Must it be freeware?
You already mentioned Umbrello, which is not freeware, so I will let myself to drop that restriction. You did not mention the operating system, so I will assume the ones on which Umbrello could run. And offer
Doxygen. It can generate UML diagrams for classes. See
diagrams documentation in Doxygen for details on how to enable that in the doxyfile.
The C++ support for Doxygen was pretty good the last time I checked. At least as far as it can be, considering that the task is provably impossible for the general case. and even in practical cases presenting C++ sources as UML diagrams is troublesome thanks to mix of paradigms, two separate levels of polymorphism, and generally poor correspondence between source and semantics it represents.