【more effective c++读书笔记】【第5章】技术(1)——将constructor和non-member functions虚化

一、将constructor虚化

1、例子：

#include<list>
#include<sstream>
#include<iostream>
using namespace std;

class NLComponent{
public:
	NLComponent(){ cout << "NLComponent()" << endl; }
	NLComponent(const NLComponent&){ cout << "NLComponent(const NLComponent&)" << endl; }
	~NLComponent(){ cout << "~NLComponent()" << endl; }

	virtual void printName(){ cout << "NLComponent" << endl; }
};
class TextBlock : public NLComponent{
public:
	TextBlock(){ cout << "TextBlock()" << endl; }
	TextBlock(const TextBlock&){ cout << "TextBlock(const TextBlock&)" << endl; }
	~TextBlock(){ cout << "~TextBlock()" << endl; }

	virtual void printName(){ cout << "TextBlock" << endl; }
};
class Graphic : public NLComponent{
public:
	Graphic(){ cout << "Graphic()" << endl; }
	Graphic(const Graphic&){ cout << "Graphic(const Graphic&)" << endl; }
	~Graphic(){ cout << "~Graphic()" << endl; }

	virtual void printName(){ cout << "Graphic" << endl; }
};
class NewsLetter{
public:
	NewsLetter(stringstream& ss){ //copy constructor
		string str;
		while (ss >> str){
			components.push_back(readComponent(str));
		}
	}
	void printNews(){
		for (list<NLComponent*>::const_iterator it = components.begin();
			it != components.end(); ++it){
			(*it)->printName();
		}
	}
private:
	list<NLComponent*> components;
	static NLComponent* readComponent(const string& str){//virtual copy constructor
		if (str == "TextBlock")
			return new TextBlock;
		if (str == "Graphic")
			return new Graphic;
		return NULL;
	}
};
int main(){
	stringstream ss("TextBlock Graphic Graphic");
	NewsLetter news(ss);
	news.printNews();

	system("pause");
	return 0;
}

上述例子中NewsLetter类的readComponent函数根据输入的字符串不同产生不同的对象。它产生新对象，所以行为好像constructor，但它能够产生不同类型的对象，所以称为一个virtual constructor。所谓virtual constructor是指能够根据输入给它的数据的不同而产生不同类型的对象。

2、有一种特别的virtualconstructor--所谓virtual copy constructor，它会返回一个指针，指向其调用者（某对象）的一个新副本。

例子：

#include<list>
#include<iostream>
using namespace std;

class NLComponent{
public:
	NLComponent(){ cout << "NLComponent()" << endl; }
	NLComponent(const NLComponent&){ cout << "NLComponent(const NLComponent&)" << endl; }
	~NLComponent(){ cout << "~NLComponent()" << endl; }

	virtual NLComponent* clone() const = 0; //声明virtual copy constructor
	virtual void printName(){ cout << "NLComponent" << endl; }
};
class TextBlock : public NLComponent{
public:
	TextBlock(){ cout << "TextBlock()" << endl; }
	TextBlock(const TextBlock&){ cout << "TextBlock(const TextBlock&)" << endl; }
	~TextBlock(){ cout << "~TextBlock()" << endl; }

	virtual TextBlock* clone() const{//virtual copy constructor
		return new TextBlock(*this);//调用copy constructor
	}
	virtual void printName(){ cout << "TextBlock" << endl; }
};
class Graphic : public NLComponent{
public:
	Graphic(){ cout << "Graphic()" << endl; }
	Graphic(const Graphic&){ cout << "Graphic(const Graphic&)" << endl; }
	~Graphic(){ cout << "~Graphic()" << endl; }

	virtual Graphic* clone() const{//virtual copy constructor
		return new Graphic(*this);//调用copy constructor
	}
	virtual void printName(){ cout << "Graphic" << endl; }
};
class NewsLetter{
public:
	NewsLetter(const list<NLComponent*>& c) :components(c){}
	NewsLetter(const NewsLetter& rhs){
		//it指向rhs.components的目前元素，然后调用该元素的clone函数取得该元素的一个副本，
		//然后将该副本加到对象的components list尾端
		for (list<NLComponent*>::const_iterator it = rhs.components.begin();
			it != rhs.components.end(); ++it){
			components.push_back((*it)->clone());
		}
	}
	void printNews(){
		for (list<NLComponent*>::const_iterator it = components.begin();
			it != components.end(); ++it){
			(*it)->printName();
		}
	}
private:
	list<NLComponent*> components;
};
int main(){
	list<NLComponent*> component;
	TextBlock* text = new TextBlock;
	cout << "-------------" << endl;
	Graphic* graphic = new Graphic;
	cout << "-------------" << endl;
	component.push_back(text);
	component.push_back(graphic);

	NewsLetter news1(component);
	news1.printNews();
	cout << "-------------" << endl;
	NewsLetter news2(news1);
	cout << "-------------" << endl;
	news2.printNews();

	system("pause");
	return 0;
}

注意上述实现手法利用“虚函数之返回类型”规则中的一个宽松点，即当derived class重新定义其base class的一个虚函数时，不再需要一定得声明与原本相同的返回类型。如果函数的返回类型是一个指向base class的指针（或引用），那么derived class的函数可以返回一个指向该base class的derived class的指针（或引用）。

二、将non-member functions虚化

1、例子

#include<iostream>  
using namespace std;

class NLComponent{
public:
	virtual ostream& operator<<(ostream& s) const = 0;//output operator的非传统声明   
};
class TextBlock : public NLComponent{
public:
	virtual ostream& operator<<(ostream& s) const{
		s << "TextBlock";
		return s;
	}
};

class Graphic : public NLComponent{
public:
	virtual ostream& operator<<(ostream& s) const{
		s << "Graphic";
		return s;
	}
};

int main(){
	TextBlock tx;
	Graphic gc;
	tx << cout << endl;//此语法与传统不符 
	gc << cout << endl;
	
	system("pause");
	return 0;
}

上述例子中必须把ostream对象放在“<<”符号的右边，和传统的output操作符习惯不符。解决方法是声明一个虚函数（如print）作为打印之用，并在TextBlock和Graphic中定义它。并定义一个operator<<的non-member function，展现出类似print虚函数一般的行为。

例子：

#include<iostream>
using namespace std;

class NLComponent{
public:
	virtual ostream& print(ostream& s) const = 0;
};
class TextBlock : public NLComponent{
public:
	virtual ostream& print(ostream& s) const{
		s << "TextBlock";
		return s;
	}
};
class Graphic : public NLComponent{
public:
	virtual std::ostream& print(ostream& s) const{
		s << "Graphic";
		return s;
	}
};
inline std::ostream& operator<<(std::ostream& s, const NLComponent& c){
	return c.print(s);
}

int main(){
	TextBlock tx;
	Graphic gc;
	cout << tx << endl;
	cout << gc << endl;
	
	system("pause");
	return 0;
}

non-member functions的虚化十分容易：写一个虚函数做实际工作，再写一个什么都不做的非虚函数，只负责调用虚拟函数。为了避免此巧妙安排蒙受函数调用所带来的成本，可以将非虚函数inline化。