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Showing content from https://en.cppreference.com/w/cpp/language/../memory/weak_ptr/../uses_allocator.html below:

std::uses_allocator - cppreference.com

template< class T, class Alloc >
struct uses_allocator;

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

template< class T, class Alloc >
constexpr bool uses_allocator_v = uses_allocator<T, Alloc>::value;

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).

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:

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.

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