define on const int, but put the value in the installed header

Question

I have a C++ library which does some numerical work. The main parameter is the number of segments. For speed it matters a factor 3 that the number of segment is const, however I would like to define it at compile time using -DSEGMENTS 32. The header looks like this:

#ifdef SEGMENTS
const int segments = SEGMENTS
#else
const int segments = 20
#endif

That works. However, programs linking with this library don't get the -DSEGMENTS and thus segments is always 20. Without the const I know the solution, but with I don't know. I can imagine some extern trick or installing the header after precompiling if that is possible with cmake.

If the header isn't getting the number of segments, then your problem is: how do i pass the number of segments ? — Yochai Timmer
– Yochai Timmer, Commented Dec 19, 2011 at 14:12
I believe C and C++ are different in their treatment of const. Tag changed to reflect the text of your question — pmg
– pmg, Commented Dec 19, 2011 at 14:17

Klas Lindbäck · Accepted Answer · 2011-12-19 14:23:33Z

5

If you want the other libraries to take the value externally you should declare it as external in the header:

extern const int segments;

You define it in one code file (.cpp) like you describe above:

#ifdef SEGMENTS
const int segments = SEGMENTS
#else
const int segments = 20
#endif

edited Dec 19, 2011 at 14:23

answered Dec 19, 2011 at 14:17

Klas Lindbäck

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