模板成员函数专业化

Question

I have this:

//forward declaration
template<typename Elem, int D1 = 1, int D2 = 1, int D3 = 1>
class matrix;

template<typename Elem, int D1, int D2, int D3>
struct matrix_deref_type_trait
{
    typedef matrix<Elem, D2 == 1 ? 1 : D1, D3 == 1 ? 1 : D2, 1> matrix_deref;
};

template<typename Elem, int D1>
struct matrix_deref_type_trait<Elem, D1, 1, 1>
{
    typedef Elem matrix_deref;
};

template<typename Elem, int D1, int D2, int D3>
class matrix:public object
{
public:
    typedef typename matrix_deref_type_trait<Elem, D1, D2, D3>::matrix_deref matrix_deref;

    inline matrix_deref operator[](int J)
    {
        ...
    }
}

And want to specialize a case for the operator[] (outside the class body):

template<typename Elem, int D1>
typename matrix<Elem, D1, 1, 1>::matrix_deref matrix<Elem, D1, 1, 1>::operator[](int J)
{
    return M_ptr[J];
}

But I'm getting this error:

error C2244: 'matrix<Elem,D1,D2,D3>::operator []' : unable to match function definition to an existing declaration

Can I override this member function without full specialization of the whole class? What should I do? Thanks.

Answer 1

You cannot specialize a non-template method of a template class. You can only specialize a template class itself. Or you can specialize template method of any (template or non-template class).

Specializing non-template method of a template class makes no sense: compiler needs to know how the whole class looks. And you're just telling: hey, I don't care about the class, but what I know is how this one method will look like.

Answer 2

just specialize you matrix class :

template<typename Elem, int D1>
class matrix<Elem,D1,1,1>
{
public:
    typedef typename matrix_deref_type_trait<Elem, D1, 1, 1>::matrix_deref matrix_deref;//this line 

    inline matrix_deref operator[](int J)
    {
        std::cout << "special\n";
    }
};

also you dont need to do the ?: check in trait class, because if D2 and D3 are 1 it will always choose the trait specialization.