MoreEffectiveC++Item35 条款25 将constructor和non-member functions虚化

class NLComponent { //用于 newsletter components
public: // 的抽象基类
... //包含至少一个纯虚函数
};
class TextBlock: public NLComponent {
public:
... // 不包含纯虚函数
};
class Graphic: public NLComponent {
public:
... // 不包含纯虚函数
};
class NewsLetter { // 一个 newsletter 对象
public: // 由 NLComponent 对象
... // 的链表组成
private:
list<NLComponent*> components;
};

class NewsLetter 
{
public:
　　NewsLetter(istream& str);
　　...
};
//此构造函数的伪代码是这样的：
NewsLetter::NewsLetter(istream& str)
{
　　while (str) 
　　{
　　　　//从 str 读取下一个 component 对象;
　　　　//把对象加入到 newsletter 的 components 对象的链表中去;
　　}
}

class NewsLetter {
public:
    NewsLetter(istream& str);
    ...
private:
    //从str读取下一个NLCompnent的数据,产生组件(compnent),并返回一个指针指向它
    static NLCompnent* readCompnent(istream& str);
    list<NLCompnent*> compnents;
    ...
};
NewsLetter::NewsLetter(istream& str){
    while (str) {
       //将readCompnent返回的指针加到compnents list尾端
       compnents.push_back(readCompnent(str));
    }
}

class NLComponent {
public:
    // 声明virtual copy constructor
    virtual NLComponent * clone() const = 0;
    ...
};
class TextBlock: public NLComponent {
public:
    virtual TextBlock * clone() const // virtual copy constructor
    { return new TextBlock(*this); }
    ...
};
class Graphic: public NLComponent {
public:
    virtual Graphic * clone() const // virtual copy constructor
    { return new Graphic(*this); } 
    ...
};

class NewsLetter {
public:
    NewsLetter(const NewsLetter& rhs);
    ...
private:
    list<NLComponent*> components;
};
NewsLetter::NewsLetter(const NewsLetter& rhs){
    //遍历rhs的list,运用每个元素的virtual copy constructor将元素复制到此对象的compnents list中.
    for (list<NLComponent*>::const_iterator it =rhs.components.begin();it != rhs.components.end();++it)
        //it指向rhs.compnents的目前元素,调用该元素的clone函数取得一个副本并加到本对象的compnents list的尾端
        components.push_back((*it)->clone());
}

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