Variadic Templates Multidimensional Array Container

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.

Faisal Vali · Accepted Answer · 2011-08-14 17:29:22Z

29

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;
}

edited Aug 14, 2011 at 17:29

answered Aug 14, 2011 at 17:21

Faisal Vali

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3 Comments

Ricky65 Over a year ago

Thank you for the reply, Faisal. That's just incredible. Very elegant. I asked mainly because I have a fixed-size multidimensional array library (Link here: code.google.com/p/fsma) which provides thin wrappers around built-in 2d and 3d arrays and is analogous to std::array. I was wondering how to implement a version which allows for an arbitrary number of dimensions using variadic templates after I read the Standard Committee Paper on them. I was unaware of the power of variadic templates until I read your reply!

CTMacUser Over a year ago

For the 2011-Aug-14 comment by @faisal-vali, I'm not sure what you're trying to construct. Wouldn't template <class T, int ...D> arrayX<T[D]...> translate to something like arrayX<long[2], long[3], long[5]> given long, 2, 3, 5 as the parameters. The variadic parameter has to expand to a comma-separated list, which nested array bounds don't qualify as.

Faisal Vali Over a year ago

@CTMacUser Yes of course you're right - my comment is non-sense and should be stricken.

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