Re: Double dispatch (makes no sense!)

From:
Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>
Newsgroups:
comp.lang.c++
Date:
Thu, 08 May 2008 18:33:47 -0700
Message-ID:
<4823bf01$0$3310$7836cce5@newsrazor.net>
saneman 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;
  }

  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?

Because your Box::collideWith(Sphere*) has a bug in it. Has nothing to
do with your double dispatch, which appears to be working perfectly.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

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