[tjr]

I have never heard of anyone not using DO-178 though.

Me neither. I can imagine that would be harder than just using DO-178, as the auditors are used to dealing with it.

Have you seen level A software written using C++?

Not personally. I have heard of a level A flight controls project that was planning to use C++, and was in the process of getting their C++ compiler (GCC C++, I believe) certified. I don't know how it turned out.