static_cast void*

// Cast a dynamically allocated string to 'void*'.
void *vp = static_cast<void*>(new std::string("it's easy to break stuff like this!"));

// Then, in the function that's using the UserEvent:
// Cast it back to a string pointer.
std::string *sp = static_cast<std::string*>(vp);//vp=void*
// You could use 'sp' directly, or this, which does a copy.
std::string s = *sp;
// Don't forget to destroy the memory that you've allocated.
delete sp;