Universal reference in a member function pointer

Question

I am having some troubles understanding why the following code cannot compile

#include <iostream>
#include <typeinfo>

#define PRINT_FUNC() {std::cout << __PRETTY_FUNCTION__ << std::endl;}

struct Obj {
    Obj(){PRINT_FUNC();}
    int run (float f, char *c) {
        PRINT_FUNC();
        return 0;
    }

    int fly () {
        PRINT_FUNC();
        return 0;
    }
};

template <typename OBJ, typename R, typename ... Args>
void call_obj_func (OBJ &&o, R(OBJ::*fn)(Args...), Args ... args) {
    PRINT_FUNC();
    (o.*fn)(args...);
}

int main () {
    Obj o;
    call_obj_func(o, &Obj::fly);
}

For the function call_obj_func I expected the type of OBJ to be used for BOTH rvlaue and lvalue types. However when calling with a lvalue type, the compiler complains that there is ambuigity of using the types: Obj and Obj&

This means that the comiler isnt sure wheter to use a copy of the obj or the reference to the obj.

I am sure there is some syntax error as I would like the function call_obj_func to be compiled with both lvalue and rvalue types.

My assumption is the member function pointer as the syntax (Obj&::*fn) and (Obj::*fn) might have different semantics. (Though I cannot find the differences anywhere).

Answer 1

You may write

template <typename OBJ, typename Method, typename ... Args>
void call_obj_func (OBJ &&o, const Method& fn, Args&& ... args) {
    PRINT_FUNC();
    (std::forward<OBJ>(o).*fn)(std::forward<Args>(args)...);
}

Live Demo

As currently, you have conflict with type deduction ( Obj& vs Obj , and you may have similar issues with args )