Question

I’m currently completing a project in PHP, but the general principles of OO still apply.

I have a parent class called Node, which is abstract – i.e. it cannot be instantiated. There are many classes that extend Node, and are instantiable. Some of these include Event, Person, and Project.

The issue I’m having is that I often need to instantiate a Node without knowing which child class it will be. All Node and node-sub-classes always takes the same type of object (a row from a database) as an argument to the constructor. To get around the problem of not knowing what type of implementing class I need, I created a simple static class that will return the correct sub-class of node. An example of this code is as follows:

<?php

class NodeClassRegistry{

    public static function init_class($node) {
        switch ($node->type) {
            case 'person':
                return new Person($node);

            case 'event':
                return new Event($node);
        }
    }

}

?>

This seems like really bad design to me – it’s kind of a factory pattern, but it seems really tied to global state. Is there a design pattern that would be appropriate for this situation?

Answer 1

What you do is a special type of the Factory Method design pattern The C++ example for this subtype in the GoF book is the following:

class Creator {
public:
  virtual Product* Create(ProductId id);
}

Product* Creator::Create(ProductId id) {
  if (id == MINE) return new MyProduct;
  if (id == YOURS) return new YourProduct;
  ...
  return 0;
}