StackOverflow2013

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POHow thread-safe is my uniquing code?
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copied!<p>I have a small value class that I create lots of instances of. Often with the same value. This class is used as a kind of identifier, so the main use is comparing instances of this class with each other (via <code>isEqual:</code>). </p> <p>To save some memory and time comparing I keep only unique instances in a <code>NSHashTable</code> and use pointer comparison instead of <code>isEqual:</code>.</p> <p>So my designated initializer looks like this:</p> <pre><code>- initWithStuff: (NSString *)stuff; { self = [super init]; if (!self) return nil; // ... (actual initialisation code omitted) hash = UniquingHashTable(); static OSSpinLock spinlock = OS_SPINLOCK_INIT; OSSpinLockLock( &spinlock ); id member = [hash member: self]; if (member) self = member; else [hash addObject: self]; OSSpinLockUnlock( &spinlock ); return self; } </code></pre> <p><code>UniquingHashTable()</code> returns the global <code>NSHashTable</code> or creates it via <code>[NSHashTable weakObjectsHashTable]</code> if it doesn’t exist yet. The <code>weakObjectsHashTable</code> is the important bit - it stores weak pointers to the objects that automatically get removed once there is no other strong reference to that object. The initialization is thread-safe thanks to <code>dispatch_once</code>.</p> <p>This works fine, peak memory usage is significantly lower and all my test cases still pass. But I am not quite sure if I can rely on pointer-comparison to test for equality. <em>Is there any case or race condition where I get two distinct instances at the same time (so the pointers are different) but they still compare equal by <code>isEqual:</code>?</em></p> <h3>To clarify what I have/want:</h3> <p>Given</p> <pre><code>MyClass *a = [[MyClass alloc] initWithStuff: stringA]; MyClass *b = [[MyClass alloc] initWithStuff: stringB]; </code></pre> <p>we always have</p> <pre><code>[a isEqual: b] == [stringA isEqual: stringB]; </code></pre> <p>This is not changed with the new code.</p> <p>What I am trying to achieve is that also</p> <pre><code>[a isEqual: b] == (a == b) </code></pre> <p>so that I can replace the <code>isEqual:</code> with a faster pointer comparison. (Yes, I measured this, this will be significant).</p> <p>For single threaded code or if I was using a <code>NSMutableSet</code> instead of a weak <code>NSHashTable</code> this always works out. I’m just not sure if there is any race condition so that I can have the case where</p> <pre><code>[a isEqual: b] && (a != b) </code></pre> <p>is true.</p>

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