Question

Is it “safe” to give macros names as arguments to other macros to simulate higher order functions?

I.e. where should I look to not shoot myself in the foot?

Here are some snippets:

#define foreach_even(ii, instr) for(int ii = 0; ii < 100; ii += 2) { instr; }
#define foreach_odd(ii, instr)  for(int ii = 1; ii < 100; ii += 2) { instr; }

#define sum(foreach_loop, accu) \
  foreach_loop(ii, {accu += ii});

int acc = 0;
sum(foreach_even, acc);
sum(foreach_odd, acc);

What about partial application, can I do that? :

#define foreach(ii, start, end, step, instr) \
  for(int ii = start; ii < end; ii += step) { instr; }

#define foreach_even(ii, instr) foreach(ii, 0, 100, instr)
#define foreach_odd(ii, instr)  foreach(ii, 1, 100, instr)

#define sum(foreach_loop, accu) \
  foreach_loop(ii, {accu += ii});

int acc = 0;
sum(foreach_even, acc);
sum(foreach_odd, acc);

And can I define a macro inside a macro?

#define apply_first(new_macro, macro, arg) #define new_macro(x) macro(arg,x)

Answer 1

If you’re into using preprocessor as much as possible, you may want to try boost.preprocessor.

But be aware that it is not safe to do so. Commas, for instance, cause a great number of problems when using preprocessors. Don’t forget that preprocessors do not understand (or even try to understand) any of the code they are generating.

My basic advice is “don’t do it”, or “do it as cautiously as possible”.