How to save a 2D array of pointers to a class attribute and access the array's contents properly.

cast_void_ptr_to_2d_ptr_array.cpp

// How to save a 2D array of pointers to a class attribute and
// access the array's contents properly.
#include <iostream>
#include <stdio.h>

// Show pointer values in array:
void show_ptr(int *a[2][2]) {
  for (int i=0; i<2; i++) {
    for (int j=0; j<2; j++) {
      printf("[%d,%d] %p\n", i, j, &a[i][j]);
    }
  }
}

// Cast for converting void pointer to 2D array of pointers:
#define ARRAY(x) ((int *(*)[2])x)

class Foo {
public:
  Foo(int *a[2][2]) {this->a = a;};
  
  void set(int v) {
    for (int i=0; i<2; i++) {
      for (int j=0; j<2; j++) {
        for (int k=0; k<2; k++) {
          ARRAY(a)[i][j][k] = v;
        }
      }
    }
  };
private:
  void *a;
};

// Show array contents:
void show(int *a[2][2]) {
  for (int i=0; i<2; i++) {
    for (int j=0; j<2; j++) {
      std::cout << "[" << i << "," << j << "] ";
      for (int k=0; k<2; k++) {
          std::cout << a[i][j][k] << " ";
      }
      std::cout << std::endl;
    }
  }
}

int main(void) {
  int *data[2][2];
  for (int i=0; i<2; i++) {
    for (int j=0; j<2; j++) {
      data[i][j] = (int *)malloc(2*sizeof(int));
    }
  }

  Foo f = Foo(data);
  f.set(0);
  show(data);
  f.set(1);
  show(data);
  
  for (int i=0; i<2; i++) {
    for (int j=0; j<2; j++) {
      free(data[i][j]);
    }
  }
}