Question

I have this D-code for reading data from a file:

int foo (SomeStruct[] somestruct_array)
{
  // get file_handle...
  for (uint i = 0; i < 1000; i++) {
    file_handle.readf ("%e %e %e %e %e %e\n",
            &(somestruct_array[i].pos_x), &(somestruct_array[i].pos_y),
            &(somestruct_array[i].pos_z), &(somestruct_array[i].vel_x),
            &(somestruct_array[i].vel_y), &(somestruct_array[i].vel_z));
  }
  return 0;
}

As you can see, the lines are getting quiet long and I keep repeating somestruct_array[i]. It’s not a big deal, but I tried to shorten that and write:

int foo (SomeStruct[] somestruct_array)
{
  // get file_handle...
  for (uint i = 0; i < 1000; i++) {
    auto elem = somestruct_array[i];
    file_handle.readf ("%e %e %e %e %e %e\n",
            &(elem.pos_x), &(elem.pos_y),
            &(elem.pos_z), &(elem.vel_x),
            &(elem.vel_y), &(elem.vel_z));
  }
  return 0;
}

This results in all the element members being nan.

So: Is it possible to get a reference to array elements?

Answer 1

The problem is that elem is a copy of the array element, so the pointers you pass to readf are pointers to the temporary value.

You have two options here, you can use a pointer:

for (uint i = 0; i < 1000; i++) {
    auto elem = &somestruct_array[i];
    file_handle.readf ("%e %e %e %e %e %e\n",
        &(elem.pos_x), &(elem.pos_y),
        &(elem.pos_z), &(elem.vel_x),
        &(elem.vel_y), &(elem.vel_z));
}

or, you can iterate the array by reference:

foreach (ref elem; somestruct_array[0..1000]) {
    file_handle.readf ("%e %e %e %e %e %e\n",
        &(elem.pos_x), &(elem.pos_y),
        &(elem.pos_z), &(elem.vel_x),
        &(elem.vel_y), &(elem.vel_z));
}

Note that if the array is 1000 elements long, you can just write:

foreach (ref elem; somestruct_array)