How to extract the argument list in variadic templates for n-dimensional array

Question

I have a template class with the following specification:

template <typename T, size_t... Dims> class Array;

And say it can be used as follows:

// Define a 2X3X4 array of integers.  Elements are uninitialized.
Array<int, 2, 3, 4> a, b;
Array<short, 2, 3, 4> c;
Array<int, 0> e1; // This line must cause a compile-time error.

How can I achieve this functionality? I thought if I could extract all the argument list I could then create the n-dimentional array as a straight forward recursive call. How can I do it now

David G · Accepted Answer · 2013-05-15T02:44:37.103

You can create a compile-time trait that does what you want:

#include <type_traits>

template <std::size_t... Ts>
struct not_zero {};

template <std::size_t N>
struct not_zero<N> : std::integral_constant<bool, N> {};

template <std::size_t N, std::size_t... Ts>
struct not_zero<N, Ts...> : std::integral_constant<bool, N && not_zero<Ts...>::value> {};

template <typename T, std::size_t... Ts>
struct Array
{
    static_assert(not_zero<Ts...>::value, "Dimension cannot be 0");
};

template struct Array<int, 3>; // OK
template struct Array<int, 3, 2, 1, 0>; // error: static assertion failed: Dimension cannot be 0

See a demo here.

stardust · Answer 2 · 2013-05-12T20:46:08.923

Something like this maybe

namespace mine {

    template<typename T, size_t first, size_t... rest>
    struct multi_array__ {
        enum { dims = sizeof...(rest) };
        static_assert(first,"dimension can not be zero!"); 
        typedef std::array< typename multi_array__<T, rest...>::type, first > type;
    };


    template<typename T, size_t first>
    struct multi_array__<T,first> {
        enum { dims = 1 };
        static_assert(first,"dimension can not be zero!"); 
        typedef std::array<T,first> type;
    };

    template <typename T, std::size_t... ds> 
    using multi_array = typename multi_array__<T, ds ...>::type;

};

You can use it like this

mine::multi_array <int,2,3,4> arr1 = {};
// multi_array<int,2,3,0,4> arr3;  // This fails
mine::multi_array <int,3,4> arr2 = {};

Assignments like this work too

arr2[0] = arr1[0][0];

Here is a simple test program.

Augment this answer with `template using Array = typename multi_array::type;`, and then you don't need to use `::type`. — Joseph Mansfield, May 12 '13 at 18:31
@sftrabbit That is actually a very nice trick. Thank you for that. didn't think of it. — stardust, May 12 '13 at 18:34

score 2 · Answer 3 · answered May 12 '13 at 18:32

Create a specialization for it:

#include <iostream>

template <typename T, std::size_t... Dims>
struct Array {};

template <typename T>
struct Array<T, 0>; // leave as an incomplete type

int main()
{
    Array<int, 3> x; // OK
    Array<int, 0> y; // error: aggregate ‘Array<int, 0u> y’ has incomplete type and cannot be defined
}

How to extract the argument list in variadic templates for n-dimensional array

3 Answers3