Accessing 2-dimensional array via pointer using pointer math: Array type required [Delphi]

Question

I’m perfectly avare that pointer math in Delphi should be used with special care, but I still don’t get where am I wrong here. Let’s take a look on the following type definition:

program Project26;
{$POINTERMATH ON}

type
  TCalibrationTable = array [0 .. 3] of byte;
  PCalibrationTable = ^TCalibrationTable;

Now I need 2 arrays, each of them having 3 elements of TCalibrationTable (ultimately they are 2-dimensional, 3 × 4 arrays). And also need a header which points to a pair of such array objects.

var
  table0: array [0 .. 2] of TCalibrationTable;
  table1: array [0 .. 2] of TCalibrationTable;

  header: array [0 .. 1] of PCalibrationTable;

Let’s initialize the header: the most comfortable way to access an array by a pointer is to assign it the first element’s address, then simply index the pointer to get an arbitrary element.

begin
  header[0] := @table0[0];
  header[1] := @table1[0];

Now, header[0] being a PCalibrationTable, I can use it to index it in order to get arbitrary TCalibrationTable typed elements from an array. In this way header[0][2] SHOULD BE a TCalibrationTable, that is, an array of 4 elements of byte.

But the following assignment will give a build error: Array type required.

  header[0][2][3] := 100;
end.

Why?

HeartWare HeartWare · Accepted Answer · 2020-10-14T10:29:18

If you remove the pointer indirection from the header, you get this:

type
  TCalibrationTable = array [0 .. 3] of byte;
  PCalibrationTable = ^TCalibrationTable;
var
  header: array [0 .. 1] of PCalibrationTable;

becomes

type
  TCalibrationTable = array [0 .. 3] of byte;
var
  header: array [0 .. 1] of TCalibrationTable;

or

var
  header: array [0 .. 1] of array [0 .. 3] of byte;

or

var
  header: array [0 .. 1,0 .. 3] of byte;

In other words, a 2-dimensional array.

What you need is this:

type
  TCalibrationTable = array [0 .. 3] of byte;
  TCalibrationArr = ARRAY[0..2] OF TCalibrationTable;
  PCalibrationArr = ^TCalibrationArr;

var
  table0: TCalibrationArr;
  table1: TCalibrationArr;

  header: array [0 .. 1] of PCalibrationArr;

you can then do

header[0] := @table0;
header[1] := @table1;
header[0][1][2]:=123;

Accessing 2-dimensional array via pointer using pointer math: Array type required [Delphi]

1 Answers