Preprocessor directives

[johne53]

Is your feature request related to a problem? Please describe.

Sorry, I've been looking to find a forum for vcpkg questions and this was the closest thing I could find!

It's not a problem - just a question that I'm curious about… i.e. whereabouts do preprocessor directives get defined in a vcpkg project? Consider the following, which we’ll assume is in a header file somewhere:-

#if defined (SOMETHING)
#define WHATEVER 1
#endif

Obviously the relevant header file is responsible for defining WHATEVER - but SOMETHING would typically be defined by the build project. So whereabouts would SOMETHING get defined for a vcpkg project?

Proposed solution

I'm not sure if this site will send me replies but I'll keep checking from time to time.

Describe alternatives you've considered

No response

Additional context

No response

[Osyotr]

Depends on what SOMETHING really is.
Sometimes it is expected to be defined manually in the user project. For example, Qt's QT_DISABLE_DEPRECATED_UP_TO. Sometimes it's a definition that controls ABI-related stuff like the export surface of a library. For example, many libraries are not defining any dllexport/dllimport related macros if they are built statically. Vcpkg usually patches library's headers to remove the condition.
Sometimes SOMETHING comes from the exported CMake config/pkg-config or even a config header. You are not expected to set it manually.

[johne53]

A good, comprehensive reply Osyotr and many thanks. In fact, deprecated functionality and dllexport stuff are what started me wondering... because (up to now at least) everything I've built seemed to build as a DLL, rather than a static lib. Nothing wrong with that of course - it's just that they're the kinda things that are rarely controlled from a header file because you'd need to be certain that every source file will #include it.

So typically, they get defined by the build project. But I couldn't see where decisions like that get defined in a vcpkg build, so it left me a bit me puzzled!