title: setDT() has unexpected side effect
body: <p>This question is not a duplicate of <a href="https://stackoverflow.com/questions/23361080/error-in-setdt-from-data-table-package">Error in <code>setDT</code> from <code>data.table</code> package</a>.</p>
  
  <pre><code>library(data.table)
  dt &lt;- iris
  str(iris) # a data.frame solely
  setDT(dt)
  str(iris) # data.frame and data.table
  </code></pre>
  
  <p>Why should <code>setDT()</code> act on an object that is not its argument?</p>
  
  <p><em>Thank you all for pointing out <strong>why</strong> iris is changed in parallel with dt. Unless one knew the answer already, there would be no way to know that the question was a duplicate.</em></p>