Re: Double dispatch (makes no sense!)

From:

saneman <ddd@sdf.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 09 May 2008 10:42:44 +0200

Message-ID:

<48240e5e$0$90273$14726298@news.sunsite.dk>

hurcan solter wrote:

I am trying to use double dispatch in the below code:

#include <iostream>

class Box;
class Sphere;

class geometry_type
{
public:
  virtual void haps(geometry_type* other)=0;
  virtual void collideWith(Box* other)=0;
  virtual void collideWith(Sphere* other) = 0;
};

class Box : public geometry_type
{
public :
  Box() {}

  virtual void haps(geometry_type* other)
  {
   other->collideWith(this);
  }

  virtual void collideWith(Sphere* other) {
    std::cout << "Sphere collision with Sphere" << std::endl;
  }

Here is a typo;it should read' Box collision with sphere'

  virtual void collideWith(Box* other) {
    std::cout << "Box collision with Box" << std::endl;
  }

};

class Sphere : public geometry_type
{
public :
  Sphere() {}

  virtual void haps(geometry_type* other)
  {
   other->collideWith(this);
  }

  virtual void collideWith(Sphere* other) {
    std::cout << "Sphere collision with Sphere" << std::endl;
  }

  virtual void collideWith(Box* other) {
    std::cout << "Sphere collision with Box" << std::endl;
  }

};

void narrow_phase(geometry_type* G1, geometry_type* G2)
{
  G1->haps(G2);
}

int main()
{
  Box* BoxA = new Box();
  Sphere* SphereA = new Sphere();

  BoxA->collideWith(BoxA);
  SphereA->collideWith(BoxA);

  // Should print box collide with box.
  narrow_phase(BoxA, BoxA);

  // Should print sphere collide with box.
  narrow_phase(SphereA, BoxA);

  // Should print box collide with sphere.
  narrow_phase(BoxA, SphereA);

  return 0;

}

But when I run main I get:

Box collision with Box
Sphere collision with Box
Box collision with Box
Sphere collision with Sphere
Sphere collision with Box

What goes on in the last two calls?

Last one is correct, you switch the parameters
inside the haps. Late there :)

thanks! Yes it was a bit late :-) I am wondering why the baseclass
should be declared as abstract. I have tried to change it to:

class geometry_type
{
public:

   virtual void haps(geometry_type* other)
   {
    other->collideWith(this);
   }

   virtual void collideWith(Box* other)
   {
     std::cout << "Box collision" << std::endl;
   }

   virtual void collideWith(Sphere* other)
   {
     std::cout << "Sphere collision" << std::endl;
   }

// virtual void haps(geometry_type* other)=0;
// virtual void collideWith(Box* other)=0;
// virtual void collideWith(Sphere* other) = 0;

};

But then I get:

test2.cpp: In member function ?virtual void
geometry_type::haps(geometry_type*)?:
test2.cpp:16: error: no matching function for call to
?geometry_type::collideWith(geometry_type* const)?
test2.cpp:19: note: candidates are: virtual void
geometry_type::collideWith(Box*)
test2.cpp:24: note: virtual void
geometry_type::collideWith(Sphere*)

Is there some rule that the baseclass must be abstract?