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.

Answer 1

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