Solved

Friend classes and methods

Posted on 2013-01-23
1
395 Views
Last Modified: 2013-01-23
Hello experts!

To declare method of class A a friend of class B this method needs to be declared above declaration of class B.

To declare entire class A a friend of class B class A does not need to be declared above declaration of class B.

Why does compiler works this way, what logic is behind it?

#include <iostream>
using namespace std;

class X;

class Y {
public:
  void print(X& x);
};

class X {
  int a, b;
  friend void Y::print(X& x);
  	  	  	  	
public:
  X() : a(1), b(2) { }
};

void Y::print(X& x) {
  cout << "a is " << x.a << endl;
  cout << "b is " << x.b << endl;
}

int main() {
  X xobj;
  Y yobj;
  yobj.print(xobj);
}

Open in new window


#include <iostream>
using namespace std;

class X {
  int a, b;
  friend class F;
public:
  X() : a(1), b(2) { }
};

class F {
public:
  void print(X& x) {
    cout << "a is " << x.a << endl;
    cout << "b is " << x.b << endl;
  }
};

int main() {
  X xobj;
  F fobj;
  fobj.print(xobj);
}

Open in new window

0
Comment
Question by:panJames
1 Comment
 
LVL 31

Accepted Solution

by:
Zoppo earned 500 total points
ID: 38809528
Hi panJames,

it is not possible to use something with friend which the compiler doesn't know yet. The second sample with friend class F; works because the compiler treats this as a forward declaration of the class F. But there's no way to do something similar to a forward declaration for class methods, so the first sample can only work if Y is completeley with all methods nad members declared before X.

Hope that helps,

ZOPPO
0

Featured Post

Free Tool: IP Lookup

Get more info about an IP address or domain name, such as organization, abuse contacts and geolocation.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
C++ assignment question 7 236
IdTelnet1DataAvailable handler freezes the Application 4 84
Template syntax for variable length arrays 9 75
Dialogbox API leak? 18 98
Article by: SunnyDark
This article's goal is to present you with an easy to use XML wrapper for C++ and also present some interesting techniques that you might use with MS C++. The reason I built this class is to ease the pain of using XML files with C++, since there is…
Introduction This article is the first in a series of articles about the C/C++ Visual Studio Express debugger.  It provides a quick start guide in using the debugger. Part 2 focuses on additional topics in breakpoints.  Lastly, Part 3 focuses on th…
The viewer will learn additional member functions of the vector class. Specifically, the capacity and swap member functions will be introduced.
The viewer will learn how to clear a vector as well as how to detect empty vectors in C++.

839 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question