I see in documentation that UIDocument is an "abstract base class".
Would someone explain to me what an abstract base class is? I understand that a class is like a blueprint for object instances.
abstract base class
"An abstract class is a class that is designed to be specifically used as a base class. An abstract class contains at least one pure virtual function. You declare a pure virtual function by using a pure specifier (= 0) in the declaration of a virtual member function in the class declaration."
A base class is a class that provides certain basic characteristics and/or behavior.
About 'class', see:
h t t p : / / blog.teamtreehouse.com/beginners-guide-objective-c-classes-objects