Programming iOS 6, 3rd Edition

Data Type Qualifiers

A variable’s data type can be declared with a qualifier before the name of the type, modifying something about how that variable is to be used. For example, the declaration can be preceded by the term const, which means (K&R 2.4) that it is illegal to change the variable’s value; the variable must be initialized in the same line as the declaration, and that’s the only value it can ever have.

You can use a const variable as an alternative way (instead of #define) to prevent “magic numbers” and similar expressions. For example:

NSString* const MYKEY = @"Howdy";

The Cocoa API itself makes heavy use of this device. For example, in some circumstances Cocoa will pass a dictionary of information to your code. The documentation tells you what keys this dictionary contains. But instead of telling you a key as a string, the documentation tells you the key as a const NSString variable name:

UIKIT_EXTERN NSString *const UIApplicationStatusBarOrientationUserInfoKey;

(Never mind what UIKIT_EXTERN means.) This declaration tells you that UIApplicationStatusBarOrientationUserInfoKey is the name of an NSString, and you are to trust that its value is set for you. You are to go ahead and use this name whenever you want to speak of this particular key, secure in the knowledge that the actual string value will be substituted. You do not have to know what that actual string value is. In this way, if you make a mistake in typing the variable name, the compiler will catch the mistake because ...