CP2001 Lecture Notes - More Basic Concepts in C++

CP2001 Data Structures and Algorithms
More Basic Concepts in C++

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Friend Functions

Chpt 7.3

A friend function of a class is a non-member function which can gain access to the private (and protected) section of the class.

For example,

	class A
	{
	  private:
		int val;
		...
	  public:
		...
		void wonderland();  //member function
		friend void alice();  //non-member function
		...
	};

allows the non-member function alice() to access class A's private members (eg, val).

Note that, since alice() is not a member function of class A, it is

not in the scope of the class (it could be just a global function),
not called with the member access operator (unless it is a member of another class).

Hence the advantage to using friend functions.

You can also make all of a class's member functions to be friends of another class:

	class X
	{
	  ...
	  friend class Y;  // make all of y's member functions friends of X
	  ...
	};

That is, allow all member functions in class Y to access the private members of class X.

We shall use this at a later stage.

Advantages of friend functions:

More efficient - eg, reduced function overheads
Avoids writing a large number of member functions to handle all possible accesses to private members of a class

Note: Use friend functions sparingly - private members should stay private (as much as possible!).

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