The Challenge

Given a macro that expands to an arbitrary list of headers, include every header in the list, in order.

#define HEADERS "stdio.h", "stdlib.h", "assert.h"
#include "include_dynamic.h"
// stdio.h, stdlib.h, and assert.h are now included and available.

Progress So Far

The C preprocessor is a powerful tool, but not a particularly versatile one. Rather, it does a few specific things, and people have taken advantage of its workings to compose those capabilities to accomplish their tasks. And sometimes, to do something ridiculous and overengineered…like this.

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In section 6.10.2, paragraph 8 of the C11 standard, this example is specifically called out as legal (if implementation-defined):

#if VERSION == 1
  #define INCFILE "vers1.h"
#elif VERSION == 2
  #define INCFILE "vers2.h" // and so on
#else
  #define INCFILE "versN.h"
#endif
#include INCFILE

Yikes! Do people really want to do this? If I encountered this in a real program, I’d try to get rid of it—in most cases I figure you can just put the #includes inside the #if and be done with it. You can’t even do any clever tricks like building up a path, at least not portably.

So of course, after telling someone else about this, my next reaction was how to take it further. (Please do not put this in a real program.)

(Continue reading…)