What is a friend function in C++?

In C++, a friend function is a special function that can access the private and protected members of a class. Although a friend function is not a member function of the class, it can access all members of the class as if it were a member function.

There are two main purposes for using friend functions:

1. Implementing operator overloading: Sometimes, we need to overload operators (e.g., the << operator) for a class, which require access to the class's private data. By declaring these operator functions as friends of the class, they can access the private and protected members.

2. Enabling tight cooperation between classes: When two or more classes need to work closely together and access each other's internal states, we can use friend functions or classes to establish this relationship.

Example

Consider a simple class Box with a private member width. We can create a friend function to access this private member.

cpp
#include <iostream>

class Box {
private:
    double width;

public:
    Box(double wid) : width(wid) {}

    // friend function
    friend void printWidth(Box box);
};

void printWidth(Box box) {
    // Access private member from the friend function
    std::cout << "Width of box: " << box.width << std::endl;
}

int main() {
    Box box(10);
    printWidth(box);
    return 0;
}

In this example, printWidth is a friend function that can access the private member width of the Box class. This would not be possible without the friend keyword, as non-member functions typically cannot access private or protected members of a class.

In summary, the friend keyword provides a flexible way to allow certain external functions or classes to access the internal members of a class while maintaining the principle of encapsulation.