title: Is empty struct defined by C++ standard?
body: <p>Is there any <code>std::empty</code> struct or something similar or do I need to define my own:</p>
  
  <pre><code>struct empty{};
  </code></pre>
  
  <p>This can be used very nice in combination with <code>std::conditional</code> or other new std features and I wonder if the standard defines it or not.</p>