预处理c++

#include <iostream>
using namespace std;
#define DEBUG
 
#define MIN(a,b) (((a)<(b)) ? a : b)
 
int main ()
{
   int i, j;
   i = 100;
   j = 30;
#ifdef DEBUG
   cerr <<"Trace: Inside main function" << endl;
#endif
 
#if 0
   /* 这是注释部分 */
   cout << MKSTR(HELLO C++) << endl;
#endif
 
   cout <<"The minimum is " << MIN(i, j) << endl;
 
#ifdef DEBUG
   cerr <<"Trace: Coming out of main function" << endl;
#endif
    return 0;
}

1.

#if SOMETHING>=100
//...
#else
//...
#endif

2.

#ifndef SOMETHING_H
#define SOMETHING_H
//...
#endif

3.

#if (defined DEBUG)&&(defined SOMETHING)
//...
#endif

4.

#ifdef SOMETHING
int func1(){/*...*/}
#else
int func1(){/*...*/}
#endif

5.

#ifdef SOMETHING
namespace space1{
#endif
//...
#ifdef SOMETHING
}//space1
#endif 还有很多。