In C++, a friend class is which can access
the "private" and "protected" data members and members
method of the class in which it is declared as a friend. On declaration of
friend class all member function of the friend class become friend of the class
in which the friend class was declared.
If class A declares class B as its friend
then class B can access all the data member including the one that declared as
protected and private. In addition class B also have access of all member method
of class A.
Example : In the following example, among two class the class Friend1 declare the class Friend2 as friend. So,
Friend2 class eligible to access the data member (x) and member
method (getdata()) of Friend1 class with help of an object of Friend1.
In this program we add two numbers where
the on number inputted from each class.
#include <iostream.h>
class
Friend1
{
private
:
friend
class Friend2;
int
x;
public:
void
getdata();
};
void
Friend1:: getdata()
{
cout<<"\n\tEnter
First No : ";
cin>>x;
}
class
Friend2
{
private:
int
y;
public:
void
read();
void
sum(Friend1 temp);
};
void
Friend2::read()
{
cout<<"\n\tEnter
Second No : ";
cin>>y;
}
void
Friend2::sum(Friend1 tmp)
{
tmp.getdata();
read();
cout<<"\n\t
Sum of "<<tmp.x<<" and "<<y<<" =
"<<tmp.x+y;
}
void
main()
{
Friend1
Fr1;
Friend2
Fr2;
Fr2.sum(Fr1);
}
Example : In this program there are two classes are TV and Remote, Where TV class
is the mutual friend of Remote to
toggle the state of a remote variable State.
#include <iostream.h>
class Remote
{
private :
friend class TV;
char State;
public:
Remote()
{
State='I';
}
void remote();
};
void Remote:: remote()
{
if(State=='N')
{
cout<<"\n\t * *
The Remote is now in Normal Mode * *\n";
}
else if(State=='I')
{
cout<<"\n\t * *
The Remote is now in Interactive Mode * *\n ";
}
}
class TV
{
private:
enum mode{OFF = 0, ON =1};
public:
void Toggle(Remote);
};
void TV::Toggle(Remote tmp)
{
int TvState;
char st;
cout<<"\n\tWanter to
Put TV on(y/n)? ";
cin>>st;
if(st=='y' || st=='Y')
{
mode(ON);
cout<<"\n\tThe
TV is On ";
while(st!='X' || st!='x')
{
cout<<"\n\tEnter
State of Remote
cin>>st;
if(st>=97
&& st<=122)
{
st-=32;
}
if(st=='I' || st=='N')
{
tmp.State=st;
tmp.remote();
}
else if(st=='X')
{
break;
}
else
{
cout<<"\n\tInvalid
Input";
}
}
}
else
{
cout<<"\n\tTv
is OFF, You Cannot Change Remote State";
}
}
void main()
{
clrscr();
Remote Rm;
TV T;
Rm.remote();
T.Toggle(Rm);
}
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