Access protected data member via friend member function

Question

EDIT:

The original program has multiple files as shown in here

I'm trying to access protected data member in my class from a friend member function of a different class.

I keep getting an access error:

9:32: error: invalid use of incomplete type 'class b' 5:7: error: forward declaration of 'class b' In member function 'void b::f(a*)': 12:13: error: 'int a::i' is protected 20:47: error: within this context

This is my code:

// Example program
#include <iostream>
#include <string>

class b;
class a{
    public:
    
    friend void b::f(a* pointer);
    
    protected:
    int i = 6;
    
    
    
};

class b{
    public:
    void f(a* pointer){std::cout<<pointer->a::i<<std::endl;}
    
    
};




int main()
{
  a a1;
  b b1;
  
  b1.f(&a1);
}

Answer 1

The issue is only superficially related to friend . It's rather due to circular dependencies of class definitions and member functions; a needs to know the definition of b because it refers to b::f . But the definition of b::f needs the definition of a , as it refers to a::i . Luckily, definitions of a class and its member functions can be separated from each other.

You can take the following measures to tackle this step by step:

1. declare class a;
2. define class b { /* ... */ }; , but without defining its member functions (as they need access to a 's definition
3. define class a { /* ... */ }; , possibly with inline member function definitions
4. and finally define b 's member functions.

This is how it looks like:

// Step 1:
class a;

// Step 2:   
class b{
    public:
    void f(a* pointer);
};

// Step 3:
class a{
    public:
    friend void b::f(a* pointer);
    
    protected:
    int i = 6;
};

// Step 4:
void b::f(a* pointer)
{
    std::cout<<pointer->a::i<<std::endl;
}

Answer 2

At the point where you try to declare the method of b a friend via friend void b::f(a* pointer); , the class b is incomplete. You cannot refer to members yet. On the other hand, b only needs a forward declaration for a :

// Example program
#include <iostream>
#include <string>

class a;
class b{
    public:
    void f(a* pointer);                
};

class a{
    public:        
    friend void b::f(a* pointer);        
    protected:
    int i = 6;
};

void b::f(a* pointer){std::cout<<pointer->a::i<<std::endl;}

int main()
{
  a a1;
  b b1;
  
  b1.f(&a1);
}

Answer 3

Try this. You are trying to reference b::f before class b is defined.

// Example program
#include <iostream>
#include <string>
class a;

class b{
    public:
    void f(a* pointer);
};
class a{
    public:
    
    friend void b::f(a* pointer);
    
    protected:
    int i = 6;        
};

void b::f(a* pointer)
{std::cout<<pointer->a::i<<std::endl;}






int main()
{
  a a1;
  b b1;
  
  b1.f(&a1);
}