c++ - generic member function pointer as a template parameter

Question

Consider this code:

#include <iostream>
using namespace std;

class hello{
public:
    void f(){
        cout<<"f"<<endl;
    }
    virtual void ff(){
        cout<<"ff"<<endl;
    }
};

#define call_mem_fn(object, ptr)  ((object).*(ptr))

template<R (C::*ptr_to_mem)(Args...)> void proxycall(C& obj){
    cout<<"hello"<<endl;
    call_mem_fn(obj, ptr_to_mem)();
}

int main(){
    hello obj;
    proxycall<&hello::f>(obj);
}

Of course this won't compile at line 16, because the compiler doesn't know what R, C and Args, are. But there's another problem: if one tries to define those template parameters right before ptr_to_mem, he runs into this bad situation:

template<typename R, typename C, typename... Args, R (C::*ptr_to_mem)(Args...)> 
                             //  ^variadic template, but not as last parameter!
void proxycall(C& obj){
    cout<<"hello"<<endl;
    call_mem_fn(obj, ptr_to_mem)();
}

int main(){
    hello obj;
    proxycall<void, hello, &hello::f>(obj);
}

Surprisingly, g++ does not complain about Args not being the last parameter in the template list, but anyway it cannot bind proxycall to the right template function, and just notes that it's a possible candidate.

Any solution? My last resort is to pass the member function pointer as an argument, but if I could pass it as a template parameter it would fit better with the rest of my code.

EDIT: as some have pointed out, the example seems pointless because proxycall isn't going to pass any argument. This is not true in the actual code I'm working on: the arguments are fetched with some template tricks from a Lua stack. But that part of the code is irrelevant to the question, and rather lengthy, so I won't paste it here.

Answer 1

You could try something like this:

template <typename T, typename R, typename ...Args>
R proxycall(T & obj, R (T::*mf)(Args...), Args &&... args)
{
    return (obj.*mf)(std::forward<Args>(args)...);
}

Usage: proxycall(obj, &hello::f);

Alternatively, to make the PTMF into a template argument, try specialization:

template <typename T, T> struct proxy;

template <typename T, typename R, typename ...Args, R (T::*mf)(Args...)>
struct proxy<R (T::*)(Args...), mf>
{
    static R call(T & obj, Args &&... args)
    {
        return (obj.*mf)(std::forward<Args>(args)...);
    }
};

Usage:

hello obj;

proxy<void(hello::*)(), &hello::f>::call(obj);

// or

typedef proxy<void(hello::*)(), &hello::f> hello_proxy;
hello_proxy::call(obj);