title: Is null reference possible?
body: <p>Is this piece of code valid (and defined behavior)?</p>
  
  <pre><code>int &amp;nullReference = *(int*)0;
  </code></pre>
  
  <p>Both g++ and clang++ compile it without any warning, even when using <code>-Wall</code>, <code>-Wextra</code>, <code>-std=c++98</code>, <code>-pedantic</code>, <code>-Weffc++</code>...</p>
  
  <p>Of course the reference is not actually null, since it cannot be accessed (it would mean dereferencing a null pointer), but we could check whether it's null or not by checking its address:</p>
  
  <pre><code>if( &amp; nullReference == 0 ) // null reference
  </code></pre>