Objective-C Fundamentals

102 CHAPTER 5 Creating classes
Falling on the side of caution
Although it may be tempting to declare all instance variables as @public, in general,
you should try to keep them as tightly restricted as possible. Once access to an
instance variable is granted, it can be hard to remove or alter the purpose of the ivar
as your application continues to grow and develop. This is especially true if your class
is designed to be part of a widely used framework or support class.
With the default visibility of @protected, a subclass can access any instance variable
declared by its superclass. Once it does, however, an updated version of the
superclass can’t easily remove that variable or alter its purpose. If it does, the subclass
might break, as it now relies on a nonexistent instance variable or at least is
potentially using it in an incorrect manner.
By making the instance variable @private and providing methods to indirectly access
its value, you can isolate the storage of the value from its behavior. If an update to
the superclass means you want to remove the instance variable and instead obtain
its value via a calculation, you can do this in the accessor methods without affecting
any users of the class. As the saying goes, most things in computer science can be
solved by adding another layer of indirection.
For the CTRentalProperty class, you want clients to be able to
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Get or set the rental price to an absolute dollar amount
Get or set the address of the property
Get or set the property type
You can easily come up with additional helpful messages that improve the usability of
the class, such as
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Increase the rental price per week by a fixed percentage
Decrease the rental price per week by a fixed percentage
The messages understood by a class are declared in the @implementation section after
the curly braces enclosing instance variables and before the @end directive. This section
is like a list of steps in a recipe. It gives you a general idea of how the final product
is put together, but it doesn’t go into great detail about how each step is achieved.
The simplest form of method declaration is one that expects no parameters and
returns a single value. For example, the following declares a method called rental-
Price, which returns a floating-point number:
- (float)rentalPrice;
The data type of the value returned by the method is enclosed in parentheses and can
be any valid Objective-C type, such as those discussed in chapter 2. A special data type,
void, can also be used to indicate that no value is returned.
Now that you have a method declared that allows you to query the current rental
price, it’s only natural to want to complement it with a method to allow you to change