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)().