http://blogs.msdn.com/b/coolbeans/archive/2009/01/07/today-s-bug-is-brought-to-you-by-don-t-mix-c-and-c.aspxTake this for example: #include &lt;iostream&gt; #include &lt;iomanip&gt; class Storage { public : Storage() : m_val( -1 ) {} int m_val; }; class Holder { public : Holder() : m_double( 50000 ) {} unionen-USTelligent Evolution Platform Developer Build (Build: 5.6.50428.7875)http://blogs.msdn.com/b/coolbeans/archive/2009/01/07/today-s-bug-is-brought-to-you-by-don-t-mix-c-and-c.aspx#9292721Thu, 08 Jan 2009 05:20:20 GMT91d46819-8472-40ad-a661-2c78acb4018c:9292721Cory Nelson<p>Guess you'll be having more fun with C++0x support then, where non-trivial classes can be part of unions!</p> <div style="clear:both;"></div><img src="http://blogs.msdn.com/aggbug.aspx?PostID=9292721" width="1" height="1">