"Can't take the address of constant expressions"

[FiftyTifty]

Yeah, it's me again.

I've made a tiny program to convert hex strings in a copy-pasted block of text, into decimal numbers. But Lazarus doesn't like me deleting characters from a TStringList, for some reason.

It's under a hundred lines, so it's not some gargantuan mess.

Problem lines:
[spoiler]

Code: [Select]

      Delete(tstrlistDest[iCounter], Pos(tstrlistDest[iCounter], strCurrentHex), iHexLength);
      Insert(strDecimal, tstrlistDest[iCounter], iPos);
[/spoiler]

Current .pas:

[spoiler]

Code: [Select]

unit hextodeclistconv;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls,
  StrUtils;

type

  { TformWindow }

  TformWindow = class(TForm)
    buttonBegin: TButton;
    editBegin: TEdit;
    editEnd: TEdit;
    groupBegin: TGroupBox;
    groupEnd: TGroupBox;
    memoSource: TMemo;
    memoResult: TMemo;
    procedure buttonBeginClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { private declarations }
  public
    { public declarations }
  end;

var
  formWindow: TformWindow;

implementation

{$R *.lfm}

{ TformWindow }

var
  tstrlistSource, tstrlistDest: TStringList;

procedure TformWindow.FormCreate(Sender: TObject);
begin
  tstrlistSource := TStringList.Create;
  tstrlistDest := TStringList.Create;

  tstrlistSource.Sorted := false;
  tstrlistDest.Sorted := false;
end;


function GetHexString(strLine: string; iPos, iNumChars: integer): string;
var
  strHex: string;
begin
  strHex := copy(strLine, iPos, iNumChars);
  Result := strHex;
end;


procedure TformWindow.buttonBeginClick(Sender: TObject);
var
  iCounter, iDecimal, iPos, iNumchars, iHexLength: integer;
  strCurrentHex, strDecimal: string;
begin

  tstrlistSource.Assign(memoSource.Lines);
  tstrlistSource.Sorted := true;
  tstrlistDest := tstrlistSource;
  tstrlistDest.Sorted := true;

  if editBegin.Text = '0' then
     editBegin.Text := '1';
  if editEnd.Text = '0' then
     editEnd.Text := '1';

  iPos := StrToInt(editBegin.Text);
  iNumChars := StrToInt(EditEnd.Text);


  for iCounter := 0 to tstrlistSource.Count - 1 do begin

      strCurrentHex := GetHexString(tstrlistSource[iCounter], iPos, iNumChars);

      iDecimal := Hex2Dec(strCurrentHex);
      strDecimal := IntToStr(iDecimal);

      iHexLength := Length(strCurrentHex);

      Delete(tstrlistDest[iCounter], Pos(tstrlistDest[iCounter], strCurrentHex), iHexLength);
      Insert(strDecimal, tstrlistDest[iCounter], iPos);

  end;

  memoResult.Lines.AddStrings(tstrlistDest);

end;


end.
[/spoiler]

Any idea what the error means?

[Blaazen]

This error message usually appears when you try to pass property as a parameter-by-reference, i.e.:

Code: Pascal  [Select][+][-]

1.   TForm1 = class(TForm)
2.     Button1: TButton;
3.     procedure Button1Click(Sender: TObject);
4.   private
5.     FMyInt: Integer;
6.   public
7.     procedure MyProc(var i: Integer);
8.     property MyInt: Integer read FMyInt;
9.   end;
10.  
11. ...
12.  
13. procedure TForm1.MyProc(var i: Integer);
14. begin
15.  
16. end;
17.  
18. procedure TForm1.Button1Click(Sender: TObject);
19. begin
20.   MyProc(MyInt);  //error message will be here
21. end;

If it's your case (although I cannot see it in your code), copy the property to a local variable.

[FiftyTifty]

Yup, that worked. Had to chuck the current line from the TStringList into a string variable, work on the string, then replace said line with the modified string.

Odd, but it works.

[lainz]

This error message usually appears when you try to pass property as a parameter-by-reference, i.e.:

Sure there's a reason for that, why that can't be done? Is a question I have.

[Blaazen]

When you pass parameter-by-reference you pass pointer to a memory. It would be possible, if property is defined by field:

Code: Pascal  [Select][+][-]

1. property MyProp: Integer read FMyProp write FMyProp;

(compiler would automatically replace MyProp with FMyProp).

But property can be defined by getter and setter:

Code: Pascal  [Select][+][-]

1. property MyProp: Integer read GetMyProp write SetMyProp;

In the second case, there's nothing to pass.

[lainz]

Thanks Blaazen. Now is clear.