Objective-C Fundamentals

106 CHAPTER 5 Creating classes
To demonstrate that classes can contain methods other than simple getters and
setters, the CTRentalProperty class also declares d two methods with the names
increaseRentalByPercent:withMaximum: and decreaseRentalByPercent:with-
Minimum:. These methods act as special setters that update the rentalPrice instance
variable by performing a simple calculation based on the provided parameters.
This class interface declaration is enough for anybody to go off and develop code
that makes use of the CTRentalProperty class. It has outlined to the compiler, and to
any interested users of the class, what kind of data you store and what kind of behavior
they can expect from you. With the classes interface specified, it’s time to provide the
compiler with the logic that implements the class.
5.3 Providing an implementation for a class
The @interface section of a class only declares what an object will look like. It doesn’t
provide details on how methods are implemented. Instead, this source code is typically
contained in a separate implementation (or method) file that commonly has the
file extension *.m.
Similar to the @interface section, the implementation of a class is started by an
@implementation directive followed by the class name and ending with an @end directive:
@implementation CTRentalProperty
... method implementations ...
@end
The class name is required after the @implementation directive because it’s possible
for a single *.m file to contain the implementations of multiple classes. With the location
of a particular class’s implementation determined, let’s look at how you specify
the behavior of a particular method.
5.3.1 Defining method implementations
You define the logic and behavior of a method by repeating its declaration. But
instead of ending the declaration with a semicolon, you use a set of curly braces and
provide the required logic that should be executed whenever the method is invoked.
The following is one possible implementation of the setRentalPrice: method:
- (void)setRentalPrice:(float)newRentalPrice {
NSLog(@"TODO: Change the rental price to $%f", newRentalPrice);
}
This implementation, however, simply logs a to-do message requesting you provide a
more practical implementation. To flesh out the implementation, you must be able
to access the instance variables associated with the current object. Luckily, this is
easy to do.
5.3.2 Accessing instance variables
When the body of an instance method is declared, the method automatically has
access to any instance variables associated with the current object. You can refer to