2.6. Friends of a Class

2.6. Friends of a Class
	Chapter 2. Top of the `class`

[ fromfile: friends.xml id: friends ]

Now that you know about accessibility rules, you need to know how to occasionally break them.
The friend mechanism makes it possible for a class to allow nonmember functions to access its private data.
The keyword friend is followed by a class or a function declaration.
friend declarations are located inside a class definition.

Here are some syntax examples.

class Timer {
   friend class Clock;
   friend void Time::toString();
   friend ostream& operator <<(ostream& os, const Timer& obj);
   [...]   
   
 private:
   long m_Elapsed;
};

A friend can be a class, a member function of another class, or a nonmember function.

Typically, friend functions are used for two purposes.

For Factory methods when you want to enforce creational rules (Section 16.1) on a class.
For global operator functions such as operator<<() and operator>>() when you do not want to make the operator a member function, or do not have write-access to the class definition.


2.5.1. UML Relationships		2.7. Constructors