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C++ 装饰器模式详解

news 来源：原创 2025/5/7 6:43:16

装饰器模式（Decorator Pattern）是一种结构型设计模式，它允许向一个现有的对象动态添加新的功能，同时不改变其结构。这种模式创建了一个装饰类，用来包装原有的类，提供了比继承更有弹性的替代方案。

核心概念

设计原则

装饰器模式遵循开闭原则（Open/Closed Principle）：

对扩展开放：可以通过装饰器添加新功能
对修改关闭：不需要修改现有代码

主要优点

灵活扩展：比继承更灵活，可以动态添加或删除功能
避免类爆炸：不需要为每种功能组合创建子类
保持单一职责：每个装饰类只关注特定功能
运行时配置：可以在运行时选择不同的装饰组合

模式结构

主要组件

Component（组件接口）
- 定义对象的接口
- 可以是抽象类或接口
ConcreteComponent（具体组件）
- 实现组件接口的基本功能
- 将被装饰的对象
Decorator（装饰器基类）
- 继承/实现组件接口
- 持有组件对象的引用

ConcreteDecorator（具体装饰器）

向组件添加具体的新功能

可以在调用前后添加行为

#include <iostream>
#include <memory>
#include <string>// ==================== 组件接口 ====================
class Beverage {
public:virtual std::string getDescription() const = 0;virtual double cost() const = 0;virtual ~Beverage() = default;
};// ==================== 具体组件 ====================
class Espresso : public Beverage {
public:std::string getDescription() const override {return "浓缩咖啡";}double cost() const override {return 1.99;}
};class HouseBlend : public Beverage {
public:std::string getDescription() const override {return "家常混合咖啡";}double cost() const override {return 0.89;}
};// ==================== 装饰器基类 ====================
class CondimentDecorator : public Beverage {
protected:std::unique_ptr<Beverage> beverage_;public:explicit CondimentDecorator(std::unique_ptr<Beverage> beverage): beverage_(std::move(beverage)) {}std::string getDescription() const override = 0;
};// ==================== 具体装饰器 ====================
class Milk : public CondimentDecorator {
public:explicit Milk(std::unique_ptr<Beverage> beverage): CondimentDecorator(std::move(beverage)) {}std::string getDescription() const override {return beverage_->getDescription() + "，加牛奶";}double cost() const override {return beverage_->cost() + 0.20;}
};class Mocha : public CondimentDecorator {
public:explicit Mocha(std::unique_ptr<Beverage> beverage): CondimentDecorator(std::move(beverage)) {}std::string getDescription() const override {return beverage_->getDescription() + "，加摩卡";}double cost() const override {return beverage_->cost() + 0.30;}
};class Whip : public CondimentDecorator {
public:explicit Whip(std::unique_ptr<Beverage> beverage): CondimentDecorator(std::move(beverage)) {}std::string getDescription() const override {return beverage_->getDescription() + "，加奶泡";}double cost() const override {return beverage_->cost() + 0.15;}
};// ==================== 客户端代码 ====================
int main() {std::cout << "=== 咖啡订单系统 ===" << std::endl;// 1. 单纯浓缩咖啡auto espresso = std::make_unique<Espresso>();std::cout << "订单1: " << espresso->getDescription() << " 价格: ¥" << espresso->cost() << std::endl;// 2. 家常混合咖啡 + 双倍摩卡 + 奶泡auto houseBlend = std::make_unique<HouseBlend>();houseBlend = std::make_unique<Mocha>(std::move(houseBlend));houseBlend = std::make_unique<Mocha>(std::move(houseBlend));houseBlend = std::make_unique<Whip>(std::move(houseBlend));std::cout << "订单2: " << houseBlend->getDescription() << " 价格: ¥" << houseBlend->cost() << std::endl;// 3. 浓缩咖啡 + 牛奶 + 奶泡auto espressoWithCondiments = std::make_unique<Espresso>();espressoWithCondiments = std::make_unique<Milk>(std::move(espressoWithCondiments));espressoWithCondiments = std::make_unique<Whip>(std::move(espressoWithCondiments));std::cout << "订单3: " << espressoWithCondiments->getDescription() << " 价格: ¥" << espressoWithCondiments->cost() << std::endl;return 0;
}

模式变体

1. 带状态的装饰器

class SizeDecorator : public CondimentDecorator {enum Size { SMALL, MEDIUM, LARGE };Size size_;public:SizeDecorator(std::unique_ptr<Beverage> beverage, Size size): CondimentDecorator(std::move(beverage)), size_(size) {}std::string getDescription() const override {std::string sizeStr;switch(size_) {case SMALL: sizeStr = "小杯"; break;case MEDIUM: sizeStr = "中杯"; break;case LARGE: sizeStr = "大杯"; break;}return beverage_->getDescription() + "，" + sizeStr;}double cost() const override {double baseCost = beverage_->cost();switch(size_) {case SMALL: return baseCost * 0.8;case MEDIUM: return baseCost;case LARGE: return baseCost * 1.2;}return baseCost;}
};

2. 接口增强装饰器

class DiscountDecorator : public Beverage {std::unique_ptr<Beverage> beverage_;double discountRate_;public:DiscountDecorator(std::unique_ptr<Beverage> beverage, double rate): beverage_(std::move(beverage)), discountRate_(rate) {}std::string getDescription() const override {return beverage_->getDescription() + "（折扣" + std::to_string(discountRate_*100) + "%）";}double cost() const override {return beverage_->cost() * (1 - discountRate_);}// 新增方法double getOriginalPrice() const {return beverage_->cost();}
};