Specify a class member function as a friend of another class?

Question

According to the C++ Primer book, the author mentioned that We can specify a class member function as a friend of another class, instead of the entire class (page 634).

Answer 1

When the compiler starts compiling the code( usually from top ) and it encounters this line:

friend void B::fB(A& a);

1. at this point, compiler has no idea about type info of B so it throws an error ( 'B' : is not a class or namespace name ).

2. by forward declaration of class B, compiler knows about type of B is Class in advance to its actual declaration with all members.

3. run below code after forward declaration of class B.

///////////////

class B;
class A
{
public:
    friend void B::fB(A& a); 
    void fA(){};
};
class B
{
public:
    void fB(A& a){};
    void fB2(A& a){};
};

Still error !!!

because forward declaration is just a declaration of an identifier for which the programmer has not yet given a complete definition. so compiler needs full definition of B before class A.

Note: class A definition dependents on type of B and also definition of B (i.e B::fB) so that forward declaration alone can not resolve, complete definition of class B needs to define before class A.

4 run this code

////////

class B
{
public:
    void fB(A& a){};
    void fB2(A& a){};
};
class A
{
public:
    friend void B::fB(A& a); 
    void fA(){}
};

Still error !!!

because class B member functions fB & fB2 having arguments of type A but compiler has no idea about type info of A so by forward declaration of class A, we can let compiler knows about type info of A. Note: class B definition only dependent on type of A not the members of A so that forward declaration of A resolve step 4.

5. final code

////////////////////////

class A;  // forward declaration of A needed by B
class B
{
public:
    void fB(A& a);
};

class A
{
    int i;
public:
    friend void fA(A& a);    //specifying function fA as a friend of A, fA is not member function of A
    friend void B::fB(A& a); //specifying B class member function fB as a friend of A
};

// fA is Friend function of A
void fA(A& a)
{
    a.i  = 11; // accessing and modifying Class A private member i
    cout<

6 Exercise:

// Cyclic dependency 
#include
using namespace std;

class A;

class B
{
public:
    void fB(A& a);
    friend void A::fA(B& b); //specifying class A's member function fA as a friend of B
};

class A
{
    int i;
public:
    void fA(B& b);  
    friend void B::fB(A& a); //specifying class B's member function fB as a friend of A
};

int main()
{
    return 0;
}