每个源文件一次扩展__COUNTER__宏

Question

I'm trying to write a macro that will expand the __COUNTER__ macro only once per source file. I understand fully how macros work with their expansion but I'm having difficulty with this one. I want to expand the __COUNTER__ macro once at the top of the file and then each reference to that define will not expand the __COUNTER__ to it's next number.

So I want to fully expand __COUNTER__ into a single value and then use that one value consistently through the current working source file.

I can only use features that are available to C .

Answer 1

The __COUNTER__ extension (I suppose you are using a compiler from the gcc family) is too restricted for such a use. The difficulty is that if you put it into another macro, say TOTO , it is not expanded at definition but only at its use. So each invocation of TOTO will give rise to a new value of the counter.

In P99 I have a portable replacement for this, that achieves that goal with some #include hackery. P99_FILEID is then an per file identifier, and P99_LINEID an ID that should be unique for all lines in your compilation unit (but use with care).

Another alternative if you just need a compile time constant and nothing in the preprocessor itself would be to use the counter in an enumeration constant.

enum { toto_id = __COUNT__, };