Objective-C Fundamentals

116 CHAPTER 5 Creating classes
It’s important to understand the two-step initialization process used by Objective-C
because there are a number of nuisances that can appear subtle but jump up and bite
you when you least expect it.
BEWARE OF LURKING GOBLINS
The code snippet that called alloc and init on separate lines has a silent but potentially
fatal flaw. Although it may not be a problem with most classes (CTRental-
Property included), it’s possible for init to return an object different from the one
created by alloc. If this occurs, the original object becomes invalid, and you can only
use the object returned by init. In coding terms this means you should always store
the return value of init; here is a bad example:
CTRentalProperty *newRental = [CTRentalProperty alloc];
[newRental init];
And here is a good example:
CTRentalProperty newRental = [CTRentalProperty alloc];
newRental = [newRental init];
Rather than worry about handling such scenarios, it’s generally easier to perform both
steps at once, as in [[CTRentalProperty alloc] init] and not give it a second thought.
You may wonder under what conditions init may return a different object from the
one you allocated via alloc. For most classes this will never occur, but in special circumstances
(implementing singletons, caches, or named instances, and so on), a class developer
may decide to more tightly control when objects are created, preferring to return
an existing object that’s equivalent rather than initialize a new one, for example.
FAILURE IS A FACT OF LIFE
It isn’t always possible for an initialization method to perform its intended task. For
example, an initWithURL: method may initialize an object with data fetched from a
website. If the URL provided is invalid or the iPhone is in flight mode, initWithURL:
may not be able to complete its task. In such cases it’s common for the init method
to free the memory associated with the new object and return nil, indicating that the
requested object couldn’t be initialized.
If there’s a chance that your initialization method may fail, you may like to explicitly
check for this condition, as demonstrated here:
CTRentalProperty newRental = [[CTRentalProperty alloc] init];
if (newRental == nil)
NSLog(@"Failed to create new rental property");
Perhaps you’ve picked up on the fact that initialization methods aren’t always called
init and that they can accept additional parameters. This is quite common, if not the
norm, so let’s take a look at the why and how behind this.
5.5.2 init is pretty dumb
An initialization method that accepts no parameters has limited practical use. You
often need to provide additional details to the initialization method in order for it to