Structural Patterns

What these patterns solve

Structural patterns deal with how pieces fit together. One interface doesn't match another, a subsystem is too wide to call directly, a feature must be layered on without editing the class, or millions of small objects use too much memory. Each pattern puts a wrapper, a tree or a shared object between the caller and the real work.

Most structural patterns look the same in code: an object that holds another object and forwards calls to it. Tell them apart by intent, not by shape.

Members

• Adapter: converts one interface into the interface a caller expects, such as wrapping a vendor SDK behind your own port.
• Bridge: splits an abstraction from its implementation so both can vary independently, avoiding a class for every combination.
• Composite: treats a single item and a tree of items through the same interface, as with files and folders.
• Decorator: wraps an object to add behavior (caching, retries, logging) while keeping its interface.
• Facade: gives a complex subsystem one small, task-oriented entry point.
• Flyweight: shares the immutable part of many similar objects to cut memory use.
• Proxy: stands in for another object with the same interface to control access: lazy loading, remote calls or permission checks.

How to choose within this category

• The interface is wrong and you don't own the other side: use Adapter.
• The interface is fine but you want extra behavior around calls: use Decorator. If the extra behavior is about whether or when the real object is reached, it's a Proxy. In languages with higher-order functions, wrapping a single function (withRetry(fetchInvoice)) often replaces a decorator class.
• The subsystem is too big for most callers: use Facade. It adds a simpler interface; it doesn't translate an incompatible one.
• You have nested parts and wholes: use Composite. A recursive union type with pattern matching can replace it in TypeScript, Rust or modern Java.
• Two dimensions vary and subclasses multiply (PdfInvoice, HtmlInvoice, PdfReceipt, ...): use Bridge.
• Profiling shows memory pressure from duplicated state: use Flyweight. Don't apply it without a measurement.

Prefer no pattern when

• You control both sides of an interface mismatch. Change one side instead of adding an adapter.
• A wrapper would forward every call unchanged. That's a Middle Man.
• Only one combination of features ever exists. Compose it directly.