Pure virtual functions

You must override pure virtual functions in derived classes.

Functions are marked as pure virtual using the = 0 initializer. For example, here's the declaration of TSlider::PaintRuler:

virtual void PaintRuler(TDC& dc) = 0;

You must override all of an abstract class pure virtual functions in a derived class before you can create an instance of that derived class. In most cases, when using the standard ObjectWindows classes, this won't be much of a problem; most of the ObjectWindows classes you could need to derive from are not abstract classes. In lieu of pure virtual functions, many ObjectWindows classes use default placeholder functions.