Template Specifications for constructor of n-dimensional array.

Question

Hello.

I am implementing an n-dimensional array class which is a template as follows (Note that the data is stored in a linear array whose length is the product of all the dimensions):

template< class valType, int rank >
class NDimensionalArray
{
public:

private:
    valType* m_data;
    int* m_dimensions;
    int m_rank;
};

So the idea is that a user (me) can specify an array of rank 2, and of a certain dimension, ie:

NDimensionalArray<double,2> matrix(10,10);

Now the difficulty is in specializing constructors for 1->n dimensions, each constructor takes n parameters where n is the rank of the array. Now I thought of using a valarray like is used in printf(), however with this defining a 1-dimensional array with 2 dimensions ie:

NDimensionalArray<double,1> matrix(10,10);

would be perfectly acceptable behavior. Is there some neat trick I can use to let the compiler do the repetition? Realistically so long as I know the rank, and have the length of each dimension the constructor can be generic:

{
    int nElements = m_dimensions[0];
    for ( int i=1 ; i<m_rank ; ++i )
        nElements *= m_dimensions[i];
    m_data = new valType[nElements];
}

Edit: Note that a similar operation will be needed for accessors.

Also I have considered the option of a constructor which looks like:

NDimensionalArray( const NDimensionalArray<int,1>& dimensions );

Which could be used like:

NDimensionalArray<int,1> dimVec(2); // Need a specification for 1-dimensional arrays.
dimVec(0) = 10;
dimVec(1) = 10;
NDimensionalArray<double,2> matrix(dimVec);

This would be a viable solution, but its ugly compared to the use I would like. Also accessing multi-dimensional arrays would become a serious pain, and seriously slow having to construct a dimension vector for each access.

Answer 1

I think the best solution is to take a vector of ints and let the constructor validate it against the template parameter 'rank'.

NDimensionalArray matrix(std::vector<int> matrixDimensions) 
{
    if (matrixDimensions.size() != rank) 
    {
        throw SomeException();
    }

    ...
}

I don't think any compiler trick can be an alternative here. (Except perhps using macros, if you can think of something, although that wouldn't be a compiler trick strictly speaking.)

Answer 2

Not a direct answer, but check out the blitz library.

Answer 3

You could take a std::tr1::array. Hmm:

#include <array>

template< class valType, int rank >
class NDimensionalArray
{
public:
   NDimensionalArray(const std::tr1::array<rank>& dims);
   // ...
};

NDimensionalArray<double,2> foo({10,10});

NDimensionalArray<double,2> bar({10}); // second dimension would be 0

NDimensionalArray<double,1> baz({10,10}); // compile error?

I'm not actually sure if that works! I'll run it through comeau.

Edited As per the comments, looks like this approach would look more like:

std::tr1::array<2> dims = {10, 10};
NDimensionalArray<double,2> foo(dims);

Answer 4

There's no good way to do it in C++ as currently standardized. In C++0x, you'll be able to use template parameter packs to approximate (I think I've got the syntax right, but not sure about expansion in requires):

template <class ValType, int Rank>
struct NDimensionalArray
{
    template <class... Args>
    requires std::SameType<Args, ValType>... && std::True<sizeof...(Args) == Rank>
    NDimensionalArray(Args... values)
    {
        ...
    }
};

Answer 5

Boost has a multi-array library that uses a custom object for constructing their multidimensional array. It's a really good way to do it; I suggest you study (or better yet, use) their code.

Answer 6

Okay, I've played with this for a while. Here's some template metaprogramming hackery that does something close to what you want. It lets you specify all dimensions inline, it doesn't do any dynamic memory allocation or other such things. In addition, with a good C++ compiler (I tested with VC++ /O2 option), the code will be fully inlined, with no copies done (in fact, for me it inlined the whole NDimensionalArray constructor at the point of the call). It will typecheck completely at compile-time, and won't let you pass too few or too many dimensions. And it can be reused for indexers. Here goes:

template<class T, int N>
class value_pack : private value_pack<T, N-1>
{
public:

 enum { size = N };

 value_pack(value_pack<T, N-1> head, const T& tail)
  : value_pack<T, N-1>(head)
  , value(tail)
 {
 }

 value_pack<T, N+1> operator() (const T& tail) const
 {
  return value_pack<T, N+1>(*this, tail);
 }

 template<int I>
 const T& get() const
 {
  return this->value_pack<T, I+1>::value;
 }

protected:

 const T value;
};

template<class T>
struct value_pack<T, 0>
{
};

struct
{
 template <class T>
 value_pack<T, 1> operator() (const T& tail) const
 {
  return value_pack<T, 1>(value_pack<T, 0>(), tail);
 }
} const values;


template <class ValType, int Rank>
struct NDimensionalArray
{
 NDimensionalArray(value_pack<ValType, Rank> values)
 {
  // ...
 }
};


int main()
{
 NDimensionalArray<int, 3> a(values(1)(2)(3));
}