incomplete type error , when using friend functions

#include <stdio.h>
class B;

class A;

c开发者_运维问答lass A
{
    int a;
    friend int B::f();
};


class B
{
    int b;
    class A x;
public:
    int f();
};

int B::f()
{
    // ...
}

main()
{
    class B b;
    b.f();
}

ERRORS:

a.cpp:9: error: invalid use of incomplete type ‘struct B’

a.cpp:2: error: forward declaration of ‘struct B’

The issue cannot be solved by placing definition of B before A as B has an object of type A.

For this example making B a friend class would do, but in my real code I have more member functions in B (so I need alternative solution).

Finally, can somebody give me links that explain what the compiler does when it comes across a forward declaration, declaration, definition.

You simply cannot do what you want to do as-is. To make that friend function declaration in class A the nature of class B needs to be known prior to the definition of class A. To make class B contain an instance of class A the nature of class A must be known prior to the definition of class B. Catch-22.

The former does not apply if you make class B a friend class of class A. The latter does not apply if you modify B to contain a pointer or reference to an instance of class A.

DefineB before A, and declare a pointer to A as member data of B:

class A; //forward declaration

class B
{
    int b;
    A  *px; //one change here - make it pointer to A
 public:
    int f();

};

class A
{    
    int a;
    friend int B::f();
};

Or, you could make the entire class B a friend of A, that way you don't have to make the member data pointer to A.

class B; //forward declaration

class A
{    
    int a;
    friend class B;
};

class B
{
    int b;
    A   x; //No change here 
 public:
    int f();

};

Forward declare class A; define class B; define class A; define B::f.

#include <cstdio>
class A;


class B
{
int b;

public:
int f();
};

class A
{
int a;
friend int B::f();
};


int B::f()
{
}

main() 
{
class B b;
b.f();
}