Question

In the C++0x Variadic Templates Proposal paper Link there is an example of a class which supports an arbitrary number of dimensions. I have copied it below:

template<typename T, unsigned PrimaryDimension, unsigned... Dimensions>
class array { /* implementation */ };

array<double, 3, 3> rotation matrix; // 3x3 rotation matrix

Sadly the implementation is not provided. As I am relatively new to variadic templates I would be interested to see an implementation of this container.

Thanks to anybody who can provide a simple implementation.

Answer 1

Here is a very simple implementation (compiled with gcc4.6.1) that demonstrates the recursion involved in getting the array type right – if there is some other specific implementation detail you are interested in, please let us know:

template<class T, unsigned ... RestD> struct array;

template<class T, unsigned PrimaryD > 
  struct array<T, PrimaryD>
{
  typedef T type[PrimaryD];
  type data;
  T& operator[](unsigned i) { return data[i]; }

};

template<class T, unsigned PrimaryD, unsigned ... RestD > 
   struct array<T, PrimaryD, RestD...>
{
  typedef typename array<T, RestD...>::type OneDimensionDownArrayT;
  typedef OneDimensionDownArrayT type[PrimaryD];
  type data;
  OneDimensionDownArrayT& operator[](unsigned i) { return data[i]; }
}; 

int main()
{
    array<int, 2, 3>::type a4 = { { 1, 2, 3}, { 1, 2, 3} };
    array<int, 2, 3> a5{ { { 1, 2, 3}, { 4, 5, 6} } };
    std::cout << a5[1][2] << std::endl;

    array<int, 3> a6{ {1, 2, 3} };
    std::cout << a6[1] << std::endl;

    array<int, 1, 2, 3> a7{ { { { 1, 2, 3}, { 4, 5, 6 } } }};
    std::cout << a7[0][1][2] << std::endl;
}