In short they come from to different conceptualizations that, in some languages where double dispatch is not natively supported, lead to the visitor pattern as a way to concatenate two (or more) single dispatch in order to have a multi-dispatch surrogate. In long The idea of multiple dispatch is – essentially – allow a call … Read more
You missed the “double” part of the double dispatch. The point of this pattern is to make sure that the right method of the processor is called – the method that accepts the right type. Since the processor is initially not aware of the type of the object that’s passed to it, you need the … Read more
When you create methods like this: virtual bool is_equal(Shape& circle) { return false; }; And in the subclass, virtual bool is_equal(Circle& circle) { return true; }; These are not the same method. You have two separate virtual methods, neither of which is overridden (they are overloaded not even overloaded, as Ben Voigt pointed out). When … Read more
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 … Read more
The JLS states in §8.4.9 Overloading: When a method is invoked (§15.12), the number of actual arguments (and any explicit type arguments) and the compile-time types of the arguments are used, at compile time, to determine the signature of the method that will be invoked (§15.12.2). If the method that is to be invoked is … Read more
The visitor pattern is a way of doing double-dispatch in an object-oriented way. It’s useful for when you want to choose which method to use for a given argument based on its type at runtime rather than compile time. Double dispatch is a special case of multiple dispatch. When you call a virtual method on … Read more