C/C++ preprocessor directive check defined and string not empty

Question

I'm trying to obtain something like:

#if (!defined(SVN_REV) || (SVN_REV==""))
   char svnrev[10]="not found";
#else
   char svnrev[16]=SVN_REV;
#endif

to handle the case that the compiler, not finding a particular environment variable, sets:

define SVN_REV ""

which for me should be treated same as the define is missing.

But it seems that this is not possible, did anybody found a way to achieve the result?

Visual C++ is complaining that it is "invalid integer constant expression", didn't try on linux gcc/g++ yet — giuspen, Jan 23 '15 at 15:28
Why not char `svnrev[]="not found";` Let it count the characters for you. — Neil Kirk, Jan 23 '15 at 16:10
What is "C/C++"? Why is this question tagged C if you are actually writing C++? — Lightness Races in Orbit, Jan 23 '15 at 16:20
@LightnessRacesinOrbit I tried to make the question generic to C/C++ because I work in both environments but still I'm interested in considering more restricted solution — giuspen, Jan 23 '15 at 16:35
@LightnessRacesinOrbit C++ uses the same preprocessor as C, so the question is the same for both languages. — James, Jul 19 '23 at 16:18

score 0 · Answer 1 · answered Jan 23 '15 at 15:29

0

No, the C preprocessor doesn't compare strings.

You can do:

#if !defined SVN_REV
#define SVN_REV "not found"
#endif
const char *svnrev = SVN_REV;

To achieve the same effect.

answered Jan 23 '15 at 15:29

unwind

5

But this does not cover the case that Visual C++ (which in the case does not find a particular environment variable) defines SVN_REV as an empty string - so it is defined but as empty which for me is like not defined. Probably it is not possible to obtain the result only with preprocessor code. – giuspen Jan 23 '15 at 15:34
2

This is not an answer to the question... :( – Zordid Mar 13 '19 at 15:37

1 Answers1