namespace std {
struct allocator_arg_t { explicit allocator_arg_t() = default; };
constexpr allocator_arg_t allocator_arg{};
}
The allocator_arg_t struct is an empty structure type used as a unique type to disambiguate constructor and function overloading. Specifically, several types (see tuple [tuple]) have constructors with allocator_arg_t as the first argument, immediately followed by an argument of a type that satisfies the Allocator requirements ([allocator.requirements]).