Carview!

member type	definition
`type`	The template parameter (`T`)
`result_type`	If `T` is a pointer to function or to member function type: the return type of `T`. If `T` is a class, and has a `result_type` member: `T::result_type`. Otherwise, not defined.
`argument_type`	If `T` is a function type or a pointer to function type taking a single argument: the type of the argument taken by `T`. If `T` is a pointer to member function: the class type of which `T` is a member (with the same const/volatile qualification as the member function). If `T` is a class, and has an `argument_type` member: `T::argument_type`. Otherwise, not defined.
`first_argument_type`	If `T` is a function type or a pointer to function type taking two arguments: the type of the first argument taken by `T`. If `T` is a pointer to member function taking a single argument: the class type of which `T` is a member (with the same const/volatile qualification as the member function). If `T` is a class, and has a `first_argument_type` member: `T::first_argument_type`. Otherwise, not defined.
`second_argument_type`	If `T` is a function type or a pointer to function type taking two arguments: The type of the second argument taken by `T`. If `T` is a pointer to member function taking a single argument: the argument taken by `T`. If `T` is a class, and has a `second_argument_type` member: `T::second_argument_type`. Otherwise, not defined.

std::reference_wrapper