title: Change the meaning of code by replacement of forward declaration with include
body: <p><a href="https://google-styleguide.googlecode.com/svn/trunk/cppguide.html">Google C++ Style Guide</a> states that</p>
  
  <blockquote>
    <p>In extreme cases, replacing an #include with a forward declaration can silently change the meaning of code.</p>
  </blockquote>
  
  <p>Can you please help me to find some examples? </p>