How do I populate Struct/Array of Records using Delphi?

Question

I want to populate an array of records, and with my recent exposure to TMappedFile, I am trying to duplicate it, w/o actually using unFileMapping or BlockRead/Write. The purpose is to leave my options open just for that. The problem I am finding is either its not initiating or Create'ing my array correctly.

From what I have seen so far, is that the DataArr remains nil after compiling. And causes an memory violation error (0x00000000) which means it's empty/null. My engineer has already addressed to me that the size of the array maybe to large for purpose, and has also mentioned that I am not able to do what I want unless I deference the pointer, ex. DataArr^[0].value1

I believe my confusion lies with SetLength which seems counter intuitive for a Dynamic Array, which is why I commented it out. Otherwise I get an error E2008 incompatible types.

type
  TDataStruct = Record
  value1 : array[0..1] of Single;
  value2 : array[0..1] of Single;
  value3 : array[0..1] of Single;
  end;

  TDataStructArray = array[0..MaxInt div SizeOf(TDataStruct) - 1] of TDataStruct;
  PDataStructArray = ^TDataStructArray;

var
  DataArr: PDataStructArray;

begin
  //SetLength(DataArr,length(DataArr)+1);
  DataArr[0].value1[0] := 2;

Other things that I have tried is used FillChar(DataArr,SizeOf(DataArr),0) and as well as FillChar(DataArr[0], SizeOf(DataArr[0]) * Length(DataArr));

The following links have been my sources

The problem is that the pointer is not pointing to anywhere. To populate an array of records, first you have to have an array of records, you have none. — Sertac Akyuz
– Sertac Akyuz, Commented Oct 13, 2016 at 18:18
As others have suggested, you should use a dynamic array. If you want to use a pointer to an array as you have then you need to get memory for that array before you use it and free the memory afterwards. e.g. for 5 records in the array you would do: GetMem(DataArr, SizeOf(TDataStruct) * 5); DataArr[0].value1[0] := 2; FreeMem(DataArr); — Graymatter
– Graymatter, Commented Oct 13, 2016 at 18:34

LU RD · Accepted Answer · 2016-10-13 18:34:37Z

10

A dynamic array is what is says: dynamic.

Dynamic arrays do not have a fixed size or length. Instead, memory for a dynamic array is reallocated when you assign a value to the array or pass it to the SetLength procedure.

Declare

DataArr: Array of TDataStruct;

Use SetLength to allocate records and initialize them at the same time.

SetLength(DataArr,Length(DataArr)+1); // Or preallocate as many as you wish to initialize

Access a record in the array:

DataArr[0].value1[0] := 2;

In your example there is no memory allocation made, hence the error. And SetLength operates on dynamic arrays, not on pointers to static arrays.

You could manage the pointer with GetMem/FreeMem and initialize with ZeroMem, but that will only cause troubles building and maintaining the code.

edited Oct 13, 2016 at 18:34

answered Oct 13, 2016 at 18:27

LU RD

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4 Comments

Sertac Akyuz Over a year ago

Another option is to use a variable, to allocate memory for the static array I mean..

LU RD Over a year ago

@SertacAkyuz, from the look of the question, it seems that the size of the array is not known at compile time.

HQSantos Over a year ago

I need to make a loop verification with this method, and everytime I do that, I receive a code violation. But if I add the [0..0] I get the error 'incompatible types' when I try to increase the array size. Do you know what can I be doing wrong?

LU RD Over a year ago

@riki481, hard to say why the dynamic array allocation fails without seeing your code. A static array is incompatible with a dynamic array and cannot be resized, hence the compiler error.

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