Member function of a class as friend to another class

Question

In this code , i have made max function of class B friend of class A.I have also done forward declaration of class B.But it is giving error.

#include<iostream>

using namespace std;

class B;

class A
{
   int a;
   public:

   void get()
   {
      cin>>a;
   }

   friend void B :: max(A , B);
};

class B
{
   int b;
   public:

   void get()
   {
      cin>>b;
   }

   void max(A x, B y)
   {
      if (x.a > y.b)
         cout<< " x is greater";
      else cout<<"y is greater";
   }
};

int main()
{
   A x;
   B y,c;
   x.get();
   y.get();
   c.max(x,y);
}

**Always** post any error text your questioning **verbatim**. — WhozCraig, Aug 23 '15 at 19:33
Did you see this question in your research: [Declare a member-function of a forward-declared class as friend](https://stackoverflow.com/questions/6310720/declare-a-member-function-of-a-forward-declared-class-as-friend) — WhozCraig, Aug 23 '15 at 19:39
You can get some tips here: [How to forward declare a member function of a class to use in another class](http://stackoverflow.com/questions/11344195/how-to-forward-declare-a-member-function-of-a-class-to-use-in-another-class?rq=1) — Bo Persson, Aug 23 '15 at 19:40

Elohim Meth · Answer 1 · 2015-08-23T19:42:23.647

2

As R Sahu already answered:

You cannot use:

friend void B :: max(A , B);

without the complete definition of B.

This is how you can achieve your goal:

#include<iostream>
using namespace std;

class A;

class B{
    int b = 2;

public: 
    void max(A x, B y);
};

class A{
    int a = 1;
public:
    friend void B :: max(A , B);
};

void B::max(A x, B y){
    if (x.a > y.b)
        cout<< " x is greater";
    else 
        cout<<"y is greater";
}

int main(){
A x;
B y,c;
c.max(x,y);
}

edited Aug 23 '15 at 19:42

answered Aug 23 '15 at 19:39

Elohim Meth

1,777
9
13

was about to answer just that :) – Evert Heylen Aug 23 '15 at 19:40

Maksim Solovjov · Accepted Answer · 2015-08-23T19:53:49.000

B is incomplete at the point where you declare B::max as a friend method. Thus, compiler does not know if there is such a method.

This means that you need to

reorder classes, so that A knows B has a method B::max and
Implement the method B::max outside of the class definition, when both classes are complete, because you access internal variables.

It is also a good idea to pass your arguments by const reference. Use const to emphasise that you are not modifying them. Pass by reference to avoid unnecessary copying.

So, with this in mind:

class A;

class B{
    int b;
public: 
    void get(){
        cin>>b;
    }
    void max(const A& x, const B& y);
};

class A{
    int a;
public:
    void get(){
        cin>>a;
    }
    friend void B :: max(const A& , const B&);
};

void B::max(const A& x, const B& y) {
    if (x.a > y.b)
       cout<< " x is greater";
    else
        cout<<"y is greater";
}

Tweaked the answer to correct myself, that point was not necessary — Maksim Solovjov, Aug 23 '15 at 19:54

score 1 · Answer 3 · answered Aug 23 '15 at 19:34

1

You cannot use:

friend void B :: max(A , B);

without the complete definition of B.

You'll need to rethink your strategy so that you can implement the functionality without using the friend declaration or move the definition of B ahead of the definition of A.

answered Aug 23 '15 at 19:34

R Sahu

204,454
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Pls tell me how i can do that. – ishan bansal Aug 23 '15 at 19:35
You could just `friend class B`, if you have no problem friending the entire class. – Evert Heylen Aug 23 '15 at 19:35
can i declare void B:: max(A,B); after forward declaration of class B; – ishan bansal Aug 23 '15 at 19:36
@ishanbansal, no, you can't do that. – R Sahu Aug 23 '15 at 19:36

Member function of a class as friend to another class

3 Answers3

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