设计模式之策略模式

所需知识

C++基础知识多态UML类图基本元素UML类图关系组合

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注意

设计模式不是针对语言的，而是针对OO编程的（所以不管你是学Java还是C++或者其他面向OOP语言），设计模式是在总结中得到的。不是发明设计模式，而是发现。

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重点

(1) 多用组合，少用继承。 (2) 针对接口编程，而不是针对实现编程。 (3) 策略模式：定义算法族，分别封装起来，让他们之间可以互相替换，此模式让算法的变化独立于使用算法的客户。

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说明

Java有接口说法，而在C++中则没有。一般说接口，那就是指示一个C++抽象类。

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模型

C++示例代码

// strategy.h #pragma once class Strategy { public: Strategy() { } virtual ~Strategy(); virtual void AlgorithmInterface() = 0; }; class ConcreteStrategyA : public Strategy { public: ConcreteStrategyA() { } virtual ~ConcreteStrategyA(); void AlgorithmInterface(); }; class ConcreteStrategyB : public Strategy { public: ConcreteStrategyB() { } virtual ~ConcreteStrategyB(); void AlgorithmInterface(); }; // strategy.cpp #include "strategy.h" #include <iostream> Strategy::~Strategy() { std::cout << "~Strategy()" << std::endl; } ConcreteStrategyA::~ConcreteStrategyA() { std::cout << "~ConcreteStrategy()" << std::endl; } void ConcreteStrategyA::AlgorithmInterface() { std::cout << "ConcreteStrategyA " "AlgorithmInterface()" << std::endl; } ConcreteStrategyB::~ConcreteStrategyB() { std::cout << "~ConcreteStrategy()" << std::endl; } void ConcreteStrategyB::AlgorithmInterface() { std::cout << "ConcreteStrategyB " "AlgorithmInterface()" << std::endl; } // context.h #pragma once class Strategy; class Context { public: Context(Strategy *stg); ~Context(); void doAction(); private: Strategy *stg_; }; // context.cpp #include "context.h" #include "strategy.h" Context::Context(Strategy *stg) : stg_(stg) { } Context::~Context() { if(!stg_) delete stg_; } void Context::doAction() { stg_->AlgorithmInterface(); }

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例子

接下来看《Head First设计模式》中策略模式之鸭子。 UML类图

代码实现

#include <iostream> class FlyBehavior { public: virtual void fly() = 0; }; class FlyWithWings : public FlyBehavior { public: void fly() { std::cout << "I'm flying!" << std::endl; } }; class FlyNoWay : public FlyBehavior { public: void fly() { std::cout << "I cann't fly!" << std::endl; } }; class QuackBehavior { public: virtual void quack() = 0; }; class Quack : public QuackBehavior { public: void quack() { std::cout << "Quack ..." << std::endl; } }; class Squeak : public QuackBehavior { public: void quack() { std::cout << "Squeak ..." << std::endl; } }; class MuteQuack : public QuackBehavior { public: void quack() { std::cout << "Mutequack ..." << std::endl; } }; class Duck { protected: FlyBehavior *flyBehavior; QuackBehavior *quackBehavior; virtual ~Duck() { delete flyBehavior; delete quackBehavior; } public: void performQuack() { quackBehavior->quack(); } void performFly() { flyBehavior->fly(); } void setFlyBehavior(FlyBehavior *f) { flyBehavior = f; } void setQuackBehavior(QuackBehavior *q) { quackBehavior = q; } }; class MallardDuck : public Duck { public: MallardDuck() : Duck() { flyBehavior = new FlyWithWings; quackBehavior = new Quack; } void dispaly() { std::cout << "I'm a real mallard duck!" << std::endl; } };

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引用

《Head Fist设计模式》，美Freeman ，中国电力出版社，2007/09/01 设计模式精解－GoF 23 种设计模式解析附 C++实现源码