Objective-C Fundamentals

118 CHAPTER 5 Creating classes
The behavior of the application would be identical. The first form is a more condensed
version of the second. Finally, the method returns the value of self, in part to
enable the condensed [[CTRentalProperty alloc] init]–style creation syntax mentioned
previously.
5.5.3 Combining allocation and initialization
Because it’s common to allocate an object and then want to immediately initialize it,
many classes provide convenience methods that combine the two steps into one.
These class methods are typically named after the class that contains them. A good
example is NSString’s stringWithFormat: method, which allocates and initializes a
new string with the contents generated by the specified format string.
To allow users to easily create a new rental property object, add the following class
method to the CTRentalProperty class:
+ (id)rentalPropertyOfType:(PropertyType)newPropertyType
rentingFor:(float)newRentalPrice
atAddress:(NSString *)newAddress;
This method draws close parallels to the initialization method (listing 5.5) you just
added. The main difference, apart from a slightly different name, is that rental-
PropertyOfType:rentingFor:atAddress: is a class method, whereas initWithAddress:
andPrice:andType: is an instance method. Being a class method, rentalPropertyOf-
Type:rentingFor:atAddress: allows you to invoke the method without first creating an
object. Implement the method as shown in the following listing.
Listing 5.6
Merging allocation and initialization into one easy-to-call method
+ (id)rentalPropertyOfType:(PropertyType)newPropertyType
rentingFor:(float)newRentalPrice
atAddress:(NSString *)newAddress
{
id newObject = [[CTRentalProperty alloc]
initWithAddress:newAddress
rentalPrice:newRentalPrice
andType:newPropertyType];
return [newObject autorelease];
}
Notice that the implementation uses the existing alloc and initWithAddress:renting-
For:andType: methods and returns the newly initialized object to the caller. Another difference
is that the object is also sent an autorelease message. This additional step deals
with a memory-related convention, which we cover in depth in chapter 9.
This just about wraps up our coverage of how to create new classes, but we have
one unpleasant task left to cover: how to dispose of, or deallocate, old and unwanted
objects. If you don’t destroy these objects, they’ll eventually consume enough memory
that the iPhone will think your application is the next Chernobyl and shut it down to
avoid impacting other phone functionality!