Question

So I’ve read up all the documentation about the Visitor pattern, and I’m still mightily confused. I’ve taken this example from another SO question, could someone help me understand? For instance when do we use a visitor design pattern? I think I may have understood some of it, but I’m just not able to see the bigger picture. How do I know when I can use it?

class equipmentVisited
{
  virtual void accept(equipmentVisitor* visitor) = 0;
}

class floppyDisk : public equipmentVisited
{
  virtual void accept(equipmentVisitor* visitor);
}

class processor : public equipmentVisited
{
  virtual void accept(equipmentVisitor* visitor);
}

class computer : public equipmentVisited
{
  virtual void accept(equipmentVisitor* visitor);
}

class equipmentVisitor
{
  virtual void visitFloppyDisk(floppyDisk* );
  virtual void visitProcessor(processor* );
  virtual void visitComputer(computer* );
}

// Some additional classes inheriting from equipmentVisitor would be here

equipmentVisited* visited;
equipmentVisitor* visitor;

// Here you initialise visited and visitor in any convenient way

visited->accept(visitor);

Answer 1

Visitor pattern is used to implement double dispatch. In plain words it means that the code that gets executed depends on runtime types of two objects.

When you call a regular virtual function, it is a single dispatch: the piece of code that gets executed depends on the runtime type of a single object, namely, the one the virtual method of which you are calling.

With the visitor pattern, the method that is being called ultimately depends on the type of two objects – the type of the object implementing the equipmentVisitor, and the type of the object on which you call accept (i.e. the equipmentVisited subclass).

There are other ways to implement double dispatch in C++. Item 31 of Scott Meyer’s “More Effective C++” treats this subject in depth.