How does double dispatch work in Visitor pattern?

Question

I was looking into other questions related to the visitor pattern but couldn't understand the implementation of double dispatch in visitor pattern.

Please refer to the link Visitor Pattern

How does double dispatch work in the Visitor pattern?

Answer 1

Single-dispatch

Assume Node is an interface class and the two sub classes are concrete implementations of the interface.

If you call GenerateCode() method on a node instance, the actual operation getting executed depends on the type of the node. It could be the method either in VariableRefNode or AssignmentNode. It's the same if you call PrettyPrint(). So the actual operation getting executed depends on name of the method you are calling and the type of the node.

Double-dispatch

This time the Node is allowing you to pass a parameter of type NodeVisitor to its method called Accept. In your program if you call Accept on a node instance, the actual operation getting executed now depends on the type of the node (VariableRefNode or AssignmentNode) AND the type of the visitor instance you passed into Accept (TypeCheckingVisitor or CodeGeneratingVisitor).