std::allocator_arg_t (3) Linux Manual Page

std::allocator_arg_t – std::allocator_arg_t

Synopsis

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

allocator_arg an object of type std::allocator_arg_t used to select allocator-aware constructors
               (constant)
(C++11)
uses_allocator checks if the specified type supports uses-allocator construction
               (class template)
(C++11)