20 General utilities library [utilities]

20.6 Memory [memory]

20.6.6 Allocator argument tag [allocator.tag]

namespace std { struct allocator_arg_t { }; constexpr allocator_arg_t allocator_arg = allocator_arg_t(); }

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]).