It's actually not a c++ problem as it defines nothing about the ABI. It's the vendors that don't get along and don't maintain backwards compatibility. But that's a curse and a blessing at the same time.
Managed c++? You mean the abomination of C++/CLR? I'm almost certain it would be impossible, if not for the sole share of 3rd party libraries required for starters. OpenGL, ICU, DirectX, Webkit, all the platform specific APIs, python and perl needed for building... nah, forget it.
C++/CLR is dying anyway. Very few people are interested in it these days. Certainly not Microsoft anymore.