Partial builds versus full builds in Visual C++

Question

For most of my development work with Visual C++, I am using partial builds, e.g. press F7 and only changed C++ files and their dependencies get rebuilt, followed by an incremental link. Before passing a version onto testing, I take the precaution of doing a full rebuild, which takes about 45 minutes on my current project. I have seen many posts and articles advocating this action, but wonder is this necessary, and if so, why? Does it affect the delivered EXE or the associated PDB (which we also use in testing)? Would the software function any different from a testing perspective?

For release builds, I'm using VS2005, incremental compilation and linking, precompiled headers.

Answer 1

The partial build system works by checking file dates of source files against the build results. So it can break if you e.g. restore an earlier file from source control. The earlier file would have a modified date earlier than the build product, so the product wouldn't be rebuilt. To protect against these errors, you should do a complete build if it is a final build. While you are developing though, incremental builds are of course much more efficient.

Edit: And of course, doing a full rebuild also shields you from possible bugs in the incremental build system.

Answer 2

The basic problem is that compilation is dependent on the environment (command-line flags, libraries available, and probably some Black Magic), and so two compilations will only have the same result if they are performed in the same conditions. For testing and deployment, you want to make sure that the environments are as controlled as possible and you aren't getting wacky behaviours due to odd code. A good example is if you update a system library, then recompile half the files - half are still trying to use the old code, half are not. In a perfect world, this would either error out right away or not cause any problems, but sadly, sometimes neither of those happen. As a result, doing a complete recompilation avoids a lot of problems associated with a staggered build process.