# 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>

### 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> (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 (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::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.

### 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)
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)

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