struct allocator_arg_t {};
Allocator arg type
This is an empty class used as the type of allocator_arg.

Values of this type are used to disambiguate constructor and function overloading: Some classes in the standard library take a value of this type in their constructor overloads just to disambiguate their signatures. The versions taking an argument of this type as first argument, take an allocator object as second argument (see tuple for an example).