[allocator.tag]

🔗

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

The allocator_arg_t struct is an empty class 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 meets the Cpp17Allocator requirements (Table 36).

20 General utilities library [utilities]

20.10 Memory [memory]

20.10.7 Allocator argument tag [allocator.tag]