Structural patterns are concerned with how classes and objects are composed to form larger structures. Structural class patterns use inheritance to compose interfaces or implementations. As a simple example, consider how multiple inheritance mixes two or more classes into one. The result is a class that combines the properties of its parent classes. This pattern is particularly useful for making independently developed class libraries work together. Another example is the class form of the Adapter (139) pattern. In general, an adapter makes one interface (the adaptee's) conform to another, thereby providing a uniform abstraction of different interfaces. A class adapter accomplishes this by inheriting privately from an adaptee class. The adapter then expresses its interface in terms of the adaptee's.

Rather than composing interfaces or implementations, structural object patterns describe ways to compose objects to realize new functionality. The added flexibility of object composition comes from the ability to change the composition at run-time, which is impossible with static class composition.

Composite (163) is an example of a structural object pattern. It describes how to build a class hierarchy made up of classes for two kinds of objects: primitive and composite. The composite objects let you compose primitive and other composite objects into arbitrarily complex structures. In the Proxy (207) pattern, a proxy acts as a convenient surrogate or placeholder for another object. A proxy can be used in many ways. It can act as a local representative for an object in a remote address space. It can represent a large object that should be loaded on demand. It might protect access to a sensitive object. Proxies provide a level of indirection to specific properties of objects. Hence they can restrict, enhance, or alter these properties.

The Flyweight (195) pattern defines a structure for sharing objects. Objects are shared for at least two reasons: efficiency and consistency. Flyweight focuses on sharing for space efficiency. Applications that use lots of objects must pay careful attention to the cost of each object. Substantial savings can be had by sharing objects instead of replicating them. But objects can be shared only if they don't define context-dependent state. Flyweight objects have no such state. Any additional information they need to perform their task is passed to them when needed. With no context-dependent state, Flyweight objects may be shared freely.

Whereas Flyweight shows how to make lots of little objects, Facade (185) shows how to make a single object represent an entire subsystem. A facade is a representative for a set of objects. The facade carries out its responsibilities by forwarding messages to the objects it represents. The Bridge (151) pattern separates an object's abstraction from its implementation so that you can vary them independently.

Decorator (175) describes how to add responsibilities to objects dynamically. Decorator is a structural pattern that composes objects recursively to allow an open-ended number of additional responsibilities. For example, a Decorator object containing a user interface component can add a decoration like a border or shadow to the component, or it can add functionality like scrolling and zooming. We can add two decorations simply by nesting one Decorator object within another, and so on for additional decorations. To accomplish this, each Decorator object must conform to the interface of its component and must forward messages to it. The Decorator can do its job (such as drawing a border around the component) either before or after forwarding a message.

Many structural patterns are related to some degree. We'll discuss these relationships at the end of the chapter.

Adapter
Discussion of Creational Patterns