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    <p>Make sure you read the discussion on this question/answer, too. <a href="https://stackoverflow.com/questions/20895214/why-should-we-separate-alloc-and-init-calls-to-avoid-deadlocks-in-objective-c/20895427#20895427">Why should we separate alloc and init calls to avoid deadlocks in Objective-C?</a></p> <hr/> <p>To expand on the race condition issue; the <em>real</em> fix is to not have indeterminate initialization within your application. <em>Indeterminate</em> or <em>lazy</em> initialization results in behavior that can easily change due to seemingly innocuous changes -- configuration, "unrelated" code changes, etc...</p> <p>Better to explicitly initialize subsystems on a known-good point in the program's lifespan. I.e. drop <code>[MyClass sharedInstance];</code> into your App delegate's <code>applicationDidFinishLaunching:</code> method if you <em>really</em> need that subsystem initialized early in the program (or move it even earlier, if you want to be extra defensive).</p> <p>Better still to move initialization out of that method entirely. I.e. <code>[MyClass initializeSharedInstance];</code> where <code>+sharedInstance</code> asserts() if that method isn't called first.</p> <p>As much as I'm a a fan of convenience, 25 years of ObjC programming has taught me that lazy initialization is a source of more maintenance and refactoring headaches than it is worth.</p> <hr/> <p><strong>While the race condition described below exists, this code doesn't fix what is described below. It did for a couple of decades when we didn't worry about concurrency in shared instance initializers. Leaving the wrong code for prosperity.</strong></p> <p>Keep in mind that for both Colin's and Harald's otherwise correct answers, there is a very subtle race condition that could lead you to a world of woe.</p> <p>Namely, if the <code>-init</code> of the class being allocated happens to call the <code>sharedInstance</code> method, it will do so before the variable is set. In both cases it will lead to a deadlock.</p> <p>This is the one time that you want to separate the alloc and the init. Cribbing Colin's code because it is the best solution (assuming Mac OS X):</p> <pre><code>+(MyClass *)sharedInstance { static MyClass *sharedInstance = nil; static dispatch_once_t pred; // partial fix for the "new" concurrency issue if (sharedInstance) return sharedInstance; // partial because it means that +sharedInstance *may* return an un-initialized instance // this is from https://stackoverflow.com/questions/20895214/why-should-we-separate-alloc-and-init-calls-to-avoid-deadlocks-in-objective-c/20895427#20895427 dispatch_once(&amp;pred, ^{ sharedInstance = [MyClass alloc]; sharedInstance = [sharedInstance init]; }); return sharedInstance; } </code></pre> <p><strong>note</strong> this only works on Mac OS X; X 10.6+ and iOS 4.0+, in particular. On older operating systems, where blocks are not available, use a lock or one of the various means of doing something once that isn't blocks based.</p> <hr/> <p>The above pattern does not actually prevent the problem described in the text and will cause a deadlock when it is encountered. The problem is that the <code>dispatch_once()</code> is not re-entrant and, thus, if the <code>init</code> calls <code>sharedInstance</code>, <em>wedge city</em>.</p>
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    1. COThanks for this one, although it took me half an hour to find the correct reference to the blocks (^) syntax http://developer.apple.com/Mac/library/documentation/Cocoa/Conceptual/Blocks/Blocks.pdf, which for some reason is not included in their Objective C documentation
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      1. USHarald Scheirich
    2. COThanks. It was the first thing too I was thinking about while seeing your answer. I did not know there is a possibility to write such blocks in objective c (like anonymous class implementation in Java or delegates in C#). This is the answer for my other problem here: http://stackoverflow.com/questions/2118728/method-signature-for-a-selector Instead of using callbacks I now use blocks. :) Perhaps, I initialize my singleton in the AppDelegate and actually don't care about thread concurrency. The instance is simple there, when needed. It is a kind of cache for few things like images etc.
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    3. CO@bbum, (1) why would `-init` call `+sharedInstance`, and (2) why do you include the line `if (sharedInstance) return sharedInstance;`?
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