Is it possible for C preprocessor macros to contain preprocessor directives?

Question

I would like to do the equivalent of the following:

#define print_max(TYPE) \\
#  ifdef TYPE##_MAX \\
     printf(\"%lld\\n\", TYPE##_MAX); \\
#  endif

prin         


        

Answer 1

The Boost Preprocessor (which works for C as well as C++, even though Boost as a whole is a C++ library) library can help with this kind of task. Instead of using an #ifdef within a macro (which isn't permitted), it helps you include a file multiple times, with different macros defined each time, so that the file can use #ifdef.

The following code, if saved to max.c, should do what you want for each of the words listed in the MAXES #define at the top of the file. However, it won't work if any of the _MAX values are floating point, since the preprocessor can't handle floating point.

(Boost Processor is a handy tool, but it's not exactly straightforward; you can decide whether or not this approach is an improvement over copy-and-paste.)

#define MAXES (SHRT)(INT)(LONG)(PATH)(DOESNT_EXIST)

#if !BOOST_PP_IS_ITERATING

/* This portion of the file (from here to #else) is the "main" file */

#include 
#include 
#include 

/* Define a function print_maxes that iterates over the bottom portion of this
 * file for each word in MAXES */
#define BOOST_PP_FILENAME_1 "max.c"
#define BOOST_PP_ITERATION_LIMITS (0,BOOST_PP_DEC(BOOST_PP_SEQ_SIZE(MAXES)))
void print_maxes(void) {
#include BOOST_PP_ITERATE()
}

int main(int argc, char *argv[])
{
    print_maxes();
}

#else

/* This portion of the file is evaluated multiple times, with
 * BOOST_PP_ITERATION() resolving to a different number every time */

/* Use BOOST_PP_ITERATION() to look up the current word in MAXES */
#define CURRENT BOOST_PP_SEQ_ELEM(BOOST_PP_ITERATION(), MAXES)
#define CURRENT_MAX BOOST_PP_CAT(CURRENT, _MAX)

#if CURRENT_MAX
printf("The max of " BOOST_PP_STRINGIZE(CURRENT) " is %lld\n", (long long) CURRENT_MAX);
#else
printf("The max of " BOOST_PP_STRINGIZE(CURRENT) " is undefined\n");
#endif

#undef CURRENT
#undef CURRENT_MAX

#endif