Is it possible to create array of pointer of 3d arrays?

Question

I have 3 arrays, A[5][3][5], B[5][4][5], C[5][2][5]. Is it possible to access them by an array of pointers, with their second dimension being different? Something like:

int A[5][3][5], B[5][4][5], C[5][2][5];
int ***D[3];
D[0] = A;
D[1] = B;
D[2] = C;

I know this is wrong, I just want to know if it's possible to access them by one array?

Answer 1

No, if the second dimension is different it won't work. The best you can do is something like this:

struct arr {
    int *p; // pointer to first element
    int x, y, z; // array size
    int &at(int i, int j, int k) {
        return p[((i*y)+j)*z+k];
    }
}

Or you can use your favorite multidimensional array library. C++ lacks built-in support for multidimensional arrays unless all but the first size is known at compile time, and C99 VLAs won't work in this situation. This is because C/C++ use the type of the array to figure out how big it is in each dimension (except the first dimension, which can be unspecified).

Answer 2

It's definitely not possible to do what you want directly due to the type system's restrictions, but you may want to consider something like the following (in C++11 syntax):

#include <vector>
#include <array>
#include <iostream>

template <typename T, size_t x, size_t z>
struct Ragged : std::array<std::vector<std::array<T, z>>, x> {
  Ragged(size_t y) {
    for (auto &i : *this) {
      i.resize(y);
    }
  }
};

int main() {
  using R5y5 = Ragged<int, 5, 5>;

  R5y5 a(3), b(4), c(2);
  vector<R5y5> d{a, b, c};
  d[1][1][2][3] = 99; // checked at() calls supported for all dimensions, too

  for (auto const &d : D) {
    for (auto const &x : d) {
      std::cout << "[";
      for (auto const &y : x) {
        std::cout << "[";
        for (auto const &z : y) {
          std::cout << z << " ";
        }
        std::cout << "]";
      }
      std::cout << "]" << std::endl;
    }
    std::cout << std::endl;
  }
}

This gets you multidimensional operator[] access to d and its elements, and allows you to put whatever y-dimensioned array inside d you want to. Note that the 3-dimensional pseudo-arrays are no longer stored completely compactly, but in potentially growable 2-D slices.

Answer 3

#include <stdio.h>

int main(){
    int A[5][3][5], B[5][4][5], C[5][2][5];
    void *D[3];
    D[0]=&A;
    D[1]=&B;
    D[2]=&C;
    B[1][2][3] = 99;
    printf("%d\n", (*(int(*)[5][4][5])D[1])[1][2][3]);//99

    return 0;
}