less verbose way to declare multidimensional std::array

Question

Short question: Is there a shorter way to do this

array,n>,m> matrix;

I was hoping for something like

Answer 1

Here's a simple, generic version:

template <typename T, size_t d1, size_t d2, size_t... ds>
struct GetMultiDimArray
{
    using type = std::array<typename GetMultiDimArray<T, d2, ds...>::type, d1>;
};

template <typename T, size_t d1, size_t d2>
struct GetMultiDimArray<T, d1, d2>
{
    using type = std::array<std::array<T, d2>, d1>;
};

template <typename T, size_t d1, size_t d2, size_t... ds>
using MultiDimArray = typename GetMultiDimArray<T, d1, d2, ds...>::type;

// Usage:
MultiDimArray<int, 3, 2> arr {1, 2, 3, 4, 5, 6};
assert(arr[1][1] == 4);

Answer 2

A palatable workaround for compilers that don't support template aliases yet is to use a simple metafunction to generate the type:

#include <cstddef>
#include <array>

template<class T, std::size_t RowsN, std::size_t ColumnsN>
struct Matrix
{
    typedef std::array<std::array<T, ColumnsN>, RowsN> type; // row major

private:
    Matrix(); // prevent accidental construction of the metafunction itself
};

int main()
{
    Matrix<int, 3, 4>::type matrix;
}

Answer 3

Solution using variadic templates (slightly more complex than the template alias, but more general purpose)

template <typename T, std::size_t thisSize, std::size_t ... otherSizes>
class multi_array : private std::array<multi_array<T, otherSizes...>, thisSize>
{
 using base_array = std::array<multi_array<T, otherSizes...>, thisSize>;

 public:
    using base_array::operator[];
    // TODO: add more using statements to make methods
    // visible.  This is less typing (and less error-prone)
    // than forwarding to the base_array type.
};

template <typename T, std::size_t thisSize>
class multi_array<T, thisSize> : private std::array<T, thisSize>
{
 using base_array = std::array<T, thisSize>;

 public:
    using base_array::operator[];
    // TODO: add more using statements to make methods
    // visible.  This is less typing (and less error-prone)
    // than forwarding to the base_array type.
};

There might be some improvement on assigning to non-leaves of the array that could be made.

I tested with a relatively recent build of clang/LLVM.

Enjoy!

Answer 4

When nested, std::array can become very hard to read and unnecessarily verbose. The opposite ordering of the dimensions can be especially confusing.

For example:

std::array < std::array <int, 3 > , 5 > arr1; 

compared to

char c_arr [5][3]; 

Also, note that begin(), end() and size() all return meaningless values when you nest std::array.

For these reasons I've created my own fixed size multidimensional array containers, array_2d and array_3d. They have the advantage that they work with C++98.

They are analogous to std::array but for multidimensional arrays of 2 and 3 dimensions. They are safer and have no worse performance than built-in multidimensional arrays. I didn't include a container for multidimensional arrays with dimensions greater than 3 as they are uncommon. In C++11 a variadic template version could be made which supports an arbitrary number of dimensions (Something like Michael Price's example).

An example of the two-dimensional variant:

//Create an array 3 x 5 (Notice the extra pair of braces) 
fsma::array_2d <double, 3, 5> my2darr = {{ 
{ 32.19, 47.29, 31.99, 19.11, 11.19}, 
{ 11.29, 22.49, 33.47, 17.29, 5.01 }, 
{ 41.97, 22.09, 9.76, 22.55, 6.22 } 
}};  

Full documentation is available here: http://fsma.googlecode.com/files/fsma.html

You can download the library here: http://fsma.googlecode.com/files/fsma.zip

Answer 5

A template alias might help out:

#include <array>

template <class T, unsigned I, unsigned J>
using Matrix = std::array<std::array<T, J>, I>;

int main()
{
    Matrix<int, 3, 4> matrix;
}