delphi pointer question

Question

I have the following code which is working, but I don't understand it 100% (please see the comments from code):

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TMyRec=record
    a:Integer;
    b:String;
  end;
  TRecArray=array of TMyRec;
  PRecArray = ^TRecArray;

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
   v1:TRecArray;
   procedure Test(a:PRecArray);
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{ TForm1 }

procedure TForm1.FormCreate(Sender: TObject);
begin
 SetLength(v1,3);
 v1[0].b:='test1';//set the first value
 Test(PRecArray(v1));//call method to change the value assigned before
end;

procedure TForm1.Test(a: PRecArray);
begin
 ShowMessage(v1[0].b);//shows test1
 try
  a^[0].b:='test2' //this is raising an error...
 except

 end;
 PRecArray(@a)^[0].b:='test3';//this is working...
 ShowMessage(v1[0].b);//shows test3
end;

end.

I don't understand why 'a^[0].b:='test2' is raising an error.

Thank you!

Please replace TPointerArrayRec with PArrayRec, as this the standard convention for declaring pointer types. — iamjoosy
– iamjoosy, Commented Aug 19, 2011 at 10:09
FWIW, it is convention to name PArrayRec = ^TArrayRec, instead of TPointerArryRec. I would personally call them TRecArray and PRecArray, i.e. Array at the end. I see @iamjoosy had the same idea at almost the same time. — Rudy Velthuis
– Rudy Velthuis, Commented Aug 19, 2011 at 10:11

Sertac Akyuz · Accepted Answer · 2011-08-19 11:11:50Z

10

Your 'Test' procedure expects a 'PRecArray', but you're passing a 'TRecArray' to it. Try calling it like

 Test(@v1);//call method to change the value assigned before

Typecasting a 'TRecArray' to a 'PRecArray' will not make it a 'PRecArray'. (Note: your 'test3' will fail then of course.)

edited Aug 19, 2011 at 11:11

answered Aug 19, 2011 at 9:51

Sertac Akyuz

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2 Comments

RBA Over a year ago

+1 yes, you're right. I must get some sleep before asking 'silly' questions

Sertac Akyuz Over a year ago

@RBA - I don't think it's silly, using pointers may always be confusing. But sleep is good :)

Rudy Velthuis · Accepted Answer · 2011-08-19 14:29:19Z

4

I see several things that are suspicious.

1

There is hardly ever a need to take a pointer to a dynamic array, as dynamic array variables are already pointers (well, references).

To pass such an array to a function or procedure, use var parameters:

procedure TForm1.Test(var a: TRecArray);

Now you don't have to use pointer syntax to access the array:

a[0].b := 'test2';

2

You call Test with:

Test(PRecArray(v1));

In your original, Test took a PRecArray, but you are not passing one (you are passing a TRecArray), so you should have done:

Test(@v1); // or Test(Addr(v1));

Applying my change above, where Test has a var parameter, simply use:

Test(v1);

3

Ok, this is probably not suspicous, but I'd like to plug my article Addressing Pointers, about pointers for Delphi programmers. It explains many of the issues you seem to have.

edited Aug 19, 2011 at 14:29

answered Aug 19, 2011 at 10:22

Rudy Velthuis

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9 Comments

RBA Over a year ago

+1 Rudy. yes, I know your article, which btw is very well done. Question was how that code is working. It wasn't wrote by me, and I didn't understand it. Thanks for the suggestions.

Marjan Venema Over a year ago

+1 Shameless plug been bookmarked a long, long time ago. Congrats by the way on picking up rep fast. At this rate you'll join David, Andreas a.o very soon. :-) See you are into (Objective-) C as well. Can't figure out where you find the time. Or does dentistry no longer occupy you full time?

Rudy Velthuis Over a year ago

@Marjan: Dentistry only occupies me during "office hours". And I don't hunt or play golf. <g> I got a Mac a few months ago and am now trying to get acquainted with Cocoa and Objective-C. And interesting language, but no Delphi. I can hardly wait for XE2.

Marjan Venema Over a year ago

@Rudy, ah yes, no hunting or golfing does save a lot of time :-)) Waiting for XE here too, especially 64 bit (work), but the x-platform as well (personal projects).

Sertac Akyuz Over a year ago

> "I see several things that are suspicious" - Are you sure point (3) is suspicious?

|

Ken White · Accepted Answer · 2011-08-19 23:27:54Z

1

You could replace the

procedure TForm1.Test(a: TPointerArrayRec);

with

   procedure TForm1.Test(var a: TArrayRec);

it's simpler and you don't have to use the ~~deprecation~~ dereference operator ^.

edited Aug 19, 2011 at 23:27

Ken White

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answered Aug 19, 2011 at 9:58

opc0de

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4 Comments

RBA Over a year ago

+1, yes I know, but I want to understand the code in the actual form

Marjan Venema Over a year ago

I think deprecation should be dereference (or dereferencing)... Deprecation is marking something as deprecate (no longer used and in danger of being removed).

opc0de Over a year ago

@Marjan whatever i said the operator ^ that is enough

Ken White Over a year ago

@opc0de: No, it's not. Marjan is correct, and you should have edited your post to correct your mistake. Unless calling you 'oopscode' would be close enough, since it starts with 'o' and ends in 'de'.

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