Pointer to friend function.

[Qbaza]

Hello

I wonder if it is possible to use pointers to friend functions. I wrote a code that tried to do it, but I wasn't able to compile it until I changed that function to regular function.

Does anyone have any experience with that?

[marembo]

is the friend function a global function or a member function of a class?
if it is global simply do this:

void *myfunctptr = myfunct; //provided it is not overloaded
//am not so sure if it is a member function of another class, you can try the following
void *myfunctptr = myclass::myfunct;

[Disch]

I must object.

It is a terrible idea to cast function pointers to void pointers.

awful awful awful.

[Qbaza]

To be more specific. Why this code doesn't want to compile?

#include <iostream>

using namespace std;

class myclass {
	private:
		int a;
		int b;
	public:
		myclass(int i, int j) : a(i), b(j) {};
		friend int sum(const myclass & obj) {
			return obj.a + obj.b;
		}
		friend int difference(const myclass & obj) {
			return obj.a - obj.b;
		}
};

void calculate(int (*fun)(const myclass & obj));

int main() {
	calculate(sum);
}

void calculate(int (*fun)(const myclass & obj)) {
	myclass temp(3,7);

	cout << fun(temp) << endl;
}

I get this error:

/C++/main.cpp: In function ‘int main()’:
/C++/main.cpp:22: error: ‘sum’ was not declared in this scope

[Albatross]

That function is missing a few arguments, that function is a member of a class that has no instance named sum, and... you know what, here's a link for you to read:
http://cplusplus.com/doc/tutorial/classes/

-Albatross

[Qbaza]

I'm sorry, but I don't get it ...

Sum is friend function not member function and as far as I know you don't invoke it by using class object.

[moorecm]

There seem to be a number of misunderstandings in the snippet posted. I suggest reading up on both friend functions and static methods to start.

As far as passing the member function as an argument, I would recommend against it altogether. It can be done but I suspect there are better solutions at your disposal.

[Qbaza]

I have not written this post without prior searching for solution in other materials. Maybe I'm missing something very basic, but I would really appreciate if someone could point that specifically ... not sending me to some other vague places ...

And once again sum is not a member function, it is friend function.

[firedraco]

friend int sum(const myclass & obj) {
			return obj.a + obj.b;
		}

Doing this creates a member function called sum, and the friend is redundant since all member functions are friends.

[moorecm]

http://www.cplusplus.com/doc/tutorial/inheritance/

Notice that you declare a friend function in a class but the function is defined outside of the class.

[Qbaza]

Thank You for responses. Indeed moving definitions out from class declaration solves compilation problem. But it is first time that I hear that friends have to be defined outside of class. Very interesting ...

[Qbaza]

OK, I looked at that a bit more and found out that neither of solutions given here so far is correct. The problem is a bit different. To show that you can simple divide the code I showed before into 3 files
* myclass.h with class declaration
* myclass.cpp with class methods and friend functions definitions
* main.cpp

In that configuration it will give the same error as the beginning.

The true problem is with friend function scope -- check Stroustrup - The C++ Programming Language 11.5.1. Basically friend functions are not in scope and solution is to put declarations of friend functions once again before main (lines 42, 43; difference doesn't second declaration because definition is in scope).

myclass.h

#ifndef MYCLASS_H
#define MYCLASS_H

class myclass {
	private:
		int a;
		int b;
	public:
		myclass(int i, int j) : a(i), b(j) {};
		friend int sum(const myclass & obj);
		friend int difference(const myclass & obj);
		friend int times(const myclass & obj) {
			return obj.a * obj.b;
		}
};

#endif // MYCLASS_H

myclass.cpp

#include "myclass.h"

int sum(const myclass & obj) {
	return obj.a + obj.b;
}

main.cpp

#include <iostream>
#include "myclass.h"

using namespace std;

int difference(const myclass & obj) {
	return obj.a - obj.b;
}

void calculate(int (*fun)(const myclass & obj));

int sum(const myclass & obj);
int times(const myclass & obj);

int main() {
	calculate(sum);
	calculate(difference);
	calculate(times);
}

void calculate(int (*fun)(const myclass & obj)) {
	myclass temp(3,7);
	cout << fun(temp) << endl;
}

[jsmith]

FWIW, a friend declaration is not a prototype.

Technically speaking, to be standard compliant, you must declare sum, difference, and times in
myclass.h after the class:

int sum( const myclass& );
int difference( const myclass& );
int times( const myclass& );