simple friend function not working

[suresh.amritapuri]

Hi,
What is wrong in this simple friend function code?

#include <iostream>
using namespace std;

class B;
class A{
public:
A():aa(10){}
void dispA(){cout << aa << endl;}
private:
int aa;
friend class B;
};

class B{
public:
B():bb(20){}
void dispB(){cout << bb + aa << endl;}
private:
int bb;
};

int main(){

A a;
B b;
b.dispB();
}

compiler (g++) says: testFriend.cpp:17: error: ‘aa’ was not declared
in this scope.

any suggestions?
thanks
suresh

 

[Ian Collins]

Hi,
What is wrong in this simple friend function code?

#include <iostream>
using namespace std;

class B;
class A{
public:
A():aa(10){}
void dispA(){cout << aa << endl;}
private:
int aa;
friend class B;
};

class B{
public:
B():bb(20){}
void dispB(){cout << bb + aa << endl;}
private:
int bb;
};

int main(){

A a;
B b;
b.dispB();
}

compiler (g++) says: testFriend.cpp:17: error: ‘aa’ was not declared
in this scope.

That's correct. There isn't a member of B called aa.

Friendship only grants a class access to another's private parts, it
doesn't include it.

 

[Jonathan Lee]

Hi,
What is wrong in this simple friend function code?

You're trying to use it like you were inheriting. "aa"
is indeed not in scope.

"aa" is not a member of class B. It is a member of class A
and, more to the point wrt B, a member of an *instance* of A.

You would need to change to either:

// Access the member aa of instance x
void dispB(const A& x) { cout << bb + x.aa << endl; }

or drop the friend and inherit A

class B : public A {
// ...
};

Depends on what you're really trying to do. A third
option would be to write a getter for aa. i.e.,
int A::get_aa() const { return aa; } // pick a better name

--Jonathan

 

[suresh.amritapuri]

You're trying to use it like you were inheriting. "aa"
is indeed not in scope.

"aa" is not a member of class B. It is a member of class A
and, more to the point wrt B, a member of an *instance* of A.

You would need to change to either:

  // Access the member aa of instance x
  void dispB(const A& x) { cout << bb + x.aa << endl; }

or drop the friend and inherit A

  class B : public A {
    // ...
  };

Depends on what you're really trying to do. A third
option would be to write a getter for aa. i.e.,
  int A::get_aa() const { return aa; } // pick a better name

--Jonathan

Thank you all, I got the point,
suresh