Objective-C Fundamentals

Providing an implementation for a class
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them by their name. The following is a possible implementation of CTRentalProperty’s
setRentalPrice: method:
- (void)setRentalPrice:(float)newRentalPrice {
rentalPrice = newRentalPrice;
}
Notice how the assignment statement can assign a new value to the rentalPrice
instance variable without any fuss. You may wonder how this statement knows which
object to update in the case that multiple CTRentalProperty objects have been created.
You don’t pass in any reference to an object of interest. The answer is that,
behind the scenes, every instance method is passed two additional hidden parameters
called self and _cmd. We leave discussion of _cmd for chapter 8, but self is the magic
that enables the method to know which object it should work with. You could explicitly
make this linkage more apparent by rewriting the setRentalPrice: method as follows:
- (void)setRentalPrice:(float)newRentalPrice {
self->rentalPrice = newRentalPrice;
}
The -> operator allows you to access the instance variables associated with the variable
referenced on the left-hand side. Because the compiler can pass different objects into
the method via the hidden self parameter, it can change which object the method
works with.
Check out your debugger window
The next time you use the Xcode debugger and hit a break point, check out the Arguments
section in the variables panel. You should be able to clearly see the self and
_cmd parameters listed.
Congratulations! You’re well on your way to becoming an Objective-C guru. Another
slightly mysterious piece of the IDE suddenly makes a little more sense.
Knowing that self is a hidden parameter that always represents the current object is
handy, but as demonstrated previously, it isn’t typically necessary to use it to access
instance variables. There is, however, another practical use of self that’s unavoidable.
5.3.3 Sending messages to self
When implementing an instance method, you may wish for it to call on the services of
other methods defined by the class. But to send a message, you must first have a reference
to the object you want to send it to. This reference is usually provided in the
form of a variable. When you want to refer to the current object, whatever it may be,
the hidden self parameter neatly fills the role, as follows:
- (void)handleComplaint {
NSLog(@"Send out formal complaint letter");
numberOfComplaints = numberOfComplaints + 1;