c++ - Syntax for specialization of nested template class

Question

I'm trying to figure out the correct syntax for explicit specialization of a nested template class. The following code will better illustrate:

struct Column_Major;
struct Row_Major;

template<size_t rows, size_t cols, typename T, typename Allocator>
class Matrix
{

    /* bunch of members */
    template <typename storage = Column_Major>
    class Iterator
    {
        /* bunch of members */
    };
};

I'd like to write an explicit specialization for template <> class Matrix<...>::Iterator<Row_Major, but the syntax is eluding me. I have a suspicion that it is not possible to explicitly specialize the Iterator class without an explicit specialization of the containing class, Matrix. But I would be very happy if there is a way to do this.

I know I could make the Iterator class a separate class, not a member of the Matrix class, but having the classes nested as such allows me full access to the template parameters and datamebers of the Matrix class, which simplifies things. I know I could work around this if I need to, but I'd first like to investigate and understand the possibilities for the nested approach.

Thanks, Shmuel

Answer 1

For explicit specialization, you need to specialize the outer class before the inner, you can see this question for example.

There is a workaround that is using partial specialization:

template<size_t rows, size_t cols, typename T, typename Allocator>
class Matrix
{

    //                           Notice the additionnal dummy parameter
    //                                       vvvvvvvvvvvvv
    template <typename storage = Column_Major, bool = true>
    class Iterator
    {
    };

    // Specialization
    template <bool dummy>
    class Iterator<Row_Major, dummy>
    {
    };
};