Question

I try to understand how double dispatch works. I created an example where a monster and a warrior derived from the abstract class Creature could fight. The class Creature has method “fight”, which is defined in derived classes, and in each derived class is defined what happens if warrior fights with warrior or with monster etc. I wrote the following code:

#include<iostream>
using namespace std;

class Monster;
class Warrior;

class Creature{
public:
    virtual void fight(Creature&) =0;
};

class Monster: public Creature{
    void fightwho(Warrior& w) {cout<<"Monster versus Warrior"<<endl; }
    void fightwho(Monster& m) {cout<<"Monster versus Monster"<<endl; }
public:
    void fight(Creature& c)  {c.fightwho(*this);}
};

class Warrior: public Creature{
    void fightwho(Warrior& w) {cout<<"Warrior versus Warrior"<<endl; }
    void fightwho(Monster& m) {cout<<"Monster versus Warrior"<<endl; }
public:
    void fight(Creature& c) {c.fightwho(*this);}
};

int main()
{
Warrior w;
Monster m;
w.fight(m);
}

This results in compiler error, which I foresee:

ex12_10.cpp: In member function ‘virtual void Monster::fight(Creature&)’: ex12_10.cpp:17:30: error: ‘class Creature’ has no member named ‘fightwho’

ex12_10.cpp: In member function ‘virtual void Warrior::fight(Creature&)’: ex12_10.cpp:24:29: error: ‘class Creature’ has no member named ‘fightwho’

But I don’t know how to proceed from here… Please help.

Answer 1

Well, obviously, you really don’t have fightwho declared in your Creature class, so you need to declare it there, and declare it as virtual.

Double dispatch works in a way that for call (this assumes Warrior& w = ..., not Warrior w):

w.fight(m);

First the virtual mechanism will chose Warrior::fight instead of Monster::fight and then the overloading mechanism will pick Monster::fightwho(Warrior& m) instead of Warrior::fightwho(Warrior& m). Note that it would make more sense if you would have:

Warrior w;
Monster m;
Creature& c1 = w;
Creature& c2 = m;
c1.fight(c2); // not w.fight(m)

Therefore, the method which will eventually be called will be dispatched according to type of the object on which you call it and type of the object sent as argument, i.e. double dispatch

Additionally, please note that this might not be the best example as your types are members of the same hierarchy. Visitor design pattern is a good example of double dispatch implementations in languages which don’t support it as first class citizens (i.e. C++ and derivatives: Java, C#…)

As @CrazyCasta correctly notes, when your class hierarchy starts to grow, this approach becomes much harder to maintain and can result in explosion of number of methods, so choose carefully…