Mismatched argument pack lengths while expanding std::index_sequence and variadic argument pack

Question

I've been dangling around with a small bare-bone multi-dimensional-tensor-array implementation; and have encountered an issue with std::make_index_sequence in combination with variadic template arguments. Give the following stripped implementation:

template <class scalar_t, std::size_t ... Dims>
class tensor {
public:
    using value_type = scalar_t;
    using size_type = std::size_t;
    using index_type = std::size_t;
    using container_type = std::array<value_type, (Dims * ...)>;
    using shape_type = std::array<size_type, sizeof...(Dims)>;
    using stride_type = std::array<index_type, sizeof...(Dims)>;

    constexpr static inline size_type size = (Dims * ...);
    constexpr static inline size_type rank = sizeof...(Dims);
    constexpr static inline shape_type shape = /* omitted for brevity */;
    constexpr static inline stride_type stride = /* omitted for brevity */;

    /* constructors omitted */

private:
    container_type m_data{};

I now wanted to have a variadic template on the call-operator operator()() to access elements within the private container:

template <class ... Indices, std::enable_if_t<sizeof...(Indices) == rank, int> = 0>
[[nodiscard]] constexpr value_type &operator()(Indices && ... index) noexcept {
    const index_type data_index = resolve_index(std::make_index_sequence<rank>(), std::forward<Indices>(index)...);
    return m_data[data_index];
}

The idea is fairly straight forward; generate a index_sequence for the rank of the tensor, then forward the request to a private helper resolve_index that resolves the index based on the stride-memory layout. Here is a working solution :

---

WORKING SOLUTION

template <class Indices>
[[nodiscard]] constexpr index_type resolve_index(const std::size_t axis, Indices && index) noexcept {
    return index * strides[axis];
}

template <std::size_t ... Axes, class ... Indices>
[[nodiscard]] constexpr index_type resolve_index(std::index_sequence<Axes...>, Indices && ... index) noexcept {
    return (resolve_index(Axes, std::forward<Indices>(index)) + ...);
}

resolve_index is overloaded. I am certain the compiler can expand the fold-expression at compile-time, however, each single invocation of resolve_index(std::size_t, Indices&&) executes at runtime (which is fine).

My pet peeve with this solution is that I normally prefer to use if constexpr (...) whenever possible to eliminate simple functions overloads as above; specifically if the return of the function doesn't have to be automatically deduced by auto or decltype(auto) .

---

Therefore I'd like to write something like the following:

template <class ... Axes, class ... Indices>
[[nodiscard]] constexpr index_type resolve_index(Axes && ... axis, Indices && ... index) noexcept {
    if constexpr (sizeof...(Indices) == 1)
        return (index + ...) * strides[(axis + ...)];
    else
        return (resolve_index(std::forward<Axes>(axis), std::forward<Indices>(index)) + ...);
}

Which unfortunately errors out at compile time with:

error: mismatched argument pack lengths while expanding ‘((tecra::tensor<scalar_t, Dims>*)this)->tecra::tensor<scalar_t, Dims>::resolve_index(forward<Axes>(axis), forward<Indices>(index))’
  108 |                 return (resolve_index(std::forward<Axes>(axis), std::forward<Indices>(index)) + ...);
      |                                                                                                    ^

Where did I go wrong? Here is a working godbolt example: https://godbolt.org/z/qsY51n8f7 (feel free to ignore the internal stuff). Thanks for anyone looking into this!

Answer 1

template <class ... Axes, class ... Indices>
[[nodiscard]] constexpr index_type
resolve_index(Axes&& ... axis, Indices&& ... index) noexcept;

has 2 issues:

• Axes&&... is non deducible (not last parameter).
• Axes&&... is either std::size_t or std::index_sequence<Is...> (so it should just be Axe&& ) but you cannot expand according to Is without an helper (function or lambda).

It would be something like:

template <class Axe, class ... Indices>
[[nodiscard]] constexpr index_type resolve_index(Axe axe, Indices && ... index) noexcept {
    if constexpr (std::is_same_v<Axe, std::size_t>) {
        static_assert(sizeof...(Indices) == 1);
        return (index + ...) * strides[axe];
    } else {
        return [&]<std::size_t...Is>(std::index_sequence<Is...>){
            static_assert(sizeof...(Indices) == sizeof...(Is));

            return (resolve_index(Is, std::forward<Indices>(index)) + ...);
            }(axe);
    }
}

Demo

I suggest to add the sequence directly in the class parameter:

template <class scalar_t, typename seq_dim, std::size_t ... Dims>
class tensor_impl;
    
template <class scalar_t, std::size_t... Is, std::size_t ... Dims>
class tensor_impl<scalar_t, std::index_sequence<Is...>, Dims...>
{
// You might directly use Is...
// simplifying your interface (you might get rid of some template)
// ...
};

template <class scalar_t, std::size_t ... Dims>
using tensor = tensor_impl<scalar_t, std::index_sequence_for<Dims...>, Dims...>;

Demo