大話設計模式C++版本-02-策略模式

概念

策略模式：定義了算法家族,分別封裝起來,讓他們之間可以互相替換,此模式讓算法的變化不會影響到用算法的客戶。
注意：與簡單工廠模式相比，使用上只需要知道 Context 類即可

應用場景

1. 算法會時常變動；
2. 用幾個類封裝了這些算法且有共同的父類；

一般步驟

1. 抽象一個算法類

   class Strategy
   {
   public:
   	virtual void Algorithm()=0;
   };
   

2. 寫具體的子類

   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");
   	}
   };
   

3. 創建一個類，根據不同條件調用不同的策略

   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;
   };
   

具體實例

#include <cstdio>
class Strategy // 抽象類-策略類
{
public:
	virtual void Algorithm()=0;
};

class ConcreateStrategyA : public Strategy // 策略A
{
public:
	void Algorithm()
	{
		printf("ConcreateStrategyA \n");
	}
};

class ConcreateStrategyB : public Strategy // 策略B
{
public:
	void Algorithm()
	{
		printf("ConcreateStrategyB \n");
	}
};

class ConcreateStrategyC : public Strategy // 策略C
{
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 // 使用策略的類
{
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

總結

策略模式是一種定義了一系列算法的方法，它可以以相同的方式調用所有的算法，減少各種算法類與使用算法類之間的耦合。 --《大話設計模式》

策略模式的 Strategy類層次爲 Context 類定義了一系列可供重用的算法和行爲。繼承有助於析取出這些算法中的公共功能。

參考資料：

《大話設計模式》