WTL for MFC Programmers(3)
ATL-style templates
Even if you can read C++ templates without getting a headache, there is two things ATL does that might trip you up at first. Take this class for example:
关于C++的模版定义，ATL有两个让人头疼的事情。注意以下的例子：
class CMyWnd : public CWindowImpl<CMyWnd>
{
...
};
That actually is legal, because the C++ spec says that immediately after the class CMyWnd part, the name CMyWnd is defined and can be used in the inheritance list. The reason for having the class name as a template parameter is so ATL can do the second tricky thing, compile-time virtual function calls.
上面的代码是合法，因为C++规定只要在class CMyWnd定义部分的后面，就可以使用CMyWnd，当然也可以将它用在基类表的定义中。这就是为什么类名可以像一个模板参数那样使用，也正是由于这样的特性，ATL就有了第二件狡猾的设计，编译时虚函数调用。
（如果在class CMyWnd的后面马上就可以使用CMyWnd，那么能不能让一个类作自己的子类呢？好像可以，其实是行不通的，因为编译器会去找基类的定义，如果自己做自己的基类，编译器会提示基类没有定义 – 蜗牛手记）
To see this in action, look at this set of classes:
注意看下面的类定义：
template <class T>
class B1
{
public:
void SayHi()
{
T* pT = static_cast<T*>(this); // HUH?? I'll explain this below
pT->PrintClassName();
}
protected:
void PrintClassName() { cout << "This is B1"; }
};
class D1 : public B1<D1>
{
// No overridden functions at all
};
class D2 : public B1<D2>
{
protected:
void PrintClassName() { cout << "This is D2"; }
};
main()
{
D1 d1;
D2 d2;
d1.SayHi(); // prints "This is B1"
d2.SayHi(); // prints "This is D2"
}
The static_cast<T*>(this) is the trick here. It casts this, which is of type B1*, to either D1* or D2* depending on which specialization is being invoked. Because template code is generated at compile-time, this cast is guaranteed to be safe, as long as the inheritance list is written correctly. (If you wrote
class D3 : public B1<D2>
you'd be in trouble.) It's safe because the this object can only be of type D1* or D2* (as appropriate), and nothing else. Notice that this is almost exactly like normal C++ polymorphism, except that the SayHi() method isn't virtual.
语句static_cast<T*>(this)是一个骗局。它通过这个特殊的调用将类型是B1*的this转换为D1*或者D2*。因为模板的代码是在编译时生成的，只要基类烈表示正确的，这种转换保证是安全的。（如果你写出这样的语句：class D3 : public B1<D2>，那你将于遇到麻烦）。这种安全是因为this对象只能根据定义转换为D1*或D2*，而不能转换为其他的什么东西。这就像标准C++的多态性一样的，只是SayHi()函数不是虚函数而已。
