Iterating through multidimensional arrays

Iterating through multidimensional arrays

With a one dimensional array, you can iterate through it like so:
for (int i : arr) {
// code
}
But why, when you are using a multidimensional array, do you have to use
references?:
for (int &row : arr) {
for (int &col : row) {
// code
}
}