What use cases necessitate #define without a token-string?

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

I agree with every answer, but I'd like to point out a small trivial thing.

Being a C purist I've grown up with the assertion that EACH AND EVERY #define should be an expression, so, even if it's common practice using:

#define WHATEVER

and test it with

#ifdef WHATEVER

I think it's always better writing:

#define WHATEVER (1)

also #debug macros shall be expressions:

#define DEBUG (xxx) (whatever you want for debugging, value)

In this way, you are completely safe from misuse of #macros and prevents nasty problems (especially in a 10 million line C project)

Answer 2

There are many uses for such a thing.

For example, one is for the macro to have different behavior in different builds. For example, if you want debug messages, you could have something like this:

#ifdef _DEBUG
  #define DEBUG_LOG(X, ...) however_you_want_to_print_it
#else
  #define DEBUG_LOG(X, ...) // nothing
#endif

Another use could be to customize your header file based on your system. This is from my mesa-implemented OpenGL header in linux:

#if !defined(OPENSTEP) && (defined(__WIN32__) && !defined(__CYGWIN__))
#  if defined(__MINGW32__) && defined(GL_NO_STDCALL) || defined(UNDER_CE)  /* The generated DLLs by MingW with STDCALL are not compatible with the ones done by Microsoft's compilers */
#    define GLAPIENTRY 
#  else
#    define GLAPIENTRY __stdcall
#  endif
#elif defined(__CYGWIN__) && defined(USE_OPENGL32) /* use native windows opengl32 */
#  define GLAPIENTRY __stdcall
#elif defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) >= 303
#  define GLAPIENTRY
#endif /* WIN32 && !CYGWIN */

#ifndef GLAPIENTRY
#define GLAPIENTRY
#endif

And used in header declarations like:

GLAPI void GLAPIENTRY glClearIndex( GLfloat c );

GLAPI void GLAPIENTRY glClearColor( GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha );

GLAPI void GLAPIENTRY glClear( GLbitfield mask );

...

(I removed the part for GLAPI)

So you get the picture, a macro that is used in some cases and not used in other cases could be defined to something on those cases and nothing to those other cases.

Other cases could be as follows:

If the macro doesn't take parameters, it could be just to declare some case. A famous example is to guard header files. Another example would be something like this

#define USING_SOME_LIB

and later could be used like this:

#ifdef USING_SOME_LIB
...
#else
...
#endif

Could be that the macro was used at some stage to do something (for example log), but then on release the owner decided the log is not useful anymore and simply removed the contents of the macro so it becomes empty. This is not recommended though, use the method I mentioned in the very beginning of the answer.

Finally, it could be there just for more explanation, for example you can say

#define DONT_CALL_IF_LIB_NOT_INITIALIZED

and you write functions like:

void init(void);
void do_something(int x) DONT_CALL_IF_LIB_NOT_INITIALIZED;

Although this last case is a bit absurd, but it would make sense in such a case:

#define IN
#define OUT

void function(IN char *a, OUT char *b);

Answer 3

One odd case that I recently dug up to answer a question turned out to be simply commentary in nature. The code in question looked like:

void CLASS functionName(){
  //
  //
  //
}

I discovered it was just an empty #define, which the author had chosen to document that the function accessed global variables in the project:

C++ syntax: void CLASS functionName()?

So not really that different from if it said /* CLASS */, except not allowing typos like /* CLAAS */...some other small benefits perhaps (?)

Answer 4

As you can see in the above responses, it can be useful when debugging your code.

#ifdef DEBUG
#define debug(msg) fputs(__FILE__ ":" (__LINE__) " - " msg, stderr)
#else
#define debug(msg)
#endif

So, when you are debugging, the function will print the line number and file name so you know if there is an error. And if you are not debugging, it will just produce no output

Answer 5

Such macro usually appears in pair and inside conditional #ifdef as:

#ifdef _DEBUG
   #define OCSTR(X)
#else
   #define OCSTR(X)  SOME_TOKENS_HERE
#endif

Another example,

#ifdef __cplusplus
   #define NAMESPACE_BEGIN(X) namespace X {
   #define NAMESPACE_END }
#else
   #define NAMESPACE_BEGIN(X) 
   #define NAMESPACE_END
#endif

Answer 6

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.

Answer 7

#define macros are simply replaced, literally, by their replacement text during preprocessing. If there is no replacement text, then ... they're replaced by nothing! So this source code:

#define FOO(x)

print(FOO(hello world));

will be preprocessed into just this:

print();

This can be useful to get rid of things you don't want, like, say, assert(). It's mainly useful in conditional situations, where under some conditions there's a non-empty body, though.

Answer 8

This can be used when you may want to silent some function. For example in debug mode you want to print some debug statements and in production code you want to omit them:

#ifdef DEBUG
#define PRINT(X) printf("%s", X)
#else
#define PRINT(X)  // <----- silently removed
#endif

Usage:

void foo ()
{
  PRINT("foo() starts\n");
  ...
}