StackOverflow2013

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copied!<p>There are several reasons that Cocoa encourages the use of <code>[Foo sharedFoo]</code> rather than a global Foo object.</p> <p>[Foo sharedFoo] can auto-instantiate the first time it's used.</p> <p>Naming the instance with a capital letter is very confusing because it looks like a class, encouraging bugs. Consistency in naming is at the heart of good Objective-C. The compiler cannot protect you against a wide variety of mistakes because ObjC is highly dynamic. Good naming and self-discipline in consistency is what leads to bug-free Cocoa.</p> <p>Parallelism:</p> <pre><code>Foo *foo = [Foo sharedFoo]; Foo *foo = [[[Foo alloc] init] autorelease]; Foo *foo = [Bar fooAtIndex:0]; </code></pre> <p>All three of those may be legal in the same program. Just because there is a singleton instance doesn't mean that there aren't other instances, too. NSNotificationCenter is a good example of this. It's a singleton, but you can make additional instances (and there are reasons to do so).</p> <p>A global variable can be globally modified. A <code>sharedInstance</code> cannot be. For instance, if <code>State</code> is a global variable (rather than a class), then <code>State=nil</code> is legal anywhere in the program. That breaks encapsulation and is an easy typo for <code>state=nil</code> that cannot be caught by the compiler. If <code>State</code> is a class, then the compiler can catch this easy error.</p> <p>The many rules of Cocoa naming are there to encourage highly readable code, and to minimize bugs in a highly dynamic and loosely typed environment. Like Perl's <code>use strict</code> we should be very careful before giving up what little safety net we have.</p>

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