Strategy模式

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Strategy模式

Strategy模式要解决的问题和Template模式相似，都是为了把算法的声明和算法的实现解耦．Template模式是通过继承来实现的，而Strategy模式是通过组合来实现的．

Strategy模式将算法封装到一个类(Context)里面，通过组合的方式将算法在组合的对象中实现，之后通过委托将抽象接口委托给组合对象来实现．其类结构图如下
这里写图片描述

实现：

//Strategy.h

//Strategy.hclass Strategy{public:    virtual void PrimitiveOperation() = 0;    virtual ~Strategy();protected:    Strategy();};class ConcreteStrategy1 :public Strategy{public:    ConcreteStrategy1();    ~ConcreteStrategy1();    void PrimitiveOperation();};class ConcreteStrategy2 :public Strategy{public:    ConcreteStrategy2();    ~ConcreteStrategy2();    void PrimitiveOperation();};

//Strategy.cpp

//Strategy.cpp#include"Strategy.h"#include<iostream>using namespace::std;Strategy::Strategy(){}Strategy::~Strategy(){}ConcreteStrategy1::ConcreteStrategy1(){}ConcreteStrategy1::~ConcreteStrategy1(){}void ConcreteStrategy1::PrimitiveOperation(){    cout << "ConcreteStrategy1 PrimitiveOperation()" << endl;}ConcreteStrategy2::ConcreteStrategy2(){}ConcreteStrategy2::~ConcreteStrategy2(){}void ConcreteStrategy2::PrimitiveOperation(){    cout << "ConcreteStrategy2 PrimitiveOperation()" << endl;}

//Context.h

//Context.h#ifndef _CONTEXT_H_#define _CONTEXT_H_class Strategy;class Context{public:    Context(Strategy*);    ~Context();    void DoAction();private:    Strategy* _strategy;};#endif

//Context.cpp

//Context.cpp#include"Context.h"#include"Strategy.h"Context::Context(Strategy* strategy){    _strategy = strategy;}Context::~Context(){    delete _strategy;}void Context::DoAction(){    _strategy->PrimitiveOperation();}

//main.cpp

//main.cpp#include"Context.h"#include"Strategy.h"int main(){    Strategy* p1 = new ConcreteStrategy1();    Strategy* p2 = new ConcreteStrategy2();    Context* c1 = new Context(p1);    Context* c2 = new Context(p2);    c1->DoAction();    c2->DoAction();    return 0;}

可以看出Template是通过继承来实现接口，而Strategy模式是通过组合来实现．这两种方式各有优缺点．

继承：优点是易于扩展和修改那些被复用的实现．
缺点有：１）破坏了封装，继承父类的实现细节暴露给子类了．２）＂白盒＂复用，因为１）．３）父类实现更改时，所有子类不得不随之改变．４）从父类继承而来的实现在运行期间不能被改变(编译期间已经确定了)．

组合：优点：１）＂黑盒＂复用，封装性好．２）实现和抽象的依赖性低，因为组合对象和被组合对象之间的依赖性很小．３）可以在运行期间修改，例如通过更改指针指向的对象．

缺点就是系统对象过多．

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