https://github.com/marketplace/circleci says C++ is supported project language but it does not appear in the add projects page in my accounts, https://circleci.com/setup-project/gh/yrnkrn/zapcc
I think the “add projects” thing is just about auto-detection. I’d say anything that can be built on a modern Linux distro can be tested, including C++. Are you wanting to compile something with gcc?
I will use g++ or clang.
Sample .yml file is shown based on the language selection
What you write makes sense, will try.