title: What's the difference between doing alloc and class_createInstance
body: <p>Wondering, what is the difference between creating a class with:</p>
  
  <pre><code>Class clazz = [NSString class];
  [clazz alloc];
  </code></pre>
  
  <p>and </p>
  
  <pre><code>class_createInstance(clazz,0);
  </code></pre>