Dynamically allocated multidimensional array using recursive templates - c++

Question

In order to read and store some results from a MATLAB program, I need to use up to 6 dimensional matrices. Instead of doing something like:
typedef std::vector<double> Row;
typedef std::vector<Row> Matrix2;
typedef std::vector<Matrix2> Matrix3;
typedef std::vector<Matrix3> Matrix4;
typedef std::vector<Matrix4> Matrix5;
typedef std::vector<Matrix5> Matrix6;
I decided to go with templates, and here's what I have so far:
template <class T, int N>
class Matrix {
public:
typedef typename Matrix<T, N - 1>::type MatrixOneDimLower;
typedef std::vector<MatrixOneDimLower> type;
type _data;
template <unsigned int dn, typename ...NT>
Matrix(unsigned int dn, NT ...drest) : _data(dn, MatrixOneDimLower(drest)) {}
MatrixOneDimLower& operator[](unsigned int index)
{
return _data[index];
}
};
template <class T>
class Matrix<T, 1> {
public:
typedef std::vector<T> type;
type _data;
Matrix(unsigned int d0) : _data(d0, T(0.0)) {}
T& operator[](unsigned int index)
{
return _data[index];
}
};
Unfortunately, I'm not very adept in variadic templates and recursive templates, and this doesn't work. For example, if I try to use this as:
Matrix<double, 4> temp(n, dim[2], dim[1], dim[0]);
I get this compile time error (Visual Studio 2017):
error C2661: 'Matrix<double,4>::Matrix': no overloaded function takes 4 arguments
I would really appreciate if you can let me know what I'm doing wrong.

Answer 1

template<class T, std::size_t I>
struct MatrixView {
MatrixView<T, I-1> operator[](std::size_t i) {
return {ptr + i* *strides, strides+1};
}
MatrixView( T* p, std::size_t const* stride ):ptr(p), strides(stride) {}
private:
T* ptr = 0;
std::size_t const* strides = 0;
};
template<class T>
struct MatrixView<T, 1> {
T& operator[](std::size_t i) {
return ptr[i];
}
MatrixView( T* p, std::size_t const* stride ):ptr(p) {}
private:
T* ptr = 0;
};
template<class T, std::size_t N>
struct Matrix {
Matrix( std::array<std::size_t, N> sizes ) {
std::size_t accumulated = 1;
for (std::size_t i = 1; i < sizes.size(); ++i) {
accumulated *= sizes[N-i];
strides[N-i] = accumulated;
}
storage.resize( strides[0] * sizes[0] );
}
MatrixView<T, N> get() { return {storage.data(), strides.data()}; }
MatrixView<T const, N> get() const { return {storage.data(), strides.data()}; }
private:
std::vector<T> storage;
std::array<std::size_t, N-1> strides;
};
this requires doing Matrix<int, 6> m{ {5,4,2,1,3,5} }; to create a matrix with 6 dimensions.
To access it you need to do m.get()[3][0][0][0][0][0] = 4.
You get get rid of that .get() but it is a bit annoying so long as you want to support tensors of first order.
The data is stored contiguously.