Function-like macro definition in C

Question

I'd like to define a function like MACRO . i.e.

#define foo(x)\
#if x>32\
 x\
#else\
 (2*x)\
#endif

that is,

if x>32, then foo(x) present x
else, foo(x) present (2*x)

but my GCC complains about:

int a = foo(31);

I think C preprocessor should be handle this correctly. since at compile time, it knows x=33. it could replace foo(33) with (2*33)

Answer 1

You can as follows

#define foo(x) ((x) > 32 ? (x) : (2 * (x)))

But that evaluates x multiple times. You can instead create a static function, which is cleaner

static int foo(int x) {
  if(x > 32) 
    return x;
  return 2 * x;
}

Then you are also able to pass things to foo that have side effects, and have the side effect happen only one time.

What you have written is using the #if, #else and #endif preprocessor directives, but you need to use language constructs if you pass variables to the macro and want to evaluate their values. Using if, and else statements as in the actual language constructs don't work either, because control flow statements don't evaluate to values. In other words, an if statement is steering control flow only ("if A, then execute B, else execute C"), not evaluating to any values.