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!

Answer 1

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.)