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：