C ++ implemented in simple C

I read several times that the early C ++ compilers converted the first C ++ code to regular C before compiling it (or perhaps a third-party C compiler was needed).

Playing with the fields of grammar / language / compilation, I am curious to see how C ++ was implemented in simple C, especially since it can be one way to implement class inheritance and call the [virtual] method.

Could you point me to a compiler that will still be available at the moment?

I know that OO code can be modeled / emulated in simple C with pointers to structures and functions, but I would like to see the actual implementation of the C ++ language in C.