A function adaptor for a constant member reference.
template<class S, class T> class const_mem_fun_ref_t<S,T> : public unary_function<T,S> explicit const_mem_fun_ref_t( S (T::*p) () const);
The template functions mem_fun_ref returns an object through which X::f can be called.
The constructor for const_mem_fun_ref_t calls the member function that it is initialized with using a given a reference argument.