C++雙重分派問題

這是第2部分，以我以前問一個問題：Is it possible to have polymorphic member overloading in C++?C++雙重分派問題

使用我創造了這個例子中，維基例子。 http://en.wikipedia.org/wiki/Double_dispatch

我的問題是，編譯的代碼永遠不會查找vtable，並始終使用基地，而不是繼承類。這裏是我的代碼：

#include <iostream> 

class xEntity; 
class xVehicle; 

class xMapObject 
{ 
    public: 
    virtual void Bump(xMapObject&) { std::cout << "MapObject Bump MapObject\n"; }; 
    virtual void Bump(xEntity&) { std::cout << "MapObject Bump Entity\n"; }; 
    virtual void Bump(xVehicle&) { std::cout << "MapObject Bump Vehicle\n"; }; 
}; 

class xEntity : public xMapObject 
{ 
    public: 
    virtual void Bump(xMapObject&) { std::cout << "Entity Bump MapObject\n"; }; 
    virtual void Bump(xEntity&) { std::cout << "Entity Bump Entity\n"; }; 
    virtual void Bump(xVehicle&) { std::cout << "Entity Bump Vehicle\n"; }; 
}; 

class xVehicle : public xEntity 
{ 
    public: 
    virtual void Bump(xMapObject&) { std::cout << "Vehicle Bump MapObject\n"; }; 
    virtual void Bump(xEntity&) { std::cout << "Vehicle Bump Entity\n"; }; 
    virtual void Bump(xVehicle&) { std::cout << "Vehicle Bump Vehicle\n"; }; 
}; 

int main(int argv, char **argc) 
{ 
    xEntity Entity; 
    xVehicle Vechile; 

    xMapObject &EntityRef = Entity; 
    xMapObject &VehicleRef = Vechile; 

    VehicleRef.Bump(EntityRef); 

    return 0; 
}

然而，輸出始終是：