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  • 装饰者模式(decorator pattern)

    装饰者模式:

        能够动态地往一个类中添加新的行为。相比继承是在编译时添加行为,装饰模式是在运行时添加行为。

    星巴克咖啡是讲述装饰者模式的一个很好的例子。

        假设现在有两种咖啡:HouseBlend、DarkRoast,同时有两种调料:Milk、Mocha。那么此时我们可以给顾客提供以下咖啡:

    HouseBlend、HouseBlendWithMilk、HouseBlendWithMocha、HouseBlendWithMilkandMocha、

    DarkRoast、DarkRoastWithMilk、DarkRoastWithMocha、DarkRoastWithMilkandMocha。如果每种搭配都是一个类,那么随着咖啡和调料种类的增加,将会出现类爆炸:

    此时装饰者模式应运而生,结构图如下:

    C++实现:

    class Coffee {
    public:
        virtual double getCost() = 0;
        virtual string getDescription() = 0;
    };
    
    class HouseBlend : public Coffee {
    public:
        double getCost() { return 1.24; }
        string getDescription() { return "HouseBlend"; }
    };
    
    class DarkRoast : public Coffee {
    public:
        double getCost() { return 2.73; }
        string getDescription() { return "DarkRoast"; }
    };
    
    class CoffeeDecorator : public Coffee {
    private:
        Coffee *decoratedCoffee;
    public:
        CoffeeDecorator(Coffee *c): decoratedCoffee(c) {}
        double getCost() { return decoratedCoffee->getCost(); }
        string getDescription() { return decoratedCoffee->getDescription(); }
    };
    
    class Milk : public CoffeeDecorator {
    public:
        Milk(Coffee *c): CoffeeDecorator(c) {}
        double getCost() { return CoffeeDecorator::getCost() + 0.5; }
        string getDescription() {return CoffeeDecorator::getDescription() + ", Milk"; }
    };
    
    class Mocha : public CoffeeDecorator {
    public:
        Mocha(Coffee *c): CoffeeDecorator(c) {}
        double getCost() { return CoffeeDecorator::getCost() + 0.7; }
        string getDescription() {return CoffeeDecorator::getDescription() + ", Mocha"; }
    };
    
    int main()
    {
        Coffee *pc = new HouseBlend;
        cout << "description: " << pc->getDescription() << "  cost: " << pc->getCost() << endl;
        pc = new Milk(pc);
        cout << "description: " << pc->getDescription() << "  cost: " << pc->getCost() << endl;
        pc = new Mocha(pc);
        cout << "description: " << pc->getDescription() << "  cost: " << pc->getCost() << endl;
        delete pc;
    }

    装饰者模式的一个特征是:装饰者和被装饰对象具有相同的超类型。

    reference:

    Decorator in C++: Before and after

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  • 原文地址:https://www.cnblogs.com/gattaca/p/4737586.html
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