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Cocoa in a Nutshell by James Duncan Davidson, Michael Beam

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Data

The Foundation framework provides many classes and protocols that extend the capabilities of the Objective-C language to represent and work with basic data types, such as strings and numbers, in an object-oriented fashion. Additionally, the Foundation framework provides application programming interfaces (APIs) for working with more complex data types, such as dates and collections.

Immutable Versus Mutable Classes

Classes such as NSString and NSArray are immutable classes; instances of these classes cannot be altered after they are initialized. Each immutable class, however, has a mutable subclass: for example, NSString has the mutable subclass NSMutableString, and NSArray has the subclass NSMutableArray. Mutable subclasses extend their superclass’s functionality to allow modification after initialization. Immutable classes are more efficient, but mutable classes are more flexible.

Basic Types

Two of the most basic data types in an application are strings and numbers . The Foundation framework provides object abstractions in the form of NSString and NSNumber, and an extensive API to manipulate them.

Strings

Foundation’s primary class used to represent and manipulate strings is NSString. Instances of NSString can be considered, at their core, an immutable array of Unicode characters, and can represent characters from the alphabets of nearly every written language, past and present. In fact, NSString is a class cluster, which shields the developer from a number of underlying ...

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