Structural patterns

In this module

• Add behaviour to an object by WRAPPING it in another object of the same type, stack as many layers as you like, at runtime.
• Convert the interface a class HAS into the interface a client EXPECTS, a wrapper that translates calls so incompatible code can work together.
• Give a complex subsystem ONE simple front door, a single class with easy methods that orchestrate all the messy internals for you.
• Build a tree of part-and-whole objects, then treat a single leaf and a whole branch THE SAME way through one interface.
• A stand-in that implements the same interface as a real object and CONTROLS access to it, to defer expensive work, check permissions, cache, or reach a remote object.
• Split one thing that varies in TWO dimensions into two separate hierarchies, joined by a reference, so each can grow independently instead of multiplying.
• When you need a HUGE number of similar objects, share their common heavy data instead of duplicating it, keep only the per-object differences.