It isn't optimal, but it used to work. I kept C++ projects on Dropbox in addition to Git because I work on multiple computers and want to be able to continue where I left off if I get interrupted. I don't like the idea of having to commit and push to git for all work in progress, even if it is on a separate branch.

The default setup in Qt Creator is to have a build folder next to the sources, but after the first big sync failure I configured Qt Creator to always build outside of Dropbox. But I experienced failed syncs after that, even with only C++ sources inside Dropbox and when building Doconce+LaTeX projects.