C++ program to add the members of two different classes using the friend function
kw.cpp
#include <iostream>
using namespace std;
class WhiteBank;
class BlackBank
{
private:
    int total_money_bb;

public:
    BlackBank() : total_money_bb(200) {}
    friend int total_amount(WhiteBank, BlackBank);
};

class WhiteBank
{
private:
    int total_money_wb;

public:
    WhiteBank() : total_money_wb(200) {}
    friend int total_amount(WhiteBank, BlackBank);
};

int total_amount(WhiteBank wb, BlackBank bb)
{
    return (wb.total_money_wb + bb.total_money_bb);
}

int main()
{
    WhiteBank wb;
    BlackBank bb;
    cout<<"Total Bank Amount "<<total_amount(wb, bb)<<endl;
    return 0;
}
Output
kodingwindow@kw:~$ g++ kw.cpp
kodingwindow@kw:~$ ./a.out
Total Bank Amount 400
kodingwindow@kw:~$
Points to Remember

1. The keyword friend is placed only in the function declaration of the friend function and not in the function definition because it is used to access the member of a class. In simple words, A friend function is a function that has no body inside its base class.

2. A friend can be a function, function template, member function, or a class, class template, in which the entire class and all of its members are friends.

3. Friend function can access public, private and protected data members of the class for which it is a friend.

4. Virtual functions and objects cannot be friends

5. Friend functions do not have a this pointer associated with it, because friends are not members of a class.

6. Friend of one class can be friend of another class or all the classes in one program, such a friend is known as global friend.

7. Friend can be declared anywhere (in public, private or protected section) of class

8. Properties of friend function:
8.1 Symmetric - if class A is a friend of class B, class B is not automatically a friend of class A.
8.2 Transitive - if class A is a friend of class B, and class B is a friend of class C, class A is not automatically a friend of class C.
8.3 Inherited - if class Base is a friend of class X, subclass Derived is not automatically a friend of class X; and if class X is a friend of class Base, class X is not automatically a friend of subclass Derived.

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