Does #ifdef (or other Preprocessor Directives) Work for Function Declarations (to Test for Function Existence)?

Question

Why doesn’t the following code work as expected?

void foobar(int);

#ifndef foobar
  printf("foobar exists");
#endif

It always prints the message; it obviously cannot detect the existence of a function as an entity. (Is it an over-loading issue?)

Why can’t #ifdef (or its variants) detect function declarations? Declarations should be available at pre-processing, so it should work, shouldn’t it? If not, is there an alternative or work-around?

Solution

Declarations should be available at pre-processing, so it should work, shouldn’t it?

The pre-processor operates before the compilation (hence the "pre") so there are no compiled symbols at that point, just text and text expansion. The pre-procesor and the compiler are distinctly separate and work independantly of each other, except for the fact that the pre-processor modifies the source that is passed to the compiler.

The typical pattern to doing something like with the pre-processor is to pair the function declaration with the function usage using the same define constant:

#define FOO

#ifdef FOO
 void foo(int);
#endif

#ifdef FOO
   printf("foo exists");
#endif

Comments

[Synetech]

I know the preprocessor works before compilation, but the function declarations are already present in the text at that point (in this case, two lines before). I suppose the problem is that the preprocessor doesn’t understand declarations; only pre-proc directives. So then there is no practical/automatic way to test for function existence? (Manually creating a pre-proc define for each function is too much work.)

[dkackman]

Yup. The pre-processor doesn't understand C or C++ at all, so a function declration is nothing more than a bit of text. What particular problem are you trying to solve? Perhaps a function pointer or something else is what you are really looking for. This works when, at runtime, you want to see if a dll exports a particular method and use it if it does. But if you are truly looking to do something like this during compilation the above is how it's done.

[Synetech]

I guess I got spoiled a bit when I did some work in JavaScript and you could do things like easily check if a function/property exists.