Solved

Friend classes and methods

Posted on 2013-01-23
1
396 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
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
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

Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

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

Suggested Solutions

Templates For Beginners Or How To Encourage The Compiler To Work For You Introduction This tutorial is targeted at the reader who is, perhaps, familiar with the basics of C++ but would prefer a little slower introduction to the more ad…
Go is an acronym of golang, is a programming language developed Google in 2007. Go is a new language that is mostly in the C family, with significant input from Pascal/Modula/Oberon family. Hence Go arisen as low-level language with fast compilation…
The goal of the video will be to teach the user the concept of local variables and scope. An example of a locally defined variable will be given as well as an explanation of what scope is in C++. The local variable and concept of scope will be relat…
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.

756 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