C/C++ conditional macros combining

Question

Can I combine macros while writing code for C or C++? If not, why? If yes, how?

I'm interested on how to solve the following (not correct and NOT compiling!!!) idea:

#define FREE(x) if((x)) {                                         \
#ifdef MEM_DEBUG_                                                 \
    fprintf(stderr, "free:%p (%s:%d)\n", (x), __FILE__, __LINE__); \
#endif                                                             \
    free((x)); }

So, what I want to achieve is this:

I want to define the macro FREE in way that it will include an extra line if I have the MEM_DEBUG defined.

I know, that for solving this I can have two defines for the FREE based on MEM_DEBUG , like:

#ifdef MEM_DEBUG
  #define FREE() something
#else 
  #define FREE() something else
#endif

but I'm just curios if there is another way!

Answer 1

Yes, you can define macros to encapsulate the idea of doing something when the flag is set.

#ifdef MEM_DEBUG
#   define IF_MEM_DEBUG( ... ) __VA_ARGS__
#   define IF_MEM_NDEBUG( ... )
#else
#   define IF_MEM_DEBUG( ... )
#   define IF_MEM_NDEBUG( ... ) __VA_ARGS__
#endif

#define FREE(x) \
if((x)) { \
    IF_MEM_DEBUG( \
        fprintf(stderr, "free:%p (%s:%d)\n", (x), __FILE__, __LINE__); \
    ) \
    free((x)); \
}

Answer 2

No, it's not possible to put preprocessing directives (lines starting with # ) inside a macro definition. So the only thing you can do is the second method you mention - put the definitions inside the conditionals.

Of course, if you're working on a bigger macro and only a part of it is #if -dependent, you can isolate the dependent part into a separate macro and use this in the bigger one, to keep the divergent definitions as small as possible.