My suggestion is to NOT use any C++ compiler for the 8051. It is a push to see C work on the 8051 architecture then yet C++. The concepts of class structures and multiple inheritance are designed for use where memory is a readily available commodity. Memory is just not a commodity on the 8051 and even if you do add up to 64K bytes of external RAM access to it is not so efficient as to be particularly good at supporting a C++ execution environment.

I think you need to ask your self why you are considering C++ for an embedded project. Then, depending on your own answer, either abandon C++ thoughts and simply use C OR find a different type of processor where the execution environment for C++ is more in tune with the CPU capabilities.
Michael Karas