std::allocator_arg_t

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Defined in header <memory>
struct allocator_arg_t {};		(since C++11)

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::promise, and std::packaged_task.

[edit] See also

allocator_arg (C++11)	an object of type std::allocator_arg_t used to select allocator-aware constructors (constant)
uses_allocator (C++11)	checks if the specified type supports uses-allocator construction (class template)