設計模式之政策模式Strategy

封裝不同的算法，使其獨立于對象改變。

代碼示例：

class ICalculate
{
	public:
	virtual int calculate(int value1, int value2) = 0;
};

class Minus : public ICalculate
{
	public:
	virtual int calculate(int value1, int value2) override
	{
		return value1 - value2;
	}
};

class Plus : public ICalculate
{
	public:
	virtual int calculate(int value1, int value2) override
	{
		return value1 + value2;
	}
};

class CalculateClient
{
	private:
	ICalculate* _strategy;
	
	public:
	CalculateClient(ICalculate* strategy)
	: _strategy(strategy)
	{
	}
	
	~CalculateClient()
	{
		if(_strategy != nullptr)
		{
			delete _strategy;
		}
	}
	
	void setStrategy(ICalculate* strategy)
	{
		if(_strategy != nullptr)
		{
			delete _strategy;
		}
		
		_strategy = strategy;
	}
	
	int calculate(int value1, int value2)
	{
		assert(_strategy != nullptr);
		return _strategy->calculate(value1, value2);
	}
};


void main()
{
	CalculateClient* client = new CalculateClient(new Minus);
	
	cout << "Minus: " << client->calculate(7, 1) << endl;
	
	//change the strategy
	client->setStrategy(new plus);
	
	cout << "Plus: " << client->calculate(7, 1) << endl;
	
	delete client;
}