使用 C++ 基本类型作为类模板特化

Question

I have a template class for D-dimensional 'geometrical' vectors:

template <size_t D>
class Vector;

I would like to use the float fundamental type as the 1-dimensional specialization of this template class.

Is it possible to define such a specialization?

Answer 1

You cannot use an existing type in place of a specialization, but you can use a typedef to do the switching:

template <std::size_t D>
class Vector_impl { };

template <std::size_t D>
using Vector = std::conditional_t<
    D == 1,
    float,
    Vector_impl<D>
>;

static_assert(std::is_same_v<Vector<1>, float>);
static_assert(std::is_same_v<Vector<2>, Vector_impl<2>>);

You could also use an intermediate type trait to perform more advanced selection logic ( à la template <std::size_t D> using Vector = typename Vector_selector<D>::type ).

Answer 2

You can use

template<>
class Vector<1>

To achieve the behavior you are looking for.

See: https://godbolt.org/z/T7ME65Wv1 for a trivial example.

If you are wanting to conditionally choose the underlying storage type, you can do this

template <size_t D>
class Vector
{
    using type = std::conditional_t<D == 1, float, T>; //T is your choice.
public:
    type data[D];
};

See:
https://godbolt.org/z/fMKMrh7Ex