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POHow to deal with double composition and inheritance?
 

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  1. POHow to deal with double composition and inheritance?
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    <p>I found this related question: <a href="https://stackoverflow.com/questions/279158/how-do-i-use-composition-with-inheritance">How do I use composition with inheritance?</a></p> <p>I would like to do the same with Objective-C, that is to say that a <code>GenericView</code> knows that its property <code>obj</code> is a GenericObject, and that a <code>SpecializedView</code> knows that the very same <code>obj</code> property is a <code>SpecializedObject</code>.</p> <p>Here is an example that will be clearer:</p> <pre><code>// // Example.m #import &lt;UIKit/UIKit.h&gt; /* HEADER */ // Electrical Machine @interface ElectricalMachine : NSObject { } - (void)plugIn; @end // Toaster @interface Toaster : ElectricalMachine { } - (float)getThermostat; @end // GenericView @interface GenericView : NSObject { ElectricalMachine *machine; } - (void)doSomethingGeneric; @property (nonatomic, retain) ElectricalMachine *machine; @end //SpecializedView @interface SpecializedView : GenericView { } - (void)doSomethingSpecialized; @end /* IMPLEMENTATION */ // GenericView @implementation GenericView @synthesize machine; - (void)doSomethingGeneric { Toaster *toaster = [[Toaster alloc] init]; [toaster plugIn]; self.machine = toaster; [toaster release]; } @end // SpecializedView @implementation SpecializedView - (void)doSomethingSpecialized { /* ERROR HERE * Incompatible types in initialization * 'ElectricalMachine' may not respond to '-getThermostat' */ float r = [machine getThermostat]; r = r; // ... } @end </code></pre> <p>As you see, I get an error at the end, because for <code>SpecializedView</code> the machine property is an <code>ElectricalMachine</code>, not a <code>Toaster</code>.</p> <p>Thank you very much for your help!</p> <p><strong>Old Question</strong></p> <p>Here is the first version of my question, which was maybe too cryptic:</p> <p>I have the following generic view:</p> <pre><code>@interface GenericView { GenericObject obj; } - (id)doSomething; </code></pre> <p>I also have the following specialized view:</p> <pre><code>@interface SpecializedView : GenericView { } - (id)doSomethingElse; </code></pre> <p>I have the following object:</p> <pre><code>@interface GenericObject { } - (id)plugIn; </code></pre> <p>and the following specialized object:</p> <pre><code>@interface SpecializedObject : GenericObject { } - (float)toastTime; </code></pre> <p>Let's say I want GenericView to handle GenericObject, and SpecializedView to handle the same object, knowing that it is SpecializedObject.</p> <p>Let me explain by showing implementations:</p> <p><strong>GenericView doSomething</strong></p> <pre><code>- (id)doSomething { [obj plugIn]; } </code></pre> <p><strong>SpecializedView doSomethingElse</strong></p> <pre><code>- (id)doSomethingElse { // ERROR here float time = [obj toastTime]; } </code></pre> <p>I will get the following warning: 'GenericObject' may not respond to '-toastBread' and the following error: Incompatible types in assignement</p> <p>Which is logical, since I have defined the type of obj as GenericObject. I want to be able to use methods from GenericObject in GenericView, and methods from SpecializedObject in SpecializedView. Is there a way to precise that obj has to be a GenericObject in GenericView to be handled, and has to be a SpecializedObject to be dealt with in SpecializedView, without adding a property? How would you do that?</p>
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