Friend_function.cpp

#include <bits/stdc++.h>
using namespace std;

/*
    Friend function
    -Has access to the private and protected members of the class (breaks encapsulation)
    -must have its prototype included withing the scope of the class prefixed with the keyword friend
    -does not have its name qualified with the class scope (MyClass:: is not required)
    -is not called with an invoking object of the class
    -can be declared friend in more than one class

    -It can be a global function
    -Any member function of a different class could also be a friend of a class
    -It can be a function template
*/


class MyClass {
    int data_;
public:
    explicit MyClass(int i) : data_(i) {}

    friend void display_friend_func(const MyClass& a);
};

//Note that display() is not a member function. This is just a global function
void display(const MyClass& a) {
    //cout << "data = " << a.data_; //Error !!! (data_ is private)
}

//This is a global function (not class member) but it's friend of MyClass
//Note :
    //Why const(const MyClass) ? Because I am accessing private variable inside it.
    //Why reference (MyClass&) ? Because, i don't want an unnecessary copy constructor
void display_friend_func(const MyClass& a) {
    cout << "data = " << a.data_;
}

int main() {
    MyClass obj(10);

    //display(obj);

    display_friend_func(obj);

return 0;
}