WebKit的智能指针分析 - Part 1：RefCounted
在WebKit的官网上有一篇使用指南：RefPtr and PassRefPtr Basics，从应用的角度讲解了RefPtr类族的由来，及使用方法。但很无厘头的是，这里面居然没有提到如何创建一个有引用计数功能的类！？（教你如何用砖头造房子却不教你如何造砖头）
在抱怨作者的粗心、懒惰、和恶搞精神后，我只能用一句俗话来安慰自己：还好有源代码，还好有源代码，⋯⋯WebKit我来了！！！
WebKit是个非常优秀的开源项目，应用范围广就不说了（safari、chrome、iphone、android、⋯⋯），代码质量也是高得一塌糊涂：清晰的模块划分、清晰的代码组织架构、晦涩（巧妙的同义词）的C++小技巧等优点完全值得有心提高内功的同志一读。
下面就是我在学习WebKit源代码里的RefPtr智能指针的一些心得。
Overview
WebKit的智能指针由类族 RefPtr 来实现，其核心由三个类组成：
- RefCounted
- RefPtr
- PassRefPtr
其中RefCounted提供了引用计数器(一个int型成员），而RefPtr和PassRefPtr则提供了自动管理引用计数器的功能。根据RefPtr and PassRefPtr Basics的说法，最初并没有RefPtr和PassRefPtr，这两个类是2005年才加入的，在它们出现之前完全是靠找死的人工管理RefCounted的引用计数。RefCounted类本身是没有问题的，但它的使用方法相当繁琐，繁琐到开发人员发现很多内存泄露都是由于对方法ref()和deref()的调用不当而造成一半以上内存泄露的程度。为了简化RefCounted的使用方法，RefPtr诞生了，而为了更高效地传递参数，开发人员又创造了PassRefPtr。
RefCounted
RefCounted的源代码在这里：RefCounted.h。
这个文件里定义了两个类：非模板类RefCountedBase和模板类RefCounted，从名字上就能看出来RefCounted继承于RefCountedBase。像维护引用计数器这么简单的活儿只使用一个类就绰绰有余了，这里之所以做成两个类是为了减少template hoisting（实例化模板导致的代码膨胀）。总而言之，最终的RefCounted完成了这样的功能：
- 定义了成员变量：int m_refCount
-
定义函数ref
// void ref() { ++m_refCount; } -
定义函数deref
// void deref() { if (derefBase()) delete static_cast<T*>(this); }derefBase的定义是：
// // Returns whether the pointer should be freed or not. bool derefBase() { if (m_refCount == 1) { return true; } --m_refCount; return false; }
上面的ref()和deref()就是RefCounted的核心功能了，不过有一点要注意的是RefCounted的析构函数是protected的，这样就不能直接定义RefCounted对象了，而是必须从RefCounted继承子类才能行。但是这里不同于一般的继承，这里玩了个小trick：
class Frame : public RefCounted<Frame> {
// ...
}
和一般的继承不一样吧，之所以要写成这样的原因在于函数deref()能够删除正确的对象！在函数deref()里，如果计数器到了1，就必须把自己销毁（即delete this），但此时的RefCounted直接来一句delete this是不对的，因为如果有一个类Foo继承于RefCounted，那在RefCounted的成员函数里调用delete this只会触发RefCounted的析构函数，而不会触发Foo的析构函数。为了让RefCounted在delete this时能确定子类的类型，类Foo就得从RefCounted<Foo>继承，这样才能为在RefCounted中delete this提供便利：
delete static_cast<T*>(this);
在编译时T的类型会被确定为Foo，因此上面这句代码会触发Foo的析构函数，另外由于Foo继承于RefCounted，RefCounted的析构函数随后会调用（虽然啥也没干）。
--EOF--
2011年1月07日 21:47
写的简明易懂，谢谢
2011年3月13日 02:26
但此时的RefCounted直接来一句delete this是不对的，因为如果有一个类Foo继承于RefCounted，那在RefCounted的成员函数里调用delete this只会触发RefCounted的析构函数，而不会触发Foo的析构函数。
对于上面这句话，有些疑问：如果把RefCounted的析构函数定义virtual的话，delete this是能够正常析构的；这里可能为了避免RefCounted使用虚函数表，故不把RefCounted的析构函数定义为virtual，转而使用模板的形式，从而使用了语句delete static_cast<T*>(this)。
2011年5月12日 13:40
@Lincoln: 说的对，我也认为如此。
很好的一篇文章。一起学习webkit吧！
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