SteveMann:
You lost me at base class, then whacked me with derived class. (The more I learn, the dumber I feel).In the example library that I am studying there is only one class, so what does 'virtual' do in this case?
Base class, derived class and virtual functions are all features that C++ offers you, so it's easy to find extensive tutorials that explain them.
It looks like you unfortunately picked a library to experiment with that's encouraging polymorphism. If your primary aim is to learn about building libraries, I'd suggest that you find another simpler one to add methods to. You can always come back to this one when you're comfortable with the base class simplest case 
Geeky anecdote:
One day I was discussing Indian bread names with an Indian colleague and wanted to know what the word for bread is in Hindi. "It depends. There's naan, paratha, poori, chapati and many others".
"No no no, what's the generic word for bread - the base class?"
"Oh I see, roti."