What's the difference between Polymorphism and Multiple Dispatch?

Question

...or are they the same thing? I notice that each has its own Wikipedia entry: Polymorphism, Multiple Dispatch, but I\'m having trouble seeing how the concepts differ.

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Answer 1

I've never heard of Multiple Dispatch before, but after glancing at the Wikipedia page it looks a lot like MD is a type of polymorphism, when used with the arguments to a method.

Polymorphism is essentially the concept that an object can be seen as any type that is it's base. So if you have a Car and a Truck, they can both be seen as a Vehicle. This means you can call any Vehicle method for either one.

Multiple dispatch looks similar, in that it lets you call methods with arguments of multiple types, however I don't see certain requirements in the description. First, it doesn't appear to require a common base type (not that I could imagine implementing THAT without void*) and you can have multiple objects involved.

So instead of calling the Start() method on every object in a list (which is a classic polymorphism example), you can call a StartObject(Object C) method defined elsewhere and code it to check the argument type at run time and handle it appropriately. The difference here is that the Start() method must be built into the class, while the StartObject() method can be defined outside of the class so the various objects don't need to conform to an interface.

This could be nice if the Start() method needed to be called with different arguments. Maybe Car.Start(Key carKey) vs. Missile.Start(int launchCode)

But both could be called as StartObject(theCar) or StartObject(theMissile)

Interesting concept...