[SOLVED] How can a friend function access a public function from a base class?

lesca · 11-16-2010, 07:38 PM

Code:

#include <iostream>
using namespace std;
class Base
{
public:
	// this function may set some output attributes
	void foo();
};

class Derived : public Base
{
public:
	friend ostream& operator <<(ostream& outs, const Derived d);
};

ostream& operator <<(ostream& outs, const Derived d)
{
	foo();			// undeclared identifier
	/* OR */
	Base::foo();		// illegal call of non-static member function

	outs << "...";
	return outs;
}

void main()
{
	Derived d;
	cout << d;
}

Any tips are appreciated!
Thanks!

Update: 16:04, 17 Nov
I did some tests, and find one possible solution: (Any other solutions are welcome, if you have one.)

Code:

ostream& operator <<(ostream& outs, const Derived d)
{
	d.foo();		// Just do this!

	outs << "...";
	return outs;
}

paulsm4 · 11-17-2010, 01:56 PM

Hi -

Of course, your "possible solution" is THE solution. The compiler error is exclaiming "?WHAT? foo?!?". Explicitly stating "d.foo()" fixes the problem.

Or you can always refactor your clases

In general, the fact that you're even USING "friend" should probably be regarded as a "yellow flag" that your design might have problems. You're usually much better off refactoring your classes than using "friend".

Here's a great link that might help:

http://www.parashift.com/c++-faq-lite/friends.html

PS:
It's uncoincidental that Java, C# and most (all?) other contemporary OO languages do *not* have a "friend" construct

PPS:
Before somebody flames me, I learned that .Net *does* have a "friend" construct:

Code:

Friend Assemblies in C# and VB.Net

Imports System.Runtime.CompilerServices
Imports System
<Assembly: InternalsVisibleTo("AssemblyB")> 

' Friend class.
Friend Class FriendClass
    Public Sub Test()
        Console.WriteLine("Sample Class")
    End Sub
End Class

' Public class with a Friend method.
Public Class ClassWithFriendMethod
    Friend Sub Test()
        Console.WriteLine("Sample Method")
    End Sub
End Class