Composite - Structural Pattern

• Composite objects into tree structures to represent part-whole hierarchies.
• Clients treat individual objects (leaves in the trees) and compositions of objects (nodes in the tree) uniformly

• Simple components are grouped into larger components, hence the need to distinguish between primitive and container objects.
• Describes how to use recursive composition so as to make primitive and container objects similar to the client.
• Abstract class represents both primitives and their containers.
• Defines virtual methods that the primitives and containers implement.
• In addition to the basic set of methods that the primitives implement, the containers also implement methods for accessing and managing its children which can be either containers or primitives.

Clients use the Component class interface to interact with objects in the composite structure. If the recipient is a Leaf, then the request is handled directly. If the recipient is a Composite, then it usually forwards requests to its child components.

Example

  Implementation

• Explicit parent references
• Sharing components - difficult when a component has more than one parent
• Maximising component interface - primitive and composite classes transparent to client
• Declaring child management operations - trade-off between safety and transparency, emphasis is on transparency over safety
• Component implement a list of components? - child pointer in Component class, space penalty for every leaf, only useful if there are very few children
• Child Ordering - use of iterators or traversers (as in Performer).
• Caching for better performance - caching of traversal and search info of children (Bounding Box)
• Composite responsible for deleting its children.

• To build a class framework for scene-graph hierarchy.
• Example, In a scene, there can be a Component Class can be Node, the leaves of the scene are the basic geometry, the Composite Class can be a Group. The Group contains methods for accessing its children/parent. Classes like Switch, LOD can be derived from Group. This structure can be seen in Performer.