title: Instance variables in extension vs implementation
body: <p>In Objective-C, for instance variables that one doesn't want to put in the header file and, one can put them either in the class extension:</p>
  
  <pre><code>@interface MyClass () {
    NSString *myInstanceVariable;
  }
  // ...
  @end
  </code></pre>
  
  <p>or in the class implementation:</p>
  
  <pre><code>@implementation MyClass {
    NSString *myInstanceVariable;
  }
  // ...
  @end
  </code></pre>
  
  <p>Is there any difference between them?</p>