Passing variadic parameters in an already template-variadic function

Question

The title is bad but I couldn't come up with anything better. Feel free to change it.

Here's a template multidimensional array class that I'm currently working on. I'm trying to optimise it as much as I can:

#include <array>

template <typename T, std::size_t... Dimensions>
class multidimensional_array
{
    public:

        using value_type = T;
        using size_type = std::size_t;

    private:

        template<typename = void>
        static constexpr size_type multiply(void)
        {
            return 1u;
        }

        template<std::size_t First, std::size_t... Other>
        static constexpr size_type multiply(void)
        {
            return First * multidimensional_array::multiply<Other...>();
        }

    public:

        using container_type = std::array<value_type, multidimensional_array::multiply<Dimensions...>()>;
        using reference = value_type &;
        using const_reference = value_type const&;
        using iterator = typename container_type::iterator;

    private:

        container_type m_data_array;

        template<typename = void>
        static constexpr size_type linearise(void)
        {
            return 0u;
        }

        template<std::size_t First, std::size_t... Other>
        static constexpr size_type linearise(std::size_t index, std::size_t indexes...)
        {
            return multidimensional_array::multiply<Other...>()*index + multidimensional_array::linearise<Other...>(indexes);
        }

    public:

        // Constructor
        explicit multidimensional_array(const_reference value = value_type {})
        {
            multidimensional_array::fill(value);
        }

        // Accessors
        reference operator()(std::size_t indexes...)
        {
            return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
        }

        const_reference operator()(std::size_t indexes...) const
        {
            return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
        }

        // Iterators
        iterator begin()
        {
            return m_data_array.begin();
        }

        iterator end()
        {
            return m_data_array.end();
        }

        // Other
        void fill(const_reference value)
        {
            m_data_array.fill(value);
        }
};

My main function is

int main(void)
{
    multidimensional_array<int, 2u, 3u, 4u, 5u, 6u> foo;
    int k = 0;

    for (auto& s : foo)
        s = k++;

    //std::cout << foo(0u, 0u, 0u, 1u, 0u) << std::endl;
    return 0;
}

The above code compilers without warning/error. As soon as I uncomment the std::cout part though, I get this:

g++-7 -std=c++17 -o foo.o -c foo.cpp -Wall -Wextra -pedantic
foo.cpp: In instantiation of ‘multidimensional_array<T, Dimensions>::value_type& multidimensional_array<T, Dimensions>::operator()(std::size_t, ...) [with T = int; long unsigned int ...Dimensions = {2, 3, 4, 5, 6}; multidimensional_array<T, Dimensions>::reference = int&; multidimensional_array<T, Dimensions>::value_type = int; std::size_t = long unsigned int]’:
foo.cpp:99:37:   required from here
foo.cpp:60:72: error: no matching function for call to ‘multidimensional_array<int, 2, 3, 4, 5, 6>::linearise<2, 3, 4, 5, 6>(std::size_t&)’
    return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
foo.cpp:38:30: note: candidate: template<class> static constexpr multidimensional_array<T, Dimensions>::size_type multidimensional_array<T, Dimensions>::linearise() [with <template-parameter-2-1> = <template-parameter-1-1>; T = int; long unsigned int ...Dimensions = {2, 3, 4, 5, 6}]
   static constexpr size_type linearise(void)
                              ^~~~~~~~~
foo.cpp:38:30: note:   template argument deduction/substitution failed:
foo.cpp:60:72: error: wrong number of template arguments (5, should be at least 0)
    return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
foo.cpp:44:30: note: candidate: template<long unsigned int First, long unsigned int ...Other> static constexpr multidimensional_array<T, Dimensions>::size_type multidimensional_array<T, Dimensions>::linearise(std::size_t, std::size_t, ...) [with long unsigned int First = First; long unsigned int ...Other = {Other ...}; T = int; long unsigned int ...Dimensions = {2, 3, 4, 5, 6}]
   static constexpr size_type linearise(std::size_t index, std::size_t indexes...)
                              ^~~~~~~~~
foo.cpp:44:30: note:   template argument deduction/substitution failed:
foo.cpp:60:72: note:   candidate expects 2 arguments, 1 provided
    return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes)];
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
Makefile:17: recipe for target 'foo.o' failed
make: *** [foo.o] Error 1

And I know why now. My question is, how can I fix linearise so that it can pass indexes without going through va_list and such? Unfortunately linearise is already a template, variadic function, so I can't use variadic template shenanigans in that regard.

Answer 1

As in preceding question the problem is that the following signatures

template<std::size_t First, std::size_t... Other>
static constexpr size_type linearise(std::size_t index,
                                     std::size_t indexes...)

reference operator()(std::size_t indexes...)

const_reference operator()(std::size_t indexes...) const

aren't what do you mean (indexes a variadic list of std::size_t) but are exactly equivalent to

template<std::size_t First, std::size_t... Other>
static constexpr size_type linearise(std::size_t index,
                                     std::size_t indexes,
                                     ...)

reference operator()(std::size_t indexes, ...)

const_reference operator()(std::size_t indexes, ...) const

where indexes is a single std::size_t followed by a C-style optional sequence of argument.

