example5.cpp

来自「Advanced Internet Programming Lecture 2 」· C++ 代码 · 共 62 行

#include <iostream>
using namespace std;

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);
  int idle(C2 b);  // now a member of C1
};

class C2 {
  int status;  // IDLE if off, INUSE if on screen
  // ...
public:
  void set_status(int state);
  friend int C1::idle(C2 b);
};

void C1::set_status(int state)
{
  status = state;
}

void C2::set_status(int state)
{
  status = state;
}

// idle() is member of C1, but friend of C2
int C1::idle(C2 b)
{
  if(status || b.status) return 0;
  else return 1;
}

int main()
{
  C1 x;
  C2 y;

  x.set_status(IDLE);
  y.set_status(IDLE);

  if(x.idle(y)) cout << "Screen can be used.\n";
  else cout << "In use.\n";
  x.set_status(INUSE);

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

  return 0;
}