Programming iOS 4

C Functions and Struct Pointers

Although your code will certainly call many Objective-C methods, it will also probably call quite a few C functions. For example, I mentioned in Chapter 1 that the usual way of initializing a CGPoint based on its x and y values is to call CGPointMake, which is declared like this:

CGPoint CGPointMake (
   CGFloat x,
   CGFloat y
);

Make certain that you can see at a glance that this is a C function, not an Objective-C method, and be sure you understand the difference in the calling syntax. To call an Objective-C method, you send a message to an object, in square brackets, with each argument following a colon in the method’s name; to call a C function, you use the function’s name followed by parentheses containing the arguments.

Moreover, many Objective-C objects and methods have lower-level C counterparts. For example, besides the Objective-C NSString, there is also something called a CFString; the “CF” stands for “Core Foundation,” which is a lower-level C-based API. A CFString is an opaque C struct (“opaque” means that the elements constituting this struct are kept secret, and that you should operate on a CFString only by means of appropriate functions). As with an NSString or any other object, in your code you’ll typically refer to a CFString by way of a C pointer; the pointer to a CFString has a type name, CFStringRef (a “reference to a CFString,” evidently). You work with a CFString in pure C, by calling functions.

You might, on occasion, actually have ...