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Forward @class Directive
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The @class directive declares a class name without defining the class. It permits you to declare a reference to a class without having compiled the @interface for that class. Following a @class directive, you can refer to the class but you cannot compile any code that assumes knowledge about that class since the compiler has none. You can t send messages to an object of that class, or access any of its properties, until the actual @interface directive for that class is compiled. As soon as you start declaring variables that are pointers to objects, you ll discover that the compiler requires that those classes be defined before it will allow you to use the class name in a statement. The natural inclination is to just #import the needed class header files for every object you reference in your @interface directive. For smaller projects, that s fine. For large projects, this becomes a burden on the compiler (translation: it will slow down your development). Every class that uses class A will import its interface (#import "A.h"). If class A has instance variables that point to objects of classes B, C, and D, it will import all of those header files. If classes B, C, and D collectively contain references to classes E through N, their header files will import all of those header files, and so on. As the complexity of your project grows, the effort needed to compile each module grows geometrically. The @class directive can be used to lighten this load considerably. An @interface directive rarely needs to know anything about a referenced class other than it exists. A typical application of the @class directive is shown in Listing 3-13.
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Listin g 3-13. Forward @class Directive
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#import "Vehicle.h" @class Engine; @class MoonRoof; @class Radio; @interface Car : Vehicle { Engine *engine; MoonRoof *moonRoof; Radio *radio; }
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CHAPTER 3 WELCOME TO OBJECTIVE-C
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- (void)startEngine; - (void)stopEngine; - (void)tuneRadio; @end The Car class contains references to Engine, MoonRoof, and Radio objects. A module that #imports this @interface can use the methods and properties of Car, but knows nothing about the other objects. The @implementation of Car, which presumably will use the other objects, begins by importing the full definition of those classes. As a rule, I #import the header file for the superclass, the class of all public objects, and the class of objects returned by methods (on the assumption that the caller will use the returned object). All remaining classes in the @interface are declared using @class. The @class directive also makes it possible to declare classes with circular references. In other words, class A has a reference to class B which has a reference to class A.
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self and super
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An Objective-C method has access to two predefined variables for referring to itself: self (this) and super (super). They work exactly like their Java counterparts, as shown in Listing 3-14.
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Listin g 3-14. Use of self and super
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#import "Person.h" @interface Voter : Person { } - (void)setAdult:(BOOL)isAdult; @end @implementation Voter - (void)setAdult:(BOOL)isAdult { if (isAdult && !self.isAdult) [VoterRegistration registerVoter:self]; [super setAdult:isAdult]; } @end The self variable is always a pointer to the reciever (the object whose method was invoked). The type of the self variable is a pointer to the class that implemented the method. It can be used to address instance variables defined or inherited by that class (i.e., self->secondName), but not a subclass variable (even if the actual class of the object is a subclass).
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CHAPTER 3 WELCOME TO OBJECTIVE-C
Caution Java programmers may initially find this bizarre, but self is a modifiable automatic variable and can
be reassigned in the body of a method. I ll explain later why you would want to do this. For now, just be careful not to change it inadvertently.
The statement [self method] sends an object a message to itself. The equivalent Java statement would be this.method() or just method(). The pseudo-variable super is identical to self, but its only practical use is when sending messages. The statement [super method] invokes the method defined by the object s superclass and is equivalent to the Java statement super.method().
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