title: Double colons before class names in Ruby?
body: <p>I have seen a lot usage of double colons in Rails before class names.</p>
  
  <p>For example:</p>
  
  <pre><code>require ::File.expand_path('../config/environment',  __FILE__)
  </code></pre>
  
  <p>I know what <code>Module::Class::Constant</code> means, but <code>::Class</code> ?</p>