Is there an elegant solution for checking whether a preprocessor symbol is defined or not

Question

Since preprocessor don't report an error when checking value of preprocessor's symbol that isn't actually defined (usually due to the lack of #include "some_header.h"), I use this cumbersome three line construction with "defined":

#if !defined(SOME_SYMBOL)
#error "some symbol isn't defined"
#endif
#if SOME_SYMBOL == 1
// Here is my conditionally compiled code
#endif

And the way with "#ifndef" is the same.
Is there a more elegant way to perform this check?

Nope, that and `#ifndef` is the only way to check if a macro is defined or not. — Some programmer dude, Nov 06 '18 at 11:30
Depends. There's [this Q&A](https://stackoverflow.com/questions/47452953/print-macro-values-without-knowing-the-amount-of-macros) that you may find applicable. — StoryTeller - Unslander Monica, Nov 06 '18 at 11:31
@StoryTeller so *there*'s where these upvotes came from ;) -- @OP: is `SOME_SYMBOL` always defined to an integer? — Quentin, Nov 06 '18 at 12:56
Just in passing, if `SOME_SYMBOL` is not defined `#if SOME_SYMBOL == 1` will evaluate to false. Undefined symbols in this context are treated as 0. — Pete Becker, Nov 06 '18 at 13:36

lhk · Accepted Answer · 2018-11-06T11:57:41.070

In your construction you could use an else block to skip the check for defined:

#if SOME_SYMBOL == 1
// Here is my conditionally compiled code
#else
// error
#endif

But in principle the comments are right. #if !defined and the shorthand #ifndef are the two available versions.

Currently, you are checking if SOME_SYMBOL is equals to 1. Do you execute different code based on that value ? If not, you could simply use:

#ifdef SOME_SYMBOL
// Here is my conditionally compiled code
#else
// error
#endif

And now it's a short step to the typical c++ include guards. Copying from that wikipedia link, here is a grandparent.h file:

#ifndef GRANDPARENT_H
#define GRANDPARENT_H

struct foo {
    int member;
};

#endif /* GRANDPARENT_H */

Now, even if you end up including this header twice, it will only be executed once.

SOME_SYMBOL can be defined not only as 1. Some other values are valid too. — Konstantin Murugov, Nov 06 '18 at 12:18

Is there an elegant solution for checking whether a preprocessor symbol is defined or not

1 Answers1