title: How to add a variable to category in Objective-C?
body: <p>It's requied to create a category with a new variable (of type NSArray).</p>
  
  <p>OriginalClass+Extension.h:</p>
  
  <pre><code>@interface OriginalClass (Extension) {
      NSArray *_array;
  }
  
  @property (nonatomic, retain) NSArray *array;
  
  @end
  </code></pre>
  
  <p>But I got the error: <code>Cannot declare variable inside @interface or @protocol</code>.</p>
  
  <p>Please help to solve the problem.</p>