A simple solution (you tagged C++17 but is available starting from C++11) is based on the use of variadic templates.

By example, as follows

template <std::size_t First, std::size_t ... Other, typename ... Ts>
static constexpr size_type linearise (std::size_t index,
                                      Ts ... indexes)
 { return multidimensional_array::multiply<Other...>() * index
        + multidimensional_array::linearise<Other...>(indexes...); }

  // Accessors
  template <typename ... Ts>
  reference operator() (Ts ... indexes)
   { return m_data_array[
        multidimensional_array::linearise<Dimensions...>(indexes...)]; }

  template <typename ... Ts>
  const_reference operator() (Ts ... indexes) const
   { return m_data_array[
        multidimensional_array::linearise<Dimensions...>(indexes...)]; }

The following is you're code, modified and compilable

#include <array>
#include <iostream>

template <typename T, std::size_t ... Dimensions>
class multidimensional_array
 {
   public:
      using value_type = T;
      using size_type  = std::size_t;

   private:
      template <typename = void>
      static constexpr size_type multiply ()
       { return 1u; }

      template <std::size_t First, std::size_t ... Other>
      static constexpr size_type multiply(void)
       { return First * multidimensional_array::multiply<Other...>(); }

   public:
      using container_type  = std::array<value_type,
               multidimensional_array::multiply<Dimensions...>()>;
      using reference       = value_type &;
      using const_reference = value_type const &;
      using iterator        = typename container_type::iterator;

   private:
      container_type m_data_array;

      template <typename = void>
      static constexpr size_type linearise ()
       { return 0u; }

      template <std::size_t First, std::size_t ... Other, typename ... Ts>
      static constexpr size_type linearise (std::size_t index,
                                            Ts ... indexes)
       { return multidimensional_array::multiply<Other...>() * index
              + multidimensional_array::linearise<Other...>(indexes...); }

   public:
      // Constructor
      explicit multidimensional_array (const_reference value = value_type{})
       { multidimensional_array::fill(value); }

      // Accessors
      template <typename ... Ts>
      reference operator() (Ts ... indexes)
       { return m_data_array[
            multidimensional_array::linearise<Dimensions...>(indexes...)]; }

      template <typename ... Ts>
      const_reference operator() (Ts ... indexes) const
       { return m_data_array[
            multidimensional_array::linearise<Dimensions...>(indexes...)]; }

      // Iterators
      iterator begin ()
       { return m_data_array.begin(); }

      iterator end ()
       { return m_data_array.end(); }

      // Other
      void fill (const_reference value)
       { m_data_array.fill(value); }
 };

int main ()
 {
   multidimensional_array<int, 2u, 3u, 4u, 5u, 6u> foo;

   int k{ 0 };

   for ( auto & s : foo )
      s = k++;

   std::cout << foo(0u, 0u, 0u, 1u, 0u) << std::endl;
 }

Bonus suggestion.

You tagged C++17 so you can use "folding".

So you can substitute the couple of multiply() template functions

  template <typename = void>
  static constexpr size_type multiply ()
   { return 1u; }

  template <std::size_t First, std::size_t ... Other>
  static constexpr size_type multiply ()
   { return First * multidimensional_array::multiply<Other...>(); }

with a single folded one

  template <std::size_t ... Sizes>
  static constexpr size_type multiply ()
   { return ( 1U * ... * Sizes ); } 

Answer 2

My approach is similar to that in this answer, except that instead of using std::tuple to store a list of types, I define my own type size_t_pack to store a (compile-time) list of size_t's.

using std::size_t;

template<size_t... values>
struct size_t_pack{};

template<size_t first_value,size_t... rest_values>
struct size_t_pack<first_value,rest_values...>{
    static constexpr size_t first=first_value;
    using rest=size_t_pack<rest_values...>;
    static constexpr size_t product=first*rest::product;
};

template<>struct size_t_pack<>{
    static constexpr size_t product=1;
};

Defines members: first, rest (in case not empty) and product (since it's not possible to specialize a function using the templates of a template argument, as far as I know, another choice is to if constexpr and make the type support checking for empty)

With that, it's easy to define the linearize function:

template<class dimensions,class... SizeTs>
static constexpr size_type linearise(std::size_t index, SizeTs... indices)
{
    using restDimensions=typename dimensions::rest;
    return restDimensions::product *index + 
    multidimensional_array::linearise<restDimensions>(indices...);
}

---

Using a std::tuple to store the list of types (SizeTs) is also possible, although struct partial specialization is still required, as far as I know.

Answer 3

You need to make indexes a parameter pack by making the operator() function a template, and expand the parameter pack when you use it by putting ... afterwards:

    template <class... DimensionType>
    const_reference operator()(DimensionType... indexes) const
    {
        return m_data_array[multidimensional_array::linearise<Dimensions...>(indexes...)];
    }

See: parameter pack expansion

The code still will not compile because of a similar problem in linearize(), but that gets you on the right track.