Access Violation with TStringList

[spitofland]

I am running Lazarus-0.9.29-25159 and fpc-2.4.1-20100504 for Win 64-bit on a Windows 7 (64-bit) machine. I passed a TStringList object by reference to a function that I created. The function works perfectly, but inside the function I get an Access Violation if I try to run this code:

Code: [Select]

function TImportForm.ParseStr(Line:String;Delim,Quote:Char;var Data:TStringList) : Boolean;
begin
  Data.Clear;   //Access violation
end;But I don't get any errors if I run:

Code: [Select]

function TImportForm.ParseStr(Line:String;Delim,Quote:Char;var Data:TStringList) : Boolean;
begin
  Data.Add('Test String');
  Data.Add('Test String2');
  Data.Add('Test String3');
end;
Then, after using this function (without using the Clear function) I get an Access Violation when I call:

Code: [Select]

  ParseStr(CurrLine,ColumnChar,QuoteChar,Fields);
  ShowMessage(Fields.Strings[1]);
What would cause an Access Violation with Clear and Strings[1], but not with Add?

[eny]

The AV is caused by passing a non-initialized object var.
I'd say that that needs fixing.
If you don't initialize the variable, it can contain anything and the results are unpredictable.

BTW: there is no reason to pass the object as a ref parameter.
Except when you really want to initialize it in your function, like so:

Code: [Select]

function TImportForm.ParseStr(Line:String;Delim,Quote:Char;var Data:TStringList) : Boolean;
begin
  Data := TStringList.Create;
  Data.Add('Test String');
  Data.Add('Test String2');
  Data.Add('Test String3');
end;


[spitofland]

Oh, thanks. I guess I should have caught that.

When you say I don't have to pass it as a var, if I pass a TStringList as a normal parameter and make changes to it, will the changes persist?

[eny]

When you say I don't have to pass it as a var, if I pass a TStringList as a normal parameter and make changes to it, will the changes persist?

You mean pass it as a value parameter: yes!
As long as you initialize the objects before using them.
Like this:

Code: [Select]

function TImportForm.ParseStr(Line:String; Delim, Quote:Char; Data:TStringList) : Boolean;
begin
  Data.Add('Test String');
  Data.Add('Test String2');
  Data.Add('Test String3');
  result := true // or false... or whatever value you need...
end;

.....

var MyList: TStringList;
begin
  MyList := TStringList.Create;
  if ParseStr('A String To Parse', ' ', '"', MyList) then ...
  ...  
end;
<<edit>> Forgot to remove the var keyword...

[mas steindorff]

don't you need to use the var keyword in the funtion prameter list?

function TImportForm.ParseStr(Line:String; Delim, Quote:Char; var Data:TStringList) : Boolean;

or else you may be working on a copy of the varable/object

[eny]

don't you need to use the var keyword in the funtion prameter list?

No, hence my added comment.
Objects are passed as a reference automatically.
You are working on a copy of the object reference. But it's referring to the same, actual object.

[spitofland]

Thanks, it's helpful to know that.