C #undef

The #undef directive is used to undefine (remove) a previously defined macro. Macros are defined using the #define directive, and they can be undefined using #undef. This is useful when you want to remove or “undefine” a macro that you no longer need or want to use in your code.

Here’s the basic syntax of the #undef directive:

#undef macro_name

• macro_name is the name of the macro that you want to undefine.

For example, suppose you have defined a macro called DEBUG like this:

#define DEBUG

You can later undefine it using the #undef directive:

#undef DEBUG

After this #undef directive, the DEBUG macro is no longer defined, and any subsequent use of DEBUG in the code will not result in macro expansion.

Here’s an example of how #undef can be used:

#include <stdio.h>

#define DEBUG // Define DEBUG macro

int main() {

    #ifdef DEBUG
        printf("Debug mode is enabled.\n");
    #else
        printf("Debug mode is disabled.\n");
    #endif
    
    #undef DEBUG // Undefine DEBUG macro
    
    #ifdef DEBUG
        printf("This will not be printed because DEBUG is undefined.\n");
    #else
        printf("Debug mode is still disabled.\n");
    #endif

    return 0;
}

In this example:

1. The DEBUG macro is defined at the beginning of the program using #define.
2. Inside the program, conditional compilation (#ifdef and #else) is used to check whether DEBUG is defined and print different messages accordingly.
3. After the first conditional block, the #undef DEBUG directive is used to undefine the DEBUG macro.
4. In the second conditional block, it’s confirmed that DEBUG is indeed undefined, and the corresponding message is printed.

By using #undef, you can selectively remove macros from your code when they are no longer needed or when their behavior should change. This can help manage code complexity and configuration in C programs.