Objective-C Fundamentals

66 CHAPTER 3 An introduction to objects
for you out of the box, and you can reuse existing classes without needing to reinvent
the wheel with each application. Reuse makes you more productive, allowing you to
focus on differentiating and perfecting your application instead of spending days
building a basic foundation.
3.5 Strings
The NSString class provides a nice object-oriented interface to represent, interact
with, and modify a string of characters. Unlike in a C-style char * null-terminated
string or char[] array, all aspects of memory allocation, text encoding, and string
manipulation are hidden from the application developer as internal implementation
details of the NSString class. This allows you to worry about the more important
and unique aspects of your application logic rather than the nitty-gritty details
of how strings are stored in a computer or how operations such as string concatenation
operate.
This abstraction also means that common sources of error, such as attempting to
store a 250-character string in a variable that has allocated space for only 200 characters,
are easily avoided in Objective-C. Let’s start our discussion of strings by learning
how to create new strings in your source code.
3.5.1 Constructing strings
The easiest way to create a new NSString instance is via the convenient @"..." language
syntax introduced in previous chapters. For example, the statement
NSString *myString = @"Hello, World!";
creates a new NSString object and initializes it with the value "Hello, World!". This
form of syntax, however, is a special case for the NSString class. It’s impossible to use
this syntax to create instances of any other class. The use of text strings in applications
is so pervasive that the Objective-C language designers decided it was beneficial to
have dedicated (and concise) syntax to create them. A more generic technique to
construct an object of a particular class is to send that class an alloc message to allocate
memory for a new object, followed by some kind of initialization message. For
example, here’s another way to create a string:
NSString *myString = [NSString alloc];
myString = [myString initWithString:@"Hello, World!"];
This code sample explicitly demonstrates the two stages of constructing a new object
in Objective-C: allocation and initialization. The first line sends the alloc (short for
allocate) message to the NSString class. This causes NSString to assign enough memory
to store a new string and returns a C-style pointer to that memory. At this stage,
however, the object is rather blank, so the next step is to call an initialization method
to initialize the object with some kind of sensible value.
Many classes provide a number of initialization messages. In this example, you sent
the initWithString: message, which initializes the new string object with the contents