Defined in header <memory>
struct allocator_arg_t {};
(since C++11)
(until C++17)
struct allocator_arg_t { explicit allocator_arg_t() = default; };
(since C++17)

std::allocator_arg_t is an empty class type used to disambiguate the overloads of constructors and member functions of allocator-aware objects, including std::tuple, std::function, std::packaged_task, (until C++17)and std::promise.

See also

an object of type std::allocator_arg_t used to select allocator-aware constructors
checks if the specified type supports uses-allocator construction
(class template)

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