Friend Function & class

FRIEND FUNCTIONS
If we want to declare an external function as a friend of class
thus allowing this function to have access to the private and protected
members of this class,we do it by declaring a prototype of this external
functions within the class and preceding it with keyword 'friend'.
For example, we could define an operator that multiplies a Matrix by a Vector . Naturally,

Vector and Matrix each hide their representation and provide a complete set of operations for

manipulating objects of their type. However, our multiplication routine cannot be a member of

both. Also, we don’t really want to provide lowlevel

access functions to allow every user to both

read and write the complete representation of both Matrix and Vector .

To avoid this, we declare the operator  a friend of both:

c l a s s M a t r i x ;

c l a s s V e c t o r

 {

f l o a t v [4 ];

f r i e n d V e c t o r o p e r a t o r *(c o n s t M a t r i x &, c o n s t V e c t o r &);

};

c l a s s M a t r i x

{

V e c t o r v

f r i e n d V e c t o r o p e r a t o r

};

reference(bjaurne.S book)

FRIEND CLASS
 we can define a class as friend of another class,by granting
that first class access to the protected and private members of
the second one.