Does anyone know of a standalone software (or plugin for Visual Studio) that can generate UML class diagrams from C++ code?
Take a look at DoxyGen.
It is widely used in industry . If you add specially formatted comments to your code, it can generate some very useful documentation. in fact the first thing I do on every new contract is to run the code through Doxygen.
In fact, even without the comments, it still gives a great deal of information, such as call trees, class diagrams and the like.
Read about its diagrams here
And read the S.O question How to use doxygen to create UML class diagrams from C++ source
Visual Studio 2017 (Community Edition) has a feature to generate class diagrams. Specifically, you load your project in VS, then right click on a header file and select "View Class Diagram".