模板继承具现化

今天在学习effective c&＃43;&＃43;中的第49个条款时&＃xff0c;遇到一个模板继承的方法&＃xff0c;让我打开眼界&＃xff0c;感慨万千啊&＃xff01;

具体的代码如下&＃xff1a;

class NewHandlerHolder<br>{<br>public:<br>explicit NewHandlerHolder(new_handler nh)<br>: handler(nh) {}<br>~NewHandlerHolder()<br>{<br>set_new_handler(handler);<br>}<br>private:<br>new_handler handler;<br>};<br>template<typename T><br>class NewHandlerSurpport<br>{<br>public:<br>static new_handler set_new_handler(new_handler) throw();<br>static void* operator new (size_t size) throw(bad_alloc);<br>private:<br>static new_handler currentHandler;<br>};<br>template<typename T><br>new_handler NewHandlerSurpport<T>::set_new_handler(new_handler p) throw()<br>{<br>new_handler oldHander &＃61; currentHandler;<br>currentHandler &＃61; p;<br>return oldHander;<br>}<br>template<typename T><br>void* NewHandlerSurpport<T>::operator new (size_t size) throw(bad_alloc)<br>{<br>NewHandlerHolder h(std::set_new_handler(currentHandler));<br>return ::operator new(size);<br>}<br>template<typename T><br>new_handler NewHandlerSurpport<T>::currentHandler &＃61; 0;<br>class Test : public NewHandlerSurpport<Test><br>{<br>public:<br>int val;<br>};

怎么样&＃xff1f;看了上面的代码&＃xff0c;大家是否有些迷惑&＃xff1f;首先&＃xff0c;类NewHandlerSurpport采用的模板类的定义&＃xff0c;其内部却没用到类型T&＃xff1b;其次&＃xff0c;它的子类在继承时&＃xff0c;用了class Test : public NewHandlerSurpport的方式&＃xff0c;将自身作为模板类型参数。怪哉怪哉&＃xff01;一般情况下&＃xff0c;大家会不采用模板类来定义NewHandlerSurpport&＃xff0c;而且子类直接继承即可&＃xff0c;为什么要这样大动干戈&＃xff0c;而又好像多此一举呢&＃xff1f;

原来&＃xff0c;这么做也是有深意的&＃xff01;因为父类中定义的是静态函数和静态变量&＃xff0c;这样按照逻辑&＃xff0c;每个继承它的子类都应该独享一份静态数据&＃xff0c;也就是独享一份父类&＃xff0c;这又怎么实现呢&＃xff1f;哈哈&＃xff0c;正如上面的模式&＃xff0c;让基类是模板类&＃xff0c;子类继承是采用子类类型作为模板参数&＃xff0c;这样在代码编译时不同的继承类就将父类具现化成各自所属的父类&＃xff0c;也就是说各自有一份具现化后的父类的拷贝的代码块&＃xff0c;各自具有自己所属的静态成员数据和函数&＃xff0c;而不会共用一份&＃xff0c;有点绕口了&＃xff01;呵呵&＃xff01;相信聪明的呢已经理解了我的意思&＃xff01;这种奇特的方式叫做“怪异的循环模板模式”&＃xff0c;在《effective c&＃43;&＃43;》中叫做“do it for me ”&＃xff0c;not do it for others&＃xff01;这个名字更贴切些&＃xff01;

发现《effective c&＃43;&＃43;》的内容真是博大精深啊&＃xff0c;这也是我看的时间最长的一本书了&＃xff01;而且每次温习都有收获&＃xff01;