C++实现策略（Strategy)模式

策略（Strategy)模式：它定义了算法家族，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，不会影响到算法的客户。下面模拟游戏中常用的打斗场面，来理解策略模式。在游戏中，通常有n个角色，每个角色都能发出动作：fight。每个角色都使用不同的武器，并且可以更换武器。下面是此场景简化的C++实现：

Strategy.h


#ifndef _STRETEGY_H_


#define _STRETEGY_H_




//定义武器接口


class WeaponBehavior






{


public:


void virtual useWeapon() = 0;


};




class Sword:public WeaponBehavior






{


public:


void useWeapon();


};




class Axe:public WeaponBehavior






{


public:


void useWeapon();


};




class Arrow:public WeaponBehavior






{


public:


void useWeapon();


};




class Knife:public WeaponBehavior






{


public:


void useWeapon();


};




//定义角色接口


class Character






{


public:


Character()






{


weapon = 0;


}


void setWeapon(WeaponBehavior *w)






{


this->weapon = w;


}


void virtual fight() = 0;


protected:


WeaponBehavior *weapon;


};




class King:public Character






{


public:


void fight();


};




class Queen:public Character






{


public:


void fight();


};




class Knight:public Character






{


public:


void fight();


};




class Troll:public Character






{


public:


void fight();


};






#endif
说明：
1.因为每个武器的使用方式不同，所以将使用武器的行为封装成接口，利用此接口调用具体武器的useWeapon行为。
2.因为每个角色都有打斗，更换武器的行为，各个角色的打斗行为不同，而更换武器的行为相同，所以将setWeapon在Character中实现，而打斗行为在各个具体的角色中实现。

Strategy.cpp


#include <iostream>


#include "Strategy.h"


using namespace std;


void Sword::useWeapon()






{


cout << "Use Sword to stuck!" << endl;


}




void Axe::useWeapon()






{


cout << "Use Axe to chop!" << endl;


}




void Knife::useWeapon()






{


cout << "Use Knife to kill!" << endl;


}




void Arrow::useWeapon()






{


cout << "Use arrow!" << endl;


}




void King::fight()






{


cout << "The king:" ;


if ( this->weapon == NULL)






{


cout << "You don't have a weapon! Please Set Weapon!" << endl;


}


else






{


weapon->useWeapon();


}


}




void Queen::fight()






{


cout << "The Queen:" ;


if ( this->weapon == NULL)






{


cout << "You don't have a weapon! Please Set Weapon!" << endl;


}


else






{


weapon->useWeapon();


}


}




void Knight::fight()






{


cout << "The Knight:" ;


if ( this->weapon == NULL)






{


cout << "You don't have a weapon! Please Set Weapon!" << endl;


}


else






{


weapon->useWeapon();


}


}




void Troll::fight()






{


cout << "The Troll:";


if ( this->weapon == NULL)






{


cout << "You don't have a weapon! Please Set Weapon!" << endl;


}


else






{


weapon->useWeapon();


}


}

main.cpp


#include <iostream>


#include "Strategy.h"


using namespace std;


int main()






{


//声明武器


WeaponBehavior *sw = new Sword();//声明剑


WeaponBehavior *axe = new Axe();//声明斧头


WeaponBehavior *arr = new Arrow();//声明弓箭


WeaponBehavior *kn = new Knife();//声明刀




//声明角色


Character *kin = new King();


Character *qu = new Queen();


Character *kni = new Knight();


Character *tr = new Troll();




//调用打斗行为


kin->fight();


qu->fight();


kni->fight();


tr->fight();


cout << endl;




//更换武器


kin->setWeapon(sw);


kin->fight();


cout << endl;




kin->setWeapon(arr);


kin->fight();




return 0;


}

策略(Strategy)模式体现了2个重要的面向对象设计原则：
1.封装变化。
2.针对接口编程。