C++ is an altered and extended form of C, thus I would say it is more accurate to call it a superset, rather than a subset. I'm not using precise definitions here.

And of course, but those will come in time. The point of it is that C++ isn't any safer (And actually, I would say is more risky in some aspects, because of the differences between (For example) the UB in C and C++), while Checked C is guaranteed to be safer, to a certain degree.