Effective C++ Item 35 考虑 virtual 函数以外的实现

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1.virtual 函数版本

class GameCharacter{
public:
	virtual int healthValue() const; //返回人物的健康指数， derived classes 可重新定义它
};

class GameCharacter{
public:
	int healthValue() const // Item 36 derived classes不重新定义它
	{
		//...
		int retVal = doHealthValue();
		//...
		return retVal;
	}
private:
	virtual int doHealthValue() const //derive classes 可重新定义它
	{
		//...  缺省算法
	}
};

//1.藉由 Function Pointers 实现 Strategy 模式


int defaultHealthCalc(const GameCharacter &gc);<span style="font-family: Arial, Helvetica, sans-serif;">//计算健康指数的缺省算法</span>

class GameCharacter{
public:
	typedef int (*HealthCalcFunc)(const GameCharacter);
	explicit GameCharacter(HealthCalcFunc hcf = defaultHealthCalc) : healthFunc(hcf){}
	int healthValue() const{
		return healthFunc(*this);
	}
	//...
private:
	HealthCalcFunc healthFunc;
};

//2.藉由 tr1::function 完成 Strategy 模式
//以 tr1::function 成员变量替换 virtual 函数，因而允许使用任何可调用物(callable entity)
搭配一个兼容于需求的签名式。这也是 Strategy 设计模式的某种形式。

class GameCharacter;
int defaultHealthCalc(const GameCharacter &gc);
class GameCharacter{
public:
	//HealthCalcFunc 可以是任何“可调用物”，可被调用并被接受任何兼容于 GameCharacter 之物，返回任何兼容于 int的东西
	typedef std::tr1::function<int (const GameCharacter&)> HealthCalcFunc;
	explicit GameCharacter(HealthCalcFunc hcf = defaultHealthCalc) : healthFunc(hcf){}
	int healthValue() const{
		return healthFunc(*this);
	}
	//...
private:
	HealthCalcFunc healthFunc;
};

class GameCharacter;
class HealthCalcFunc{
public:
	//...
	virtual int calc(const GameCharacter &gc) const{...}
};


HealthCalcFunc defaultHealthCalc;
class GameCharacter{
public:
	explicit GameCharacter(HealthCalcFunc *phcf = &defaultHealthCalc) : pHealthCalc(phhcf){}
	int healthValue() const {return pHealthCalc->calc(*this);}
private:
	HealthCalcFunc *pHealthCalc;
};

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Effective C++ Item 35 考虑 virtual 函数以外的实现

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原文地址：http://blog.csdn.net/zhengsenlie/article/details/37807181