The usage of #undef and its meaning

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1. #include <iostream>  
2. usingnamespace std;   
3.   
4. intmain  ()  
5. {  
6. #define MODI 10  
7.     cout << MODI << endl;  
8. #undef MODI  
9.     cout << MODI + 1 << endl;  
10.     return 0;  
11. }  

The compiler will report an error to the line cout << MODI + 1 << endl;.

The reason is what #undef does:

When you run out of a macro, such as MODI, you don't want the following code to use this MODI, then you can #undef it, then if you use the MODI macro again, the compiler will report an error.

A common practice is:

[cpp]  view plain copy  

 

1. #define MAX 50  
2. #include "common.h"  
3. #undef  MAX  

This makes the macro MAX available only in common.h.