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    <blockquote> <p>Why is id a weak reference pointer?</p> </blockquote> <p><code>id</code> is not a weak reference pointer, at least not in the ARC ownership sense. Whether an <code>id</code>-typed reference to an object is weak or not depends on the reference having been declared <code>__weak</code> (and variations) and the object’s class actually supporting weak references.</p> <p>However, you could say that <code>id</code> provides <em>weak</em> typing, although I think that dynamic/duck typing is a more accurate description. Since an <code>id</code>- typed reference contains no compile-time class-type information, the compiler isn’t able to, for example, determine if the underlying object can respond to a given selector, which could lead to runtime errors.</p> <blockquote> <p>How is it able to handle any class type pointer?</p> </blockquote> <p>That’s part of the definition of <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocObjectsClasses.html">the Objective-C language</a>. The compiler recognises <code>id</code> as being the supertype of every Objective-C class, and it treats <code>id</code> differently. See the answer below as well.</p> <blockquote> <p>At runtime, how can we detect that which type of class pointer is assigned to id?</p> </blockquote> <p>In Apple’s Objective-C runtime, the first bytes in the memory allocated to an object must point to that object’s class. You might see this referenced elsewhere as the <code>isa</code> pointer, and that’s how Apple’s runtime finds out the class of every<sup>1</sup> object. The <code>id</code> type is defined to have this information as well. In fact, its only attribute is the <code>isa</code> pointer, which means that all<sup>1</sup> Objective-C objects conform to this definition.</p> <p>If you have an <code>id</code> reference and want to discover the class of the referenced object, you can send it <code>-class</code>:</p> <pre><code>id someObject; // Assign something to someObject // Log the corresponding class Class c = [someObject class]; NSLog(@"class = %@", c); // Test whether the object is of type NSString (or a subclass of NSString) if ([someObject isKindOfClass:[NSString class]]) { NSLog(@"it's a string"); } </code></pre> <p><sup>1</sup><a href="http://objectivistc.tumblr.com/post/7872364181/tagged-pointers-and-fast-pathed-cfnumber-integers-in">Tagged pointers</a> are a notable deviation of this structure, and (partly) because of them one shouldn’t access the <code>isa</code> pointer directly.</p>
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