Discussing design patterns from your C++ work

Which object-oriented design patterns have you applied in C++ code?

Pick two or three you have actually used and explain why. Common ones in pricing code are factory (create payoffs or models from configuration), strategy or bridge (swap random generators or payoffs), template method, decorator, singleton (with its drawbacks) and the virtual-constructor clone idiom. For each, describe the problem it solved and the trade-off; a pattern list without context scores poorly.