My dad has had trouble understanding classes for decades, but he had mostly stopped programming during that timeframe as well so it wasn't something he was going to put much time into learning. Now, he's returned to programming more regularly but still is having trouble with classes. I figured the big problem for him is exactly what the problem for you was, the hidden this pointer.
I'd started working on manually writing the same code is both C++ and C, but your approach of using something automated is an even better idea. Showing the implicit this pointer isn't hard to do manually, but polymorphism is a bit more of a pain. But I think the best part about using a tool is that he can change the C++ code and see how it affects the emitted C. Being able to tinker with inputs and see how they affect the output is huge when it comes to learning how something works.
I'd started working on manually writing the same code is both C++ and C, but your approach of using something automated is an even better idea. Showing the implicit this pointer isn't hard to do manually, but polymorphism is a bit more of a pain. But I think the best part about using a tool is that he can change the C++ code and see how it affects the emitted C. Being able to tinker with inputs and see how they affect the output is huge when it comes to learning how something works.