partial specialization with dependent name (typename)

Question

I have the following simple strinToTypeImpl function which converts any kind of string into the template type. The problem I am concerned about is the fact that the compiler tells me for the partial specialization for typename MyMatrix<T>::Vector3 :

template parameter T not used in partial specialization

Can't I use dependent names in the specialization?

namespace details
{

    template<typename T>
    struct stringToTypeImpl{
        bool operator()(T& t, const std::string& s)
        {
          std::istringstream iss(s);
          return !(iss >> t).fail();
        }
    };


    template<typename T>
    struct stringToTypeImpl< typename MyMatrix<T>::Vector3  >{ 

        // Replacing typename MyMatrix<T>::Vector3  by 
        // Eigen::Matrix<T,3,1> WORKS but why?

        bool operator()(typename MyMatrix<PREC>::Vector3 & t, const std::string& s)
        {
          stringToVector3<PREC>(t,s);
        }
    };
}

Answer 1

This is just another form of the problem that has been discussed many times: There's no one-to-one mapping from types X to types T such that MyMatrix<T>::Vector3 == X.

Simple example:

MyMatrix<double> { typedef Vector3 int; };
MyMatrix<float> { typedef Vector3 int; };

stringToTypeImpl<int> // Help, what is "T"?

Answer 2

Kerreck's answer and my comment there explain the problem. There is no way for the type system to map a member to a parent-of-member, so the :: operator stops the deduction process which needs to match T.

The simple way to a solution would be to bring Vector3 outside and specialize it on the matrix type, then make the member MyMatrix< T >::Vector3 a typedef.

template< typename Matrix >
struct Vector3 {
    typename Matrix::ValueType x, y, z;
};

template< typename T >
struct MyMatrix {
    typedef Vector3< MyMatrix > Vector3;
};

The partial specialization of StringToTypeImpl needs to be on the final template type, not the typedef. Partial specialization cannot match across a set of typedefs, although specialization can match type to the type aliased by a single typedef name.