Friends function and_classes

OBJECT ORIENTED PROGRAMMINGOBJECT ORIENTED PROGRAMMING
Friends Classes and Functions

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

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

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();
}

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

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;
}
};

void main()
{
alpha aa;
beta bb;
bb.func(aa);
getch();
}

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);
};

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;
}

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

Friends function and_classes

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