Saturday, December 22, 2012

visitor pattern - overloading vs overriding (again

In standard visitor, you have a hierarchy of visitable data classes (eg Product and 30 subclasses) + a bunch of visitor classes. In
this case, i have just one visitor class, which defines overloaded methods
Visitor.visit(Option)
Visitor.visit(Bond)
Visitor.visit(Futures) etc

Now, I am given a random Product object myProduct. It might be an Option, a Bond, a Futures... but given the multitude of Product
subtypes, it's too tedious to type-test.

I already have a myVisitor object, so I try calling myVisitor.visit(myProduct). I want it bound to the correct visit() among the
overloads, but guess what -- this won't compile because ... (hold your breath).... there's no Visitor.visit(Product) defined.

Reason -- overloaded method call is resolved at compile time based on declared type of myProduct

Solution -- define
abstract Product.accept(Visitor)
Bond.accept(Visitor){v.visit(this);}
Option.accept(Visitor v) {v.visit(this);}

Now, myProduct.accept(myVisitor) would bind the method call to the correct visit(). Why? Well, accept() is resolved at runtime and
binds to Bond.accept() assuming myProduct is a Bond. Inside Bond.accept(), visit(this) is always visit(Bond) -- resolved at compile
time like all Overloaded methods.

In conclusion, Visitable.accept(Visitor) is resolved dynamically and Visitor.visit(Visitable) is resolved statically.

What if I create a FriendlyVisitor type? Is accept() dynamic-and-static?

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