预编译理解

废话少说，直接上代码：

#include <iostream>
using namespace std;

#define  MAX 0

class Time{
public:
	int hour;
	int minute;
	int sec;
};
void set_time(Time&t,int hour,int minute,int sec){
	t.hour = hour;
	t.minute = minute;
	t.sec = sec;
}

void show_time(Time& t){
	cout<<t.hour<<":"<<t.minute<<":"<<t.sec<<endl;
}
int main(){
	void set_time(Time&,int hour = 10,int minute = 0,int sec = 0);
	void show_time(Time&);
#if MAX 
	Time t1;
	set_time(t1,12,20,30);
	show_time(t1);
#else
	Time t2;
	set_time(t2);
	show_time(t2);
	
#endif
	
return 0;

}

此时输出：

当把

#define  MAX 0

改成 

#define  MAX 1

代码如下：

#include <iostream>
using namespace std;

#define  MAX 1

class Time{
public:
	int hour;
	int minute;
	int sec;
};
void set_time(Time&t,int hour,int minute,int sec){
	t.hour = hour;
	t.minute = minute;
	t.sec = sec;
}

void show_time(Time& t){
	cout<<t.hour<<":"<<t.minute<<":"<<t.sec<<endl;
}
int main(){
	void set_time(Time&,int hour = 10,int minute = 0,int sec = 0);
	void show_time(Time&);
#if MAX 
	Time t1;
	set_time(t1,12,20,30);
	show_time(t1);
#else
	Time t2;
	set_time(t2);
	show_time(t2);
	
#endif
	
return 0;

}

此时输出：