title: When to use extern in C++
body: <p>I'm reading "Think in C++" and it just introduced the <code>extern</code> declaration. For example: </p>
  
  <pre><code>extern int x;
  extern float y;
  </code></pre>
  
  <p>I think I understand the meaning (declaration without definition), but I wonder when it proves useful. </p>
  
  <p>Can someone provide an example?</p>