条件编译

#include <iostream>
using namespace std;
#define DEBUG

#define MIN(a,b) (((a)<(b))?a:b)

int main()
{
    int i,j;
    i=100;
    j=30;

//如果没有定义符号常量DEBUG,会进行编译
#ifdef DEBUG
    cerr<<"Trace: Inside main function"<<endl;
#endif

#if 0
    /*注释部分 不进行编译的代码*/
    cout<<MKSTR(HELLO C++)<<endl;
#endif

    cout<<"The minimum is"<<MIN(i,j)<<endl;

#ifdef DEBUG //调试时进行编译
    cerr<<"Trace: coming out of main function"<<endl;
#endif
    system("pause");
    return 0;
}