title: iVars references strong, weak or what?
body: <p>In Obj-C, properties can be configured to be weak/strong. Instance variables. like following -</p>
  
  <pre><code>@interface MyClass {
  NSObject *a;
  }
  </code></pre>
  
  <p>Does the MyClass's object keep weak reference to <code>a</code> or strong, or something else? I think iVar is not released until its object is released.
  Why don't we specify weak/strong for iVar like properties?</p>