title: "Strong" qualify in objc
body: <p>I create a new project using xcode4.2, and view the AppDelegate:</p>
  
  <pre><code>
  @property (strong, nonatomic) UIWindow *window;
  </code></pre>
  
  <p>I know the <code>strong </code> is a new qualify in Xcode for ARC.However I didn't select using ARC while creating the project. As a result it is boring me.
  Moreover,what does the below mean:</p>
  
  <pre><code>
  @synthesize window = _window;
  </code></pre>
  
  <p>Is <code>_window</code> a instance variable? But it didn't declare in the header file.I can understand if <code>_window</code> declared in the header but failed in this style.
  Is it kind of modern obj-c runtime?</p>
  
  <p>thanks!</p>