Hi, The boost::spirit::hold_any implementation includes the following functions defined in the detail sub-namespace: // member of fxns<mpl::true_> specialisation, line 99 static void move(void* const* src, void** dest) { reinterpret_cast<T*>(dest)->~T(); *reinterpret_cast<T*>(dest) = *reinterpret_cast<T const*>(src); } // member of fxns<mpl::false_> specialisation, line 99 static void move(void* const* src, void** dest) { (*reinterpret_cast<T**>(dest))->~T(); **reinterpret_cast<T**>(dest) = **reinterpret_cast<T* const*>(src); } As I understand it, the object is destroyed _and then assigned to_; does it not need to be constructed at this point, i.e. have something like this (for the first specialisation): new (reinterpret_cast<T*>(dest)) T( *reinterpret_cast<T const*>(src) ); Thanks & apologies if I've misunderstood --rob <http://theotherbranch.wordpress.com/>