title: type conversion PTLib PString -> std::string
body: <p>How??
  In documentation of PTLib:</p>
  
  <blockquote>
    <p>PString::operator std::string     () const [inline]</p>
    
    <p>Cast the PString to a std::string.</p>
  </blockquote>
  
  <p>I try:</p>
  
  <pre><code>m_string= string(m_pstring);
  m_string = (string) m_pstring;
  </code></pre>
  
  <p>But get 
  error: call of overloaded 'basic_string(PString&amp;)' is ambiguous</p>
  
  <p>gcc version 4.6.3</p>