This document provides an overview of objects, classes, messaging, and the Objective-C runtime system. It discusses key concepts including:
- Objects associate instance variables (data) with methods (operations). Objects encapsulate their data and methods.
- The id type can hold any object regardless of class. Objects are dynamically typed at runtime based on their class.
- Messages in the form [receiver message] are used to send objects messages to invoke their methods. Method names in messages are called selectors.
- Polymorphism and dynamic binding allow objects to respond differently to the same message depending on their class. The runtime looks up the appropriate method implementation at runtime based on the object's class.