So, to start off, here's the code, with actual names switched for generic ones to limit confusion.
/* Get the list of Hotkey commands */
#define A_COMMANDS_MACRO(a, b, c, d) a = b ,
enum {
#include "commandsFile.def"
} ;
#undef A_COMMANDS_MACRO
This is a snippet from some source code I have been looking over and considering forking as a way to familiarize myself with the intricacies of the C programming language. So, to my untrained eye, this appears to do nothing. To my brain, defining something and then immediately undefining it would seem to cancel each other out.
Obviously, I realize that I'm wrong. But why am I wrong?
What you see there is usually called X-MACRO. The technique consists in defining macros via #define, and then including a file that makes use of them with #include.
As a very simple example, you could have a header (say myheader.h) that declared 2 functions in the form of:
int foo(MYTYPE a, MYTYPE_PTR b);
void bar(MYTYPE a, MYTYPE_PTR b);
And then, in your code:
#define MYTYPE int
#define MYTYPE_PTR int*
#include "myheader.h"
#undef MYTYPE
#undef MYTYPE_PTR
The #undefs are sometimes in the included file as well.
For more information, take a look at this Wikipedia link.