概念
策略模式：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用算法的客户。 注意：与简单工厂模式相比，使用上只需要知道 Context 类即可
应用场景
算法会时常变动；用几个类封装了这些算法且有共同的父类；
一般步骤
抽象一个算法类
class Strategy
{
public:
virtual void Algorithm()=0;
};
写具体的子类
class ConcreateStrategyA : public Strategy
{
public:
void Algorithm()
{
printf("ConcreateStrategyA \n");
}
};
class ConcreateStrategyB : public Strategy
{
public:
void Algorithm()
{
printf("ConcreateStrategyB \n");
}
};
class ConcreateStrategyC : public Strategy
{
public:
void Algorithm()
{
printf("ConcreateStrategyC \n");
}
};
创建一个类，根据不同条件调用不同的策略
class Context
{
public:
Context(char algorithm
)
{
this->algorithm
= algorithm
;
switch (algorithm
)
{
case 'A':
this->strategy
= new ConcreateStrategyA();
break;
case 'B':
this->strategy
= new ConcreateStrategyB();
break;
case 'C':
this->strategy
= new ConcreateStrategyC();
break;
default:
this->strategy
= NULL;
break;
}
}
~Context()
{
printf("delete strategy%c\n",algorithm
);
delete strategy
;
}
void ContextInterface()
{
strategy
->Algorithm();
}
private:
Strategy
*strategy
;
char algorithm
;
};
具体实例
#include <cstdio>
class Strategy
{
public:
virtual void Algorithm()=0;
};
class ConcreateStrategyA : public Strategy
{
public:
void Algorithm()
{
printf("ConcreateStrategyA \n");
}
};
class ConcreateStrategyB : public Strategy
{
public:
void Algorithm()
{
printf("ConcreateStrategyB \n");
}
};
class ConcreateStrategyC : public Strategy
{
public:
void Algorithm()
{
printf("ConcreateStrategyC \n");
}
};
#if 0
class Context
{
public:
Context(Strategy
*strategy
)
{
this->strategy
= strategy
;
}
~Context()
{
delete strategy
;
}
void ContextInterface()
{
strategy
->Algorithm();
}
private:
Strategy
*strategy
;
};
int main()
{
Context
*context
= NULL;
context
= new Context(new ConcreateStrategyA());
context
->ContextInterface();
context
= new Context(new ConcreateStrategyB());
context
->ContextInterface();
context
= new Context(new ConcreateStrategyC());
context
->ContextInterface();
return 0;
}
#else
class Context
{
public:
Context(char algorithm
)
{
this->algorithm
= algorithm
;
switch (algorithm
)
{
case 'A':
this->strategy
= new ConcreateStrategyA();
break;
case 'B':
this->strategy
= new ConcreateStrategyB();
break;
case 'C':
this->strategy
= new ConcreateStrategyC();
break;
default:
this->strategy
= NULL;
break;
}
}
~Context()
{
printf("delete strategy%c\n",algorithm
);
delete strategy
;
}
void ContextInterface()
{
strategy
->Algorithm();
}
private:
Strategy
*strategy
;
char algorithm
;
};
int main()
{
Context
*context
= NULL;
context
= new Context('A');
context
->ContextInterface();
delete context
;
context
= new Context('B');
context
->ContextInterface();
delete context
;
context
= new Context('C');
context
->ContextInterface();
delete context
;
return 0;
}
#endif
总结
策略模式是一种定义了一系列算法的方法，它可以以相同的方式调用所有的算法，减少各种算法类与使用算法类之间的耦合。 --《大话设计模式》
策略模式的 Strategy类层次为 Context 类定义了一系列可供重用的算法和行为。继承有助于析取出这些算法中的公共功能。
参考资料：
《大话设计模式》
