Programming iOS 6, 3rd Edition

C Functions

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.

You might even have reason to write your own C functions as part of a class, instead of writing a method. A C function has lower overhead than a full-fledged method; so even though it lacks the object-oriented abilities of a method, it is sometimes useful to write one, as when some utility calculation must be called rapidly and frequently. Also, once in a while you might encounter a Cocoa method or function that requires you to supply a C function as a “callback.”

An example is the NSArray method sortedArrayUsingFunction:context:. The first parameter is typed like this:

NSInteger (*)(id, id, void *)

That expression denotes, in the rather tricky C syntax used for these things, a pointer to a function that takes three parameters and returns an NSInteger. The ...