What is an ABC (“abstract base class”)?
At the design level, an ABC corresponds to an abstract concept. If you asked a Mechanic if he repaired Vehicles, he’d probably wonder what KIND-OF Vehicle you had in mind. Chances are he doesn’t repair space shuttles, ocean liners, bicycles, or nuclear submarines. The problem is that the term “Vehicle” is an abstract concept (e.g., you can’t build a “vehicle” unless you know what kind of vehicle to build). In C++, class Vehicle would be an ABC, with Bicycle, SpaceShuttle, etc, being subclasses (an OceanLiner is-a-kind-of-a Vehicle). In real-world OOP, ABCs show up all over the place. As programming language level, an ABC is a class that has one or more pure virtual member functions (see next FAQ). You cannot make an object (instance) of an ABC.
