映射整數類型在特定情況下

我試圖編譯下面基於模板的代碼在VC失敗++ 2005映射整數類型在特定情況下

#include <iostream> 
using namespace std; 


/* 
* T is a template which maps an integer to a specific type. 
* The mapping happens through partial template specialization. 
* In the following T<1> is mapped to char, T<2> is mapped to long 
* and T<3> is mapped to float using partial template specializations 
*/ 
template <int x> 
struct T 
{ 
public: 
}; 

template<> 
struct T<1> 
{ 
public: 
    typedef char xType; 
}; 


template<> 
struct T<2> 
{ 
public: 
    typedef long xType; 
}; 

template<> 
struct T<3> 
{ 
public: 
    typedef float xType; 
}; 

// We can easily access the specific xType for a specific T<N> 
typedef T<3>::xType x3Type; 

/*! 
* In the following we are attempting to use T<N> inside another 
* template class T2<R> 
*/ 

template<int r> 
struct T2 
{ 
    //We can map T<r> to some other type T3 
    typedef T<r> T3; 
    // The following line fails 
    typedef T3::xType xType; 
}; 

int main() 
{ 
    T<1>::xType a1; 
    cout << typeid(a1).name() << endl; 
    T<2>::xType a2; 
    cout << typeid(a2).name() << endl; 
    T<3>::xType a3; 
    cout << typeid(a3).name() << endl; 
    return 0; 
}

存在其中不編譯的代碼的特定行：

typedef T3::xType xType;

如果我刪除此行，編譯去細，其結果是：

char 
long 
float

如果我保留這一行，則會發現編譯錯誤。

main.cpp(53) : warning C4346: 'T<x>::xType' : dependent name is not a type 
    prefix with 'typename' to indicate a type 
    main.cpp(54) : see reference to class template instantiation 'T2<r>' being compiled 
main.cpp(53) : error C2146: syntax error : missing ';' before identifier 'xType' 
main.cpp(53) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

我無法弄清楚如何確保T :: xType可以被視爲T2模板內的類型。任何幫助，高度讚賞。

來源

2010-06-14 Shailesh Kumar