Class non-static method pointer to global function - c++

Question

I'm trying to point a class method to a global function, i've seen this
but how i can't do it without instance?.
Consider this:
class x
{
public:
int(x::*GetVal)(int);
};
int RtX(int a)
{
return a * 4;
}
// declaration
int(x::*GetVal)(int) = (int(x::*)(int))&::Rtx; // :: global? // error
int main()
{
x a;
cout << (a.*GetVal)(4) << endl;
}
This returns me the error:
[Error] invalid cast from type 'int ()(int)' to type 'int
(x::)(int)'

Answer 1

x::GetX is a pointer to member. These are deeply complicated beasts, and you can't get them to point to non-member functions. The following code will work:
#include <iostream>
int RtX(int a) // Global non-member function
{
return a * 4;
}
class x
{
public:
int(x::*GetVal)(int);
// This is an instance member function which acts as a proxy and calls the
// the global function
int RtX(int a) { return ::RtX(a); }
};
int main()
{
x a;
a.GetVal =&x.RtX; // Assign the member variable. Could do this in the
// constructor. **NOTE** No casts!
std::cout << (a.*GetVal)(4) << std::endl;
}
If you find yourself reaching for a cast when dealing with function pointers and pointers-to-member-functions, stop - you are almost certainly doing it wrong, and while it will compile, it is very likely not to run properly.
Alternatively, as noted in the comments, use std::function.
#include <iostream>
#include <functional>
int RtX(int a)
{
return a * 4;
}
class x
{
public:
std::function<int(int)> GetVal;
// Initialize GetVal in the constructor.
x() : GetVal(RtX)
{}
// Alternatively, you can initialize the variable in with a default
// initialization. You can also declare the member const if you don't want to
// change it later.
const std::function<int(int)> gv = RtX;
/*
};
int main()
{
x a;
std::cout << a.GetVal(4) << std::endl;
}

Answer 2

Non static member functions need an instance in order to be called. You might consider using a static function member instead and if you also use std::function, you might get a simple code to assign your member function without instance:
#include <iostream>
#include <functional>
int RtX(int a)
{
return a * 4;
}
class x
{
public:
static std::function<int(int)> GetVal;
};
std::function<int(int)> x::GetVal = RtX;
int main()
{
x a;
std::cout << (a.GetVal)(4) << std::endl;
}