait
                                         (function template)
(C++11)

std::uses_allocator<std::stack> specializes the std::uses_allocator type trait
                                         (function template)
(C++11)

std::uses_allocator<std::function> specializes the std::uses_allocator type trait
                                         (class template specialization)
(C++11) (until C++17)

std::uses_allocator<std::promise> specializes the std::uses_allocator type trait
                                         (class template specialization)
(C++11)

std::uses_allocator<std::packaged_task> specializes the std::uses_allocator type trait
                                         (class template specialization)
(C++11)(until C++17)

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.