title: Difference between using self.variable and _variable when init these variables
body: <p>I know instance variable and property. I often see people init a <code>UILabel</code> like this </p>
  
  <pre><code>self.label = [[UILabel alloc] init]; //and 
  _label = [[UILabel alloc] init];
  </code></pre>
  
  <p>So, what's the difference between using <code>self.label</code> and <code>_label</code> to set a object?</p>