title: Why can't I use the property of a superclass in a subclass (Objective-C)
body: <p>I declared one property in a class:</p>
  
  <pre><code>@property double x;
  </code></pre>
  
  <p>Then I made a <strong>subclass</strong> of that class in which I declared two more properties:</p>
  
  <pre><code>@property double a, b;
  </code></pre>
  
  <p>Then I made an init method in the subclass:</p>
  
  <pre><code>-(id)initWithAValue:andBValue:
  </code></pre>
  
  <p>And I am allowed to use:</p>
  
  <pre><code>_a;  // same for b
  [self setValue a]; // same for b
  self.a;  // same for b and x
  </code></pre>
  
  <p>Now, when I try to use the x property, I can only use [self setValue x] and I can't use _x. Why?</p>