TStringList in an array in Lazarus

Question

I have a problem with TStringLists in Lazarus. I have an array called 'trans' of records called 'TTrans' which contain, among other things, TStringList called 'meebetalers'. So when I need to know for example the amount of lines in that StringList I would have to write this right? trans[i].meebetalers.Count;

Anyways, I first create a stringlist and put the selected strings from a checklistbox in it, and that works (i.e. the program returns 3 when I ask for the Count, which is correct).

In this piece of code I add values to the StringList:

slmeebetalers := TStringList.Create;
for i:= 0 to Form6.CLBox.Count-1 do begin
        if Form6.CLBox.Checked[i] then begin
          slmeebetalers.Add(Form6.CLBox.Items[i]);
          end;
        end;

Then I put the stringlist a procedure, and in that procedure I assign my first created StringList to the stringlist I mentionned before (trans[i].meebetalers), see my piece of code next.

Unit6.VoegTransToe(Form6.TransNaam.Text,
                         Form6.TrComboBox.Text,
                         bedrag,
                         slmeebetalers,
                         Form6.CalendarDialog1.Date);

But when I then ask for the count, it returns 0.

procedure VoegTransToe(naam, betaalpers: string; bedrag: currency;
  meebetalers: TStringList; datum: TDateTime);
begin
  aantaltrans:= aantaltrans+1;
  trans[aantaltrans].naam:=naam;
  trans[aantaltrans].pers.naam:=betaalpers;
  trans[aantaltrans].bedrag:=bedrag;
  trans[aantaltrans].datum:=datum;
  meebetalers:= TStringList.Create;

    trans[aantaltrans].meebetalers:= TStringList.Create;
    trans[aantaltrans].meebetalers.Assign(meebetalers);
  meebetalers.Free;
  //trans[aantaltrans].meebetalers.Free;
end; 

note The difference in name of the variable is because they are in different units

With this code I don't get an error, but it returns 0. When I say //meebetalers.Free; the same happens. But when I add //trans[aantaltrans].meebetalers.Free; I don't get an error while compiling, but when I call the procedure. Then I get this error: Project project1 raised exception class 'External: SIGSEGV'.

I think there is something wrong with the Create and Free function, but I don't know what. When I implement the try...finally...end it returns the same error. Can anybody help me?

Answer 1

The problem is that your VoegTransToe() procedure is ignoring the populated TStringList object that is passed in via its meebetalers parameter. You are resetting meebetalers to point at a newly created empty TStringList object just before assigning meebetalers to trans[aantaltrans].meebetalers.

procedure VoegTransToe(naam, betaalpers: string; bedrag: currency;
  meebetalers: TStringList; datum: TDateTime);
begin
  aantaltrans:= aantaltrans+1;
  trans[aantaltrans].naam:=naam;
  trans[aantaltrans].pers.naam:=betaalpers;
  trans[aantaltrans].bedrag:=bedrag;
  trans[aantaltrans].datum:=datum;
  // meebetalers:= TStringList.Create; // <-- GET RID OF THIS!

  trans[aantaltrans].meebetalers:= TStringList.Create;
  trans[aantaltrans].meebetalers.Assign(meebetalers);
  //meebetalers.Free; // <-- AND THIS!
end; 

Don't forget to Free() the input TStringList object when you are done using it:

slmeebetalers := TStringList.Create;
try
  for i := 0 to Form6.CLBox.Count-1 do begin
    if Form6.CLBox.Checked[i] then begin
      slmeebetalers.Add(Form6.CLBox.Items[i]);
    end;
  end;
  Unit6.VoegTransToe(..., slmeebetalers, ...);
finally
  slmeebetalers.Free;
end;