std::uses_allocator - cppreference.com (original) (raw)

| | | | | --------------------------------------------------------- | | ------------- | | 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

• 1 Helper variable template
• 2 Inherited from std::integral_constant
  • 2.1 Member constants
  • 2.2 Member functions
  • 2.3 Member types
  • 2.4 Uses-allocator construction
  • 2.5 Specializations
  • 2.6 Notes
  • 2.7 See also

[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

| | true if T uses allocator Alloc, false otherwise (public static member constant) | | ---------------------------------------------------------------------------------- |

Member functions

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

Member types

[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).

[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:

[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