10.5 - The Creator Pattern

Table of Contents

• 10.5.1 The Creator Pattern Specification

The Creator pattern lets you replace a single class in a system with a specialized subclass.

You might do this to fix a bug, or to add or change the behavior of specific functions. This is usually done through the Factory Method pattern. Unfortunately, if an application allows multiple parallel hierarchies (such as two display engines in the same application), the Factory Method pattern fails, because it indiscriminately sets the override class for all users. To implement similar behavior, but allow for changes to a class implementation tied to a specific root object, VR-Vantage uses the Creator pattern.

The Creator pattern involves two classes, the class which can be replaced (called the object class) and a class that does the creation (called the creator class). The creator instance is a Rooted Singleton of a given type registered with the root display engine. It can be accessed by anyone who has a root object pointer. All users of the object class use a Rooted Singleton of the creator class to create an instance of the object class.

The exampleCreator example uses the Creator pattern to cause a derived channel to get created when the display engine creates a channel. For details, please see the exampleCreator example.

10.5.1 The Creator Pattern Specification

The Creator pattern has the following requirements and behavior:

• The Creator pattern involves two classes, the class that can be replaced (called the object class), and a class that does the creation (called the creator class).
• All users of the object class use a ‘pseudo-global' instance of the creator class to create an instance of the object class. The creator instance is pseudo-global because a single instance of a given type is registered with the root object and can be accessed by anyone who has a root object pointer (in this case the root object is the display engine object).
• The object class should not have a public constructor.
• The object class should use mostly virtual functions.
• The object class should specify the creator as a friend class.
• The creator class should provide a virtual create function.
• The creator class should provide a static creator instance accessor to easily get the creator instance from the root object.
• The creator instance must be explicitly registered with the root object before anyone attempts to create a class object.
• The creators should be registered during initialization.
• The lifetime of a creator is linked to the lifetime of the root object.
• To override the object class, sub-class the creator and implement the virtual create function to return a new subclass instance.

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