Friend Classes: Cannot Access private members?

Question

I was just learning C++ friend classes. As it says on books and tuts, a friend class can access all the members (private and protected) too. But does not happen to be that in my case.

I know there's this stupid error that i cannot see. Please help me find it :D

My code:

#include <iostream>
using namespace std;

class A;
class B {
private:
    int num;
public:
    B(int n=0):num(n){}

    friend int add(A, B);
    friend int mul(A, B);
    friend int sub(A, B);
    void showthis(A);
    friend class A;
};

class A{
private:
    int num;
public:
    A(int n=0):num(n){}

    friend int add(A, B);
    friend int mul(A, B);
    friend int sub(A, B);   
};

int add(A a, B b){
    return a.num+b.num;
}

int sub(A a, B b){
    return a.num-b.num;
}

int mul(A a, B b){
    return a.num*b.num;
}


void B::showthis(A a){
    cout<<a.num<<endl;
}

int main(){
    A a(3);
    B b(6);
    cout<<add(a,b)<<endl;
    cout<<mul(a,b)<<endl;
    cout<<sub(a,b)<<endl;
    b.showthis(a);
}

The error:

q17.cpp: In member function ‘void B::showthis(A)’:
q17.cpp:20:6: error: ‘int A::num’ is private
  int num;
      ^
q17.cpp:43:10: error: within this context
  cout<<a.num<<endl;

Answer 1

You declared neither B::showthis(A) nor class B as class A's friend.

You could add either

friend B::showthis(A);

or

friend class B;

into class A.

Answer 2

You can see A is friend of B, but B is not friend of A. So you need declare friend class B in A.

In class A, add this line, like @timrau answer.

friend class B;