Pass By Reference Multidimensional Array With Unknown Size

Question

How to pass by reference multidimensional array with unknown size in C or C++?

EDIT:

For example, in main function I have:

int main(){
    int          


        

Answer 1

H2CO3's solution will work for C99 or a C2011 compiler that supports VLAs. For C89 or a C2011 compiler that doesn't support VLAs, or (God forbid) a K&R C compiler, you'd have to do something else.

Assuming you're passing a contiguously allocated array, you can pass a pointer to the first element (&a[0][0]) along with the dimension sizes, and then treat it as a 1-D array, mapping indices like so:

void foo( int *a, size_t rows, size_t cols )
{
  size_t i, j;

  for (i = 0; i < rows; i++)
  {
    for (j = 0; j < cols; j++)
    {
      a[i * rows + j] = some_value();
    }
  }
}

int main( void )
{
  int arr[10][20];

  foo( &arr[0][0], 10, 20 );
  ...
  return 0;
}

This will work for arrays allocated on the stack:

T a[M][N];

and for dynamically allocated arrays of the form:

T (*ap)[N] = malloc( M * sizeof *ap );

since both will have contiguously allocated rows. This will not work (or at least, not be guaranteed to work) for dynamically allocated arrays of the form:

T **ap = malloc( M * sizeof *ap );
if (ap)
{
  size_t i;
  for (i = 0; i < M; i++)
  {
    ap[i] = malloc( N * sizeof *ap[i] );
  }
}

since it's not guaranteed that all the rows will be allocated contiguously to each other.