我對類型列表感興趣。 在這個URL http://drdobbs.com/184403813有一個如何使用類型列表創建訪問者模式的很好的例子。類型列表訪客模式示例
我對這個例子有兩個問題。我的兩個問題在本主題的末尾。
代碼如下考慮:
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";
}
}
此代碼的incovenients根據Alexandrescu的是:
除了是徹底難看,上面的代碼具有不能概念問題趕在編譯時「忘記處理這種類型的」
所以來了類型串:
#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-我們還有dynamic_cast和一個虛函數。所以我不太明白引入類型列表的好處嗎?
2 - 在本文Alexandrescu的給予一定的結束建議,以改善這個代碼,但我沒有看得很清楚如何實現這些,可能有人能幫助我在這?
謝謝
你可以看看洛基庫,你會發現類型串的完整implentation。另外這本書可以幫助:http://www.amazon.co.uk/Modern-Design-Applied-Generic-Patterns/dp/0201704315 – 2011-04-07 09:26:40