less verbose way to declare multidimensional std::array - c++

Question

Short question:
Is there a shorter way to do this
array<array<atomic<int>,n>,m> matrix;
I was hoping for something like
array< atomic< int>,n,m> matrix;
but it doesnt work...

Answer 1

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;
}

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

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);