C++ – Combining source files, like header files

Aren't you actually coupling your implementation to a directory structure on the filesystem?

That point aside, maybe the problem has gone beyond what you can satisfactorily accomplish with #define and it's time to write some code to handle it.

I'm thinking a simple string GetMyFilename(Conditions condition) so you can utilise all the power of your development language and not just the macro-preprocessor to handle what file you need for your tests.

My general point is that tests are first class citizens of the code base, don't be afraid to use all your skills and disciplines on them.

They can, as shown by new languages that do.

But a design decision was made all those years ago (when the C compiler was multiple independent stages) and now to maintain compatibility the pre-processor has to act in a certain way to make sure old code compiles as expected.

As C++ inherits the way it processes header files from C it maintained the same techniques. We are supporting a old design decision. But changing the way it works is too risky lots of code could potentially break. So now we have to teach new users of the language how to use include guards.

There are a couple of tricks with header files were you deliberately include it multiple times (this does actually provide a useful feature). Though if we redesigned the paradigm from scratch we could make this the non-default way to include files.