Question

I have encountered the #define pre-processor directive before while learning C, and then also encountered it in some code I read. But apart from using it to definite substitutions for constants and to define macros, I’ve not really understook the special case where it is used without a “body” or token-string.

Take for example this line:

#define OCSTR(X)

Just like that! What could be the use of this or better, when is this use of #define necessary?

Answer 1

This is used in two cases. The first and most frequent involves
conditional compilation:

#ifndef XYZ
#define XYZ
//  ...
#endif

You’ve surely used this yourself for include guards, but it can also be
used for things like system dependencies:

#ifdef WIN32
//  Windows specific code here...
#endif

(In this case, WIN32 is more likely defined on the command line, but it
could also be defined in a "config.hpp" file.) This would normally
only involve object-like macros (without an argument list or
parentheses).

The second would be a result of conditional compilation. Something
like:

#ifdef DEBUG
#define TEST(X) text(X)
#else
#define TEST(X)
#endif

That allows writing things like:

TEST(X);

which will call the function if DEBUG is defined, and do nothing if it
isn’t.