friendfunc.cpp

/*
Friend Class : A friend class can access private and protected members of other class in which it is declared as friend. It is sometimes useful to allow a particular class to access private members of other class.

Friend Function : Like friend class, a friend function can be given a special grant to access private and protected members. A friend function can be: 
a) A member of another class 
b) A global function 

Following are some important points about friend functions and classes: 
1) Friends should be used only for limited purpose. too many functions or external classes are declared as friends of a class with protected or private data, it lessens the value of encapsulation of separate classes in object-oriented programming.
2) Friendship is not mutual. If class A is a friend of B, then B doesn't become a friend of A automatically.
3) Friendship is not inherited (See this for more details)
4) The concept of friends is not there in Java.

Source : https://www.geeksforgeeks.org/friend-class-function-cpp/
*/

#include<iostream>
using namespace std;

class ClassB;

class ClassA {
    
    public:
        // constructor to initialize numA to 12
        ClassA() : numA(12) {}
        
    private:
        int numA;
        
         // friend function declaration
         friend int add(ClassA, ClassB);
};

class ClassB {

    public:
        // constructor to initialize numB to 1
        ClassB() : numB(1) {}
    
    private:
        int numB;
 
        // friend function declaration
        friend int add(ClassA, ClassB);
};

// access members of both classes
int add(ClassA objectA, ClassB objectB) {
    return (objectA.numA + objectB.numB);
}

int main() {
    ClassA objectA;
    ClassB objectB;
    cout << "Sum: " << add(objectA, objectB);
    return 0;
}