Structural Patterns

Source: Gang of Four — Design Patterns (1994)

Pain Signals — You Are in This Branch When:

External library or API types appear directly in domain or business logic
A subsystem has a complex multi-step call sequence that callers frequently get wrong
You need combinations of behaviours (logging + caching + retry) but subclassing every combo is impractical
Access to an object needs to be deferred, controlled, or instrumented without changing the caller
A domain naturally forms a hierarchy where leaves and containers need identical treatment
Many objects duplicate the same large chunk of read-only data, causing memory pressure
Two independent dimensions of variation are creating a matrix of subclasses