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copied!<p>Instance method is a method you can call on an object (as opposed to a class). Each object is an <em>instance</em> of a certain class (just as each of us is an instance of a human<sup>1</sup>), that’s why we talk about <em>instance methods</em>. For example when you say <code>[someArray count]</code>, you are calling an instance method called <code>count</code> on some array object (= instance of the <code>NSArray</code> class).</p> <p>Class methods are called on classes, for example <code>[UIApplication sharedApplication]</code> is a class method of the <code>UIApplication</code> class. You can’t call an instance method on a class, nor can you call a class method on an object.</p> <p>Category is a way to extend behaviour of existing classes, a kind of plugin you stick on an existing class. For example by writing this:</p> <pre><code>@interface NSObject (SampleCategory) - (void) sayFoo; @end @implementation NSObject (SampleCategory) - (void) sayFoo { NSLog(@"Foo!"); } @end </code></pre> <p>…you make it possible to call <code>[anObject sayFoo]</code> on any object derived from <code>NSObject</code>. And now we are getting to the point of the sentence: <code>NSObject</code> seems to be special, because when you declare an instance method of <code>NSObject</code> using a category (such as our <code>sayFoo</code>), you can call this method even as a class method: <code>[NSObject sayFoo]</code>, <code>[NSView sayFoo]</code>, etc.</p> <hr> <p>1] Sorry, Googlebot!</p>

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