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Comment by foota | Hacker News Reader

The difference confused me for a bit, but consider the following:

  Print(Vehicle v){
    print("vehicle");
  }

  Print(Car c){
    print("car");
  }

  Print(Boat b){
    print("boat");
  }

  Vehicle test = Boat();
  
  Print(test);

In a language with multi dispatch I believe this prints boat, whereas with single dispatch it prints vehicle.

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Not quite. Multiple dispatch (as mentioned in sibling comments) is basically 'virtual on both object and argument types', so like:

  class Vehicle {
    virtual void Collide(Vehicle other);
  }

  class Car {
    void Collide(Truck other) { /* hit a truck */ }
    void Collide(Car other) { /* hit another car */ }
  }

  class Truck {
    void Collide(Truck other) { /* hit another truck */ }
    void Collide(Car other) { /* hit a car */ }
  }

  Vehicle c = Car();
  Vehicle t = Truck();

  c.Collide(t); // with multiple dispatch, calls Car::Collide(Truck)

You are using static overloading on the argument rather than dynamic dispatch. If the static type of truck is obscured to a super class, your behavior won't be as expected. In general, overloading shouldn't be used as a replacement for dynamic dispatch, rather it should be used to accommodate multiple incompatible types (e.g. foo(X) and foo(Y) but never foo(Z) if X <: Z and Y <: Z unless foo(Z) maintains the behavior of foo(X) and foo(Y) given an x or a y under subsumption).

Multiple dispatch is typically meant to be purely dynamic in meaning, as wiki puts it:

> Multiple dispatch or multimethods is a feature of some programming languages in which a function or method can be dynamically dispatched based on the run-time (dynamic) type or, in the more general case some other attribute, of more than one of its arguments.[1]

(https://en.wikipedia.org/wiki/Multiple_dispatch)

The static type of both c and t is Vehicle, but by dynamic multiple dispatch Car::Collide(Truck) is selected. That's not how C++ works, but the example was exactly about illustrating how C++ doesn't work.

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