Friend functions and classes allow non-member functions or other classes to access the private and protected members of a class. A friend function is defined outside the class but has access to private members, and can be used when two classes need to access each other's private data. A class can declare another class as a friend, making all of its member functions friends. This allows them access to the private members of the other class. Friendship violates encapsulation but is useful when two classes are tightly coupled.

1. OBJECT ORIENTED PROGRAMMINGOBJECT ORIENTED PROGRAMMING
Friends Classes and Functions

2. Friend functionsFriend functions
 A friend function of a class is defined outside that class's
scope, yet has the right to access the non-public (and
public) members of the class
 Let’s say you want a function to operate on objects of
two different classes and access their private data, you
would need to use a friend function
CPS235:Inheritance 2

3. Example ofExample of friendfriend functionsfunctions
class beta; //req for frifunc declaration
class alpha
{
private:
int data;
public:
alpha():data(3) {}
friend int frifunc(alpha, beta); //friend
func
declaration
};
class beta
{
private:
int data;
public:
beta():data(7) {}
friend int frifunc(alpha,beta);
//friend func
declaration
};CPS235:Inheritance 3
A class cannot be referred to
until it has been declared

4. Example ofExample of friendfriend functionsfunctions
int frifunc(alpha a, beta b) //friend func
defined
{
return (a.data + b.data);
}
void main()
{
alpha aa;
beta bb;
cout<<frifunc(aa,bb)<<endl; //friend func
called
getch();
}
CPS235:Inheritance 4

5. friendfriend classesclasses
 The member functions of a class can all be made friends
at the same time when you make the entire class a
friend
 If class alpha declares class beta as a friend class, then all
member functions of class beta can access private data
of class alpha
 It is also possible to declare only one member function
of another class to be a friend
CPS235:Inheritance 5

6. friendfriend classesclasses
class beta;
class alpha
{
private:
int data;
public:
alpha():data(3) {}
friend class beta; //friend class
declaration
};
class beta
{
public:
void func(alpha a)
{
cout<<"alpha's data:
"<<a.data;
}
};
CPS235:Inheritance 6

7. friendfriend classesclasses
void main()
{
alpha aa;
beta bb;
bb.func(aa);
getch();
}
CPS235:Inheritance 7

8. Class member functions asClass member functions as
friendsfriends
class CSquare;
class CRectangle {
int width, height;
public:
int area ()
{return (width * height);}
void convert (CSquare a);
};
class CSquare {
private:
int side;
public:
void set_side (int a)
{side=a;}
friend void CRectangle::convert(CSquare);
};
CPS235:Inheritance 8

9. Class member functions asClass member functions as
friendsfriends
void CRectangle::convert (CSquare a) {
width = a.side;
height = a.side;
}
int main () {
CSquare sqr;
CRectangle rect;
sqr.set_side(4);
rect.convert(sqr);
cout << rect.area();
getch();
return 0;
}
CPS235:Inheritance 9

10. Summary (friends)Summary (friends)
 Even though the prototypes for friend functions appear
in the class definition, friends are not member functions
 Friend declarations can be placed anywhere in a class
definition either in public, private or protected sections
 Violates the data hiding principle of classes, so it should
be avoided as much as possible
 Friendship is granted, not taken i.e., for class B to be a
friend of class A, class A must explicitly declare that
class B is its friend
 The friendship relation is neither symmetric nor
transitive
CPS235:Inheritance 10