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copied!<pre><code>// See Mike Ash "Care and Feeding of Singletons" // See Cocoa Samurai "Singletons: You're doing them wrong" +(MySingleton *)singleton { static dispatch_once_t pred; static MySingleton *shared = nil; dispatch_once(&pred, ^{ shared = [[MySingleton alloc] init]; shared.someIvar = @"blah"; }); return shared; } </code></pre> <p>Be aware that <a href="https://stackoverflow.com/a/7997698/412916">dispatch_once is not reentrant</a>, so calling itself from inside the dispatch_once block will deadlock the program.</p> <p>Don't try to code defensively against yourself. If you are not coding a framework, treat your class as normal then stick the singleton idiom above. Think of the singleton idiom as a convenience method, not as a defining trait of your class. You want to treat your class as a normal class during unit testing, so it's OK to leave an accessible constructor. </p> <p>Don't bother using <code>allocWithZone:</code></p> <ul> <li>It ignores its argument and behaves exactly like <code>alloc</code>. Memory zones are no longer used in Objective-C so <code>allocWithZone:</code> is only kept for compatibility with old code. </li> <li>It doesn't work. You can't enforce singleton behavior in Objective-C because more instances can always be created using <code>NSAllocateObject()</code> and <code>class_createInstance()</code>. </li> </ul> <p>A singleton factory method always returns one of these three types:</p> <ul> <li><code>id</code> to indicate the return type is not fully known (case where you are building a class cluster).</li> <li><code>instancetype</code> to indicate that the type returned is an instance of the enclosing class. </li> <li>The class name itself (<code>MySingleton</code> in the example) to keep it simple.</li> </ul> <p>Since you tagged this iOS, an alternative to a singleton is saving the ivar to the app delegate and then using a convenience macro that you can redefine if you change your mind:</p> <pre><code>#define coreDataManager() \ ((AppDelegate*)[[UIApplication sharedApplication] delegate]).coreDataManager </code></pre>

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