Friend Function Program in C++

/*A friend function allows you to generate more efficient

code. The following program illustrates this concept:*/

#include <iostream.h>

#include <conio.h>

const int IDLE = 0;

const int INUSE = 1;

class C2; // forward declaration

class C1 {

int status; // IDLE if off, INUSE if on screen

// ...

public:

void set_status(int state);

friend int idle(C1 a, C2 b);

};

class C2 {

int status; // IDLE if off, INUSE if on screen

// ...

public:

void set_status(int state);

friend int idle(C1 a, C2 b);

};

void C1::set_status(int state)

{

status = state;

}

void C2::set_status(int state)

{

status = state;

}

int idle(C1 a, C2 b)

{

if(a.status || b.status) return 0;

else return 1;

}

int main()

{

clrscr();

C1 x;

C2 y;

x.set_status(IDLE);

y.set_status(IDLE);

if(idle(x, y)) cout << "Screen can be used.\n";

else cout << "In use.\n";

x.set_status(INUSE);

if(idle(x, y)) cout << "Screen can be used.\n";

else cout << "In use.\n";

getch();

return 0;

}