Friend function in C++

• If a function is defined as a friend function then, the private and protected data of class can be accessed from that function.
• The complier knows a given function is a friend function by its keyword friend.
• The declaration of friend function should be made inside the body of class (can be anywhere inside class either in private or public section) starting with keyword friend.

Syntax:

class class_name
{
    ......  ....   ........
    friend return_type function_name(arguments);
    ......  ....   ........
}

• Now, you can define friend function of that name and that function can access the private and protected data of that function.
• No keywords in used in function definition of friend function.

Example:

#include <iostream>

class Distance
{
    private:
        int meter;
    public:
        Distance(): meter(0){ }
        friend int func(Distance);  //friend function
};
int func(Distance d)            //function definition
{
    d.meter=5;         //accessing private data from non-member function
    return d.meter;
}
void main()
{
    Distance D;
    cout<<"Distace: "<<func(D);
    getch();
}

Output:

Distance: 5

• Here, friend function func() is declared inside Distance class. So, the private data can be accessed from this function.
• Though this example gives you what idea about the concept of friend function, this program doesn't give you idea about when friend function is helpful.

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