std::allocator_arg

Defined in

memory

Since C++17

inline constexpr std::allocator_arg_t allocator_arg = std::allocator_arg_t();

Since C++11

struct allocator_arg_t { explicit allocator_arg_t() = default; };

constexpr std::allocator_arg_t allocator_arg = std::allocator_arg_t();  // Until 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. std::allocator_arg is a constant of it.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2510	C++11	the default constructor was non-explicit, which could lead to ambiguity	made explicit