Mapping integers to types using C++ template fails in a specific case
- by Shailesh Kumar
I am attempting to compile the following template based code in VC++ 2005.
#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;
}
There is a particular line in the code which doesn't compile:
typedef T3::xType xType;
If I remove this line, compilation goes fine and the result is:
char
long
float
If I retain this line, compilation errors are observed.
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
I am not able to figure out how to make sure that T::xType can be treated as a type inside the T2 template. Any help is highly appreciated.
