C++ Friend Function Tutorial

A friend function of class is defined outside that class's scope, yet has the right to access the non-public (and public) members of the class. Even though the prototype for friend function appear in the class definition, friends are not members functions. 

Using friend functions can enhance performance. A friend can be a function, function template, or member function, or a class or class template, in which case the entire class and all of its members are friends.

To declare a function as friend of a class, precede the function prototype in the class definition with keyword friend as below:

class student {
	private:
		int ST_Id;
		string ST_Name;
	public
		friend void DisplayName(student ST); //_prototype friend function
		void setName(string Name);	
}

To declare all member function of a class as friend with other class, place a following declaration in the definition of class: friend class ClassName;
Example: we have ClassA and ClassB and we want all member functions of ClassB as friends of ClassA, declaration as below definition of class ClassA: friend class ClassB;

Click Here to download example file for practice. You have to download Code Block to open project file.

Reference: C Plus Plus How to Program 8th Edition by Paul Deitel