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Friend Functions and Friend Classes in C++

1. Friend Functions

A friend function is a function that is not a member of a class but has access to its private and protected members. It is declared inside the class using the friend keyword.

Friend functions are useful when you need a non-member function to access the internal details of a class, such as when overloading operators or implementing complex interactions between classes.

#include <iostream>

class Example {
private:
    int data;
public:
    Example() : data(0) {}
    // Declare the friend function
    friend void showData(const Example& obj);
};

// Definition of the friend function
void showData(const Example& obj) {
    std::cout << "Data: " << obj.data << std::endl;
}

int main() {
    Example ex;
    showData(ex); // Friend function accessing private data
    return 0;
}
                    

In this example, showData is a friend function of the Example class. It can access the private data member data of the Example class, allowing it to print the data even though it is not a member of the class.

2. Friend Classes

A friend class is a class that can access the private and protected members of another class. It is declared using the friend keyword within the class whose members are to be accessed.

Friend classes are useful when you need another class to have access to the private or protected members of your class for purposes like data manipulation or encapsulating complex interactions.

#include <iostream>

class Example {
private:
    int data;
public:
    Example() : data(0) {}
    // Declare the friend class
    friend class FriendClass;
};

class FriendClass {
public:
    void showData(const Example& obj) {
        std::cout << "Data: " << obj.data << std::endl;
    }
};

int main() {
    Example ex;
    FriendClass fc;
    fc.showData(ex); // Friend class accessing private data
    return 0;
}
                    

In this example, FriendClass is a friend class of the Example class. It can access the private data member data of the Example class, allowing it to print the data even though it is not a member of the Example class.