title: What's the point in passing I/O as parameters?
body: <p>I am learning C++ Primer, Fifth Edition; and I still don't get it what's the point of passing a reference to a stream in the <code>Sales_data</code> class</p>
  <pre class="lang-cpp prettyprint-override"><code>Sales_data(std::istream &amp;is)
  </code></pre>
  <p>Why do this if you can just use <code>std::cout</code> and <code>std::cin</code> explicitly?</p>