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    <p>Luc is right on with using the Visitor pattern, I'm just expanding on that by giving an example of how you could do it.</p> <pre><code>#include &lt;iostream&gt; #include &lt;conio.h&gt; using namespace std; class SpaceObject; class SpaceShip; class GiantSpaceShip; class Asteroid; class ExplodingAsteroid; class SpaceObject { public: virtual void CollideWith(SpaceObject*) {} virtual void CollideWith(SpaceShip*) {} virtual void CollideWith(GiantSpaceShip*) {} virtual void CollideWith(Asteroid*) {} virtual void CollideWith(ExplodingAsteroid*) {} }; class Asteroid : public SpaceObject { public: virtual void CollideWith(SpaceObject* o) { o-&gt;CollideWith(this); } virtual void CollideWith(SpaceShip *) { cout &lt;&lt; "Asteroid hit a SpaceShip" &lt;&lt; endl; } virtual void CollideWith(GiantSpaceShip *) { cout &lt;&lt; "Asteroid hit a GiantSpaceShip" &lt;&lt; endl; } }; class ExplodingAsteroid : public Asteroid { public: virtual void CollideWith(SpaceObject* o) { o-&gt;CollideWith(this); } virtual void CollideWith(SpaceShip *) { cout &lt;&lt; "ExplodingAsteroid hit a SpaceShip" &lt;&lt; endl; } virtual void CollideWith(GiantSpaceShip *) { cout &lt;&lt; "ExplodingAsteroid hit a GiantSpaceShip" &lt;&lt; endl; } }; class SpaceShip : public SpaceObject { public: virtual void CollideWith(SpaceObject* o) { o-&gt;CollideWith(this); } virtual void CollideWith(Asteroid* o) { o-&gt;Asteroid::CollideWith(this); } virtual void CollideWith(ExplodingAsteroid* o) { o-&gt;ExplodingAsteroid::CollideWith(this); } }; class GiantSpaceShip : public SpaceShip { public: virtual void CollideWith(SpaceObject* o) { o-&gt;CollideWith(this); } virtual void CollideWith(Asteroid* o) { o-&gt;Asteroid::CollideWith(this); } virtual void CollideWith(ExplodingAsteroid* o) { o-&gt;ExplodingAsteroid::CollideWith(this); } }; int main() { SpaceObject* s = new GiantSpaceShip(); SpaceObject* a = new ExplodingAsteroid(); a-&gt;CollideWith(s); getch(); return 0; } </code></pre>
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    1. POWhy does double dispatch not work in C++?
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    1. COCan you explain why your example usage includes three dispatches instead of just two? (It's not called *triple dispatch*, after all.)
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    2. COLooks like pdehaan found a weakness of double dispatch. Where do you put the implementation? Since both objects are involved either class seems a valid location. Personally, I put the implementation in the heavier class even it creates 3 calls (2 for double dispatch and a 3rd to forward the invocation to the preferred class). My 3rd call is not virtual, but a direct call to minimize the overhead. If both classes are equally weighty I have left the implementations as free friend functions, though I've never needed to do this in production code.
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    3. CO@Rob: Are you referring to the main call resulting in three calls before resolving, or the fact that I define CollideWith 3 times per sub-class? Passing 'this' as an argument passes as the type 'this' is when the function is defined. If I don't define the method inside each class, it reverts to the parent version, which only passes itself as the parent type. With each class having methods for both the base type and specific types they care about, I can make any two classes interact without knowing exactly which class they are. There's probably better ways for specific cases, but this works.
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