when_present

Given a source file and line number, identifies which preprocessor conditional(s) must be true or false for the line to be included in compilation

Usage

when_present accepts a single file path and one or more line numbers. E.g. to specify a single line number:

when_present --file foo.h --lines 3

Or multiple line numbers:

when_present --file foo.h --lines 3 8 42

Limitations

There are some obvious limitations here. For one, #includes are not followed. E.g. if you are looking at the conditions for a line in bar.h, but your code is including foo.h which conditionally includes bar.h, then this will not capture the conditions from foo.h. For such a scenario, you would need to run this executable twice: once for bar.h for the line number(s) that you care about and again for foo.h for the line numbers that #include "bar.h".

Another limitation - or perhaps better phrased as a missing feature - is that macros are not expanded and dependencies are not simplified. For example, consider the following:

#define FOO
#if LIBRARY_VERSION > 8
#define BAR 42
#endif
#if defined(FOO) && defined(BAR)
void CoolFunction(); // Line 42
#endif

If you were to run when_present for line 42 on the above, it would only say that #if defined(FOO) && defined(BAR) must be true. That is, even though FOO is unconditionally defined and even though BAR's definition depends on the value of LIBRARY_VERSION, these dependencies are not explored.