mem_fun_ref_t
template<class R, class T>
struct mem_fun_ref_t : public unary_function<T *, R> {
explicit mem_fun_t(R (T::*pm)());
R operator()(T& x);
};
The template class stores a copy of pm, which must be a pointer to a member function of class T, in a private member object. It defines its member function operator() as returning (x.*Pm)().