Reset __COUNTER__ macro to zero

Question

is it possible to reset the __COUNTER__ macro at the start of a header file to make its evaluation within the header file consistent over several compile units?

Answer 1

How about an enum?

enum { COUNTER_BASE = __COUNTER__ };

#define LOCAL_COUNTER (__COUNTER__ - COUNTER_BASE)

Answer 2

You can set BASE to __COUNTER__ at the top of your header file, and then use __COUNTER__ - BASE later on.

However, do this after you've included all necessary headers, because else thee result would depend on the use of __COUNTER__ within the header guards of those nested include files.

Answer 3

No, there is no way to reset that value.

Take a look at the GCC source that increments the counter:

case BT_COUNTER:
    if (CPP_OPTION (pfile, directives_only) && pfile->state.in_directive)
    cpp_error (pfile, CPP_DL_ERROR,
        "__COUNTER__ expanded inside directive with -fdirectives-only");
    number = pfile->counter++;
    break;

And if you look arount this library, nowhere is the counter modified again. It is default initialized to 0 and then incremented at every use.

Note that the pfile, where the counter variable resides, represents the the preprocessor input, that in this case is the current compilation unit, not the actual file.