打字员访问模式示例

我喜欢打字员。在此url://drdobbs.com/184403813有一个很好的例子，说明如何使用typelist来创建访问者模式。

考虑以下代码:

void SomeOperation(DocumentItem* p)
{
    if (TextArea* pTextArea = dynamic_cast<TextArea*>(p))
    {
        ... operate on a TextArea object ...
    }
    else if (VectorGraphics* pVectorGraphics =
        dynamic_cast<VectorGraphics*>(p))
    {
        ... operate on a VectorGraphics object ...
    }
    else if (Bitmap* pBitmap = dynamic_cast<Bitmap*>(p))
    {
        ... operate on a Bitmap object ...
    }
    else
    {
        throw "Unknown type passed";
    }
}

根据Alexandrescu的说法，这个代码的不便之处是:

除了非常丑陋之外，上面的代码还有一个概念性问题，即无法在编译时捕获"忘记处理此类型"

未来的打字员也是如此:

    #include<iostream>
class null_typelist {};
template <class H, class T>
struct typelist
{
    typedef H head;
    typedef T tail;
};
template<class T1, class T2=null_typelist, class T3=null_typelist, class T4=null_typelist> struct cons;
template <class T1>
struct cons<T1, null_typelist, null_typelist,null_typelist>
{
    typedef typelist<T1, null_typelist> type;
};
template <class T1, class T2>
struct cons<T1, T2, null_typelist, null_typelist>
{
    typedef typelist<T1, typelist<T2,null_typelist> > type;
};
template <class T1, class T2, class T3>
struct cons<T1, T2, T3, null_typelist>
{
    typedef typelist<T1, typelist<T2, typelist<T3,null_typelist> > > type;
};
template <class T1, class T2, class T3, class T4>
struct cons
{
    typedef typelist<T1, typelist<T2, typelist<T3,typelist<T4, null_typelist> > > > type;
};

template <class tlist> class AdHocVisitor;
template <class H, class T>
class AdHocVisitor< typelist<H, T> > : public AdHocVisitor<T>
{
public:
    virtual void Visit(H*) = 0;
    template <class SomeClass>
    void StartVisit(SomeClass* p)
    {
        if (H* pFound = dynamic_cast<H*>(p))
        {
            Visit(pFound);
        }
        else
        {
            AdHocVisitor<T>::StartVisit(p);
        }
    }
};
template <class H>
class AdHocVisitor< typelist<H, null_typelist> > 
{
public:
    virtual void Visit(H*) = 0;
    template <class SomeClass>
    void StartVisit(SomeClass* p)
    {
        if (H* pFound = dynamic_cast<H*>(p))
        {
            Visit(pFound);
        }
        else
        {
            throw "Unknown type passed";
        }
    }
};
struct DocElement{virtual ~DocElement(){};};
struct TextArea: DocElement{};
struct Bitmap: DocElement{};
struct VectorGraphics: DocElement{};
int main()
{
    typedef cons<TextArea,Bitmap,VectorGraphics>::type MyHierarchy;
    DocElement *p = new Bitmap;
    struct ConcreteVisitor : AdHocVisitor<MyHierarchy>
    {
        void Visit(TextArea* p){std::cout << "I'm a textarea" << "n";}
        void Visit(VectorGraphics* p){std::cout << "I'm a VectorGraphics" << "n";}
        void Visit(Bitmap* p){std::cout << "I'm a Bitmap" << "n";}
    };
    ConcreteVisitor visitor;
    visitor.StartVisit(p);
    delete p;
    std::cin.get();
}

1-仍然有dynamic_cast和虚函数。所以我看不出引进打字员有什么好处?

2-在这篇文章的最后Alexandrescu给出了一些建议，以改进这段代码，但我不太清楚如何实现这些，有人可以帮助我吗?

谢谢