title: C++ double dispatch "extensible" without RTTI
body: <p>Does anyone know a way to have double dispatch handled correctly in C++ <em>without</em> using RTTI and dynamic_cast&lt;> and also a solution, in which the class hierarchy is extensible, that is the base class can be derived from further and its definition/implementation does not need to know about that? <br>
  I suspect there is no way, but I'd be glad to be proven wrong :)</p>