Programming iOS 4

Variable Declaration, Initialization, and Data Types

C is a strongly typed language. Every variable must be declared, indicating its data type, before it can be used. Declaration can also involve explicit initialization; a variable that is declared but not explicitly initialized is of uncertain value (and should be regarded as dangerous until it is initialized). In K&R C, declarations must precede all other statements, but in modern versions of C, this rule is relaxed so that you don’t have to declare a variable until just before you start using it. The usual convention is thus to declare a variable and assign it a value as it makes its first appearance on the scene:

int height = 2;
int width = height * 2;
height = height + 1;
int area = height * width;

The basic built-in C data types are all numeric: char (one byte), int (four bytes), float and double (floating-point numbers), and varieties such as short (short integer), long (long integer), unsigned short, and so on. iOS makes use of some further numeric types derived from the C numeric types (by way of the typedef statement, K&R 6.7); the most important of these are NSInteger (along with NSUInteger) and CGFloat. You don’t need to use these explicitly unless an API tells you to, and even when you do, just think of NSInteger as int and CGFloat as float, and you’ll be fine.

To cast (or typecast) a variable’s value explicitly to another type, precede the variable’s name with the other type’s name in parentheses:

int height = 2;
float fheight = (float)height;

In that particular example, the explicit cast is unnecessary because the integer value will be cast to a float implicitly as it is assigned to a float variable, but it illustrates the notation. You’ll find yourself typecasting quite a bit in Objective-C, mostly in order to subdue the worries of the compiler (examples appear in Chapter 3).

Another form of numeric initialization is the enum (K&R 2.3). It’s a way of assigning names to a sequence of numeric values and is useful when a value represents one of several possible options. The Cocoa API uses this device a lot. For example, the three possible types of status bar animation are defined like this:

typedef enum {
   UIStatusBarAnimationNone,
   UIStatusBarAnimationFade,
   UIStatusBarAnimationSlide,
} UIStatusBarAnimation;

That definition assigns the value 0 to the name UIStatusBarAnimationNone, the value 1 to the name UIStatusBarAnimationFade, and the value 2 to the name UIStatusBarAnimationSlide. The upshot is, however, that you can use the suggestively meaningful names without caring about, or even knowing, the arbitrary numeric values they represent. It’s a useful idiom, and you may well have reason to define enums in your own code.

There appears to be a native text type (a string) in C, but this is something of an illusion; behind the scenes, it is actually a null-terminated array of char. For example, in C you can write a string literal like this:

"string"

But in fact this is stored as 7 bytes, the numeric (ASCII) equivalents of each letter followed by a byte consisting of 0 to signal the end of the string. This data structure, called a C string, is rather tricky, and if you’re lucky you’ll rarely or never encounter one while programming iOS. In general, when working with strings, you’ll use an Objective-C object type called NSString. An NSString is totally different from a C string; it happens, however, that Objective-C lets you write a literal NSString in a way that looks very like a C string:

@"string"

Notice the at-sign! This expression is actually a directive to the Objective-C compiler to form an NSString object. A common mistake is forgetting the at-sign, thus causing your expression to be interpreted as a C string, which is a completely different animal.

Because the notation for literal NSStrings is modeled on the notation for C strings, it is worth knowing something about C strings, even though you won’t generally encounter them. For example, K&R lists a number of escaped characters (K&R 2.3), which you can also use in a literal NSString, including the following:

\n

A Unix newline character

\t

A tab character

\"

A quotation mark (escaped to show that this is not the end of the string literal)

\\

A backslash

Note

NSStrings are natively Unicode-based, but because Objective-C is C, including non-ASCII characters in a literal NSString was, until quite recently, remarkably tricky, and you needed to know about such things as the \x and \u escape sequences. Now, however, it is perfectly legal to type a bullet or any other non-ASCII character directly into an NSString literal, and you should ignore old Internet postings (and even an occasional sentence in Apple’s own documentation) warning that it is not.

K&R also mention a notation for concatenating string literals, in which multiple string literals separated only by white space are automatically concatenated and treated as a single string literal. This notation is useful for splitting a long string into multiple lines for legibility, and Objective-C copies this convention for literal NSStrings as well, except that you have to remember the at-sign:

@"This is a big long literal string "
@"which I have broken over two lines of code.";