Is it possible in c++ for a class to have a member which is a multidimensional array whose dimensions and extents are not known until runtime?

Question

I originally asked using nested std::array to create an multidimensional array without knowing dimensions or extents until runtime but this had The XY Problem of trying to a

Answer 1

Just avoid multidimensional arrays:

template
class Matrix
{
    public:
    Matrix(unsigned m, unsigned n)
    :   n(n), data(m * n)
    {}

    T& operator ()(unsigned i, unsigned j)  {
        return data[ i * n + j ];
    }

    private:
    unsigned n;
    std::vector data;
};

int main()
{
    Matrix m(3, 5);
    m(0, 0) = 0;
    // ...
    return 0;
}

A 3D access (in a proper 3D matrix) would be:

T& operator ()(unsigned i, unsigned j, unsigned k)  { 
    // Please optimize this (See @Alexandre C) 
    return data[ i*m*n + j*n + k ]; 
}

Getting arbitrary dimensions and extent would follow the scheme and add overloads (and dimensional/extent information) and/or take advantage of variadic templates.

Having a lot of dimensions you may avoid above (even in C++11) and replace the arguments by a std::vector. Eg: T& operator(std::vector indices). Each dimension (besides the last) would have an extend stored in a vector n (as the first dimension in the 2D example above).