title: Bad practice to declare names in the standard namespace?
body: <p>I was looking through the <a href="http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml" rel="noreferrer">Google C++ style guide</a>, and came across this:</p>
  
  <p>"Do not declare anything in namespace std, not even forward declarations of standard library classes. Declaring entities in namespace std is undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file."</p>
  
  <p>Could someone explain what this means and why this is undefined behavior using example code?  </p>