C++ 装饰模式讲解和代码示例
装饰是一种结构设计模式， 允许你通过将对象放入特殊封装对象中来为原对象增加新的行为。
由于目标对象和装饰器遵循同一接口， 因此你可用装饰来对对象进行无限次的封装。 结果对象将获得所有封装器叠加而来的行为。
复杂度：
流行度：
使用示例： 装饰在 C++ 代码中可谓是标准配置， 尤其是在与流式加载相关的代码中。
识别方法： 装饰可通过以当前类或对象为参数的创建方法或构造函数来识别。
概念示例
本例说明了装饰设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
main.cc: 概念示例
/** * The base Component interface defines operations that can be altered by * decorators. */ class Component { public: virtual ~Component() {} virtual std::string Operation() const = 0; }; /** * Concrete Components provide default implementations of the operations. There * might be several variations of these classes. */ class ConcreteComponent : public Component { public: std::string Operation() const override { return "ConcreteComponent"; } }; /** * The base Decorator class follows the same interface as the other components. * The primary purpose of this class is to define the wrapping interface for all * concrete decorators. The default implementation of the wrapping code might * include a field for storing a wrapped component and the means to initialize * it. */ class Decorator : public Component { /** * @var Component */ protected: Component* component_; public: Decorator(Component* component) : component_(component) { } /** * The Decorator delegates all work to the wrapped component. */ std::string Operation() const override { return this->component_->Operation(); } }; /** * Concrete Decorators call the wrapped object and alter its result in some way. */ class ConcreteDecoratorA : public Decorator { /** * Decorators may call parent implementation of the operation, instead of * calling the wrapped object directly. This approach simplifies extension of * decorator classes. */ public: ConcreteDecoratorA(Component* component) : Decorator(component) { } std::string Operation() const override { return "ConcreteDecoratorA(" + Decorator::Operation() + ")"; } }; /** * Decorators can execute their behavior either before or after the call to a * wrapped object. */ class ConcreteDecoratorB : public Decorator { public: ConcreteDecoratorB(Component* component) : Decorator(component) { } std::string Operation() const override { return "ConcreteDecoratorB(" + Decorator::Operation() + ")"; } }; /** * The client code works with all objects using the Component interface. This * way it can stay independent of the concrete classes of components it works * with. */ void ClientCode(Component* component) { // ... std::cout << "RESULT: " << component->Operation(); // ... } int main() { /** * This way the client code can support both simple components... */ Component* simple = new ConcreteComponent; std::cout << "Client: I've got a simple component:\n"; ClientCode(simple); std::cout << "\n\n"; /** * ...as well as decorated ones. * * Note how decorators can wrap not only simple components but the other * decorators as well. */ Component* decorator1 = new ConcreteDecoratorA(simple); Component* decorator2 = new ConcreteDecoratorB(decorator1); std::cout << "Client: Now I've got a decorated component:\n"; ClientCode(decorator2); std::cout << "\n"; delete simple; delete decorator1; delete decorator2; return 0; } Output.txt: 执行结果
Client: I've got a simple component: RESULT: ConcreteComponent Client: Now I've got a decorated component: RESULT: ConcreteDecoratorB(ConcreteDecoratorA(ConcreteComponent)) 
