Safe Unity Builds.

Question

I work on a large C++ project which makes use of Unity builds. For those unfamiliar with the practice, Unity builds #include multiple related C++ implementation files into one large translation unit which is then compiled as one. This saves recompiling headers, reduces link times, improves executable size/performance by bring more functions into internal linkage, etc. Generally good stuff.

However, I just caught a latent bug in one of our builds. An implementation file used library without including the associated header file, yet it compiled and ran. After scratching my head for bit, I realized that it was being included by an implementation file included before this one in our unity build. No harm done here, but it could have been a perplexing surprise if someone had tried to reuse that file later independently.

Is there any way to catch these silent dependencies and still keep the benefits of Unity builds besides periodically building the non-Unity version?

Answer 1

I've used UB approaches before for frozen projects in our source repository that we never planned to maintain again. Unfortunately, I'm pretty sure the answer to your question is no. You have to periodically build all the cpp files separately if you want to test for those kinds of errors.

Probably the closest thing you can get to an automagic solution is a buildbot which automatically gathers all the cpp files in a project (with the exception of your UB files) and builds the regular way periodically from the source repository, pointing out any build errors in the meantime. This way your local development is still fast (using UBs), but you can still catch any errors you miss from using a unity build from these periodic buildbot builds which build all cpps separately.