title: c++ convert string into void pointer
body: <p>I use a library that have a callback function where one of the parameters is of type <code>void *</code>. (I suppose to let to send value of any type.)</p>
  
  <p>I need to pass a string (<code>std::string</code> or a <code>char[]</code> is the same).</p>
  
  <p>How can I do this?</p>