Objective-C Fundamentals

Declaring the interface of a class
101
Although declaring instance variables looks similar to declaring normal variables,
there’s a little more to their syntax than what we’ve discussed. Just as an employee may
not want her boss to know she’s searching for a new job, an object may want to restrict
access to some of its internal data. The declaration of an instance variable can be preceded
by an optional directive such as @private to alter who can access (or alter) its
current value. Table 5.1 outlines the available directives. Once a visibility directive is
specified, it applies to all additional instance variables declared until another accessibility
directive is found. By default, instance variables are @protected.
Table 5.1
Standard visibility directives available for instance variables in Objective-C
Visibility level
@private
@protected
@public
@package
Description
Accessible only by the class that declared it
Accessible only by the class that declared it or by any subclass
Accessible from anywhere
Accessible from anywhere in the current code image (the application or static
library in which the class is compiled)
To see why the concept of instance variable visibility is important, imagine that you
had access to a class that represented a loan in a banking system. The bank wouldn’t
be happy if anyone with access to a loan object could reach inside and change the status
instance variable from “rejected” to “accepted” or change the interest rate to –10%
so the bank would pay the loan holder for the privilege of having a loan! Access and
modification of this data would need to be tightly controlled, and this is exactly the
intent of directives such as @protected and @private.
At the other end of the spectrum, you could mark an instance variable as @public,
which means any piece of code can directly access the instance variable with no checks
or balances to ensure it’s done correctly. This practice is usually discouraged because
it provides less control and flexibility over how instance variables are utilized. If direct
access is discouraged, then how are you meant to access or change their values? The
answer leads nicely onto the subject of how to declare methods in a class.
5.2.2 Method declarations
Because allowing direct access to instance variables is discouraged, you must find
another way to allow users to query and alter their values. This alternative technique
should allow you to tightly control access and optionally provide other services such as
logging and validation. One solution is to provide a set of messages that the object can
be sent in order to update or access the instance variables on your behalf. Because the
code implementing the method is part of the class, it’ll have access to the instance
variables no matter their protection level.