Defined in header <memory>
struct allocator_arg_t { explicit allocator_arg_t() = default; };
(since C++11)
constexpr std::allocator_arg_t allocator_arg = std::allocator_arg_t();
(since C++11)
(until C++17)
inline constexpr std::allocator_arg_t allocator_arg = std::allocator_arg_t();
(since 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

See also

checks if the specified type supports uses-allocator construction
(class template)

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