【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 constructorstring 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 constructorif (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 constructorvirtual 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 constructorreturn 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 constructorreturn 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。