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    <p>As <code>NSObject</code> provides <code>isEqual:</code>, and all your objects are descendants of <code>NSObject</code>, then the the simple answer is that a default implementation is provided.</p> <p>Now you are concerned over the algorithm this default uses, and in a comment write <em>"I wouldn't be sure simply by testing"</em>. Let's look at testing, just for fun ;-)</p> <p>Now <code>isEqual:</code> is a rather fundamental method, if Apple decided to change its semantics the consequences could be significant and not good. So while Apple is free to change how it is implemented provided the <em>semantics</em> remain the same, which means the same objects compare equal after the change as before. Now you've mentioned three possible algorithms <code>isEqual:</code> could use:</p> <ol> <li>Pointer comparison - is it the <em>exact same</em> object</li> <li>Shallow comparison - do the fields of the object have the same value compared directly</li> <li>Deep comparison - do the non-pointer-valued fields compared directly have the same value, and do the pointer-valued fields compare equal using <code>isEqual:</code></li> </ol> <p>These all have different semantics, whichever one Apple has chosen it can't change without breaking a lot of code. And different semantics means you can test...</p> <p>Coding as I type, errors expected! Only important bits included:</p> <pre><code>@implementation A - (BOOL) isEqual:(id)other { NSLog(@"A.isEqual called"); return self == other; // true iff same object } @end @interface B @property (readwrite) int anInteger; @property (readwrite) A *anA; @end @implementation B @synthesize anInteger, anA; @end // Let's test the algorithm A *myA = [A new]; B *bOne = [B new]; B *bTwo = [B new]; bOne.anInteger = 42; bOne.anA = myA; bTwo.anInteger = 42; bTwo.anA = myA; // What output is produced (all of it!) NSLog(@"[bOne isEqual:bTwo] -&gt; %@", [bOne isEqual:bTwo] ? @"Yes" : @"No"); </code></pre> <p>HTH a little.</p>
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    1. PODoes Apple Provide default implementation of isEqual:
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    1. COTo test apple's default implementation you must not implement one your self and see if it works. I guess it's obvious by now that it's not even documented and shouldn't be count on at all. +1
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    2. COIf you actually test it for fun I'll select this one as the answer. I just want to move on to other things as it's obviously not documented and can't be relied on.
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    3. CO@HaryantoCiu - The only logical choice as the default is pointer comparison, that is what `NSObject`'s `equalTo` uses, and it is documented - though not obviously (e.g. see <http://developer.apple.com/library/ios/#documentation/General/Conceptual/CocoaEncyclopedia/Introspection/Introspection.html>). I was suggesting you can test and rely on the result.
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