注释的位置应与被描述的代码相邻

 1 #include <iostream>
 2 #define   PI   3.1416
 3 /* run this program using the console pauser or add your own getch, system("pause") or input loop */
 4 using namespace std;
 5 int main(int argc, char** argv) {
 6         int i=100;
 7 #if 1
 8     cout<<"i="<<i<<endl;
 9 #endif
10 
11 #ifdef PI
12     cout<<"1  PI="<<PI<<endl;
13 #endif
14 
15 #ifndef PI
16     cout<<"2  PI="<<PI<<endl;   //此语句不被编译执行
17 #endif
18     return 0;
19 }