C++ Double Dispatch problems

Question

This is part 2 to a problem I previously asked: Is it possible to have polymorphic member overloading in C++?

Using the Wiki example I created this example. http://en.wikipedia.org/wiki/Double_dispatch

My problem is that the compiled code never looks up the vtable, and always uses the base instead of the inherited class. Here is my code:

#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;
}

However, the output is always:

Vehicle Bump MapObject

Any help on this mystery is greatly appreciated.

Answer 1

You only did single dispatch, not double dispatch. The idea is that in xVehicle, taking an xMapObject&, you would call ref.bump(*this); which is double dispatch.