Friend2.cpp

#include<iostream>
using namespace std;

class B;                                              //Forward declaration of class B

class A{
    private:
    int num1;

    public:
    A(){
        this->num1=10;
    }

    A(int num1){
        this->num1=num1;
    }

    friend void sum(A aobj, B bobj);
};

class B
{
private:
    int num2;

public:
    B(){
        this->num2=20;
    }

    B(int num2){
        this->num2=num2;
    }

    friend void sum(A aobj, B bobj);

};

void sum(A aobj, B bobj){                              //friend fn defined outside the class //friend keyword not required
    int res=aobj.num1+bobj.num2;
    cout<<"Result = "<<res<<endl;
}

int main(){
    A a1(200);
    B b1(150);
    sum(a1,b1);

    A a2;
    B b2;
    sum(a2,b2);

    return 0;
}