Hello List,

Since 2.6.0, when you write something like this:

procedure foo(AClass : TStrings);
foo(MyStringList); // Will return an error that TStrings is expected ...

I can not use TStringList as the parameter without casting it to TStrings. But I do not understand why this limitation added. Can you please explain it to me ?

What can be done in order to make it supported again ? should it be declared as an abstract class or something ?

Thanks,
Ido

It works here with 2.6.0 RC1.

Can it be that you used "var" or "out"? Cause than this is expected: http://wiki.freepascal.org/User_Changes_2.6.0#Passing_derived_classes_to_var-_and_out-parameters

Regards,
Sven

ik wrote

Since 2.6.0, when you write something like this:

I think it was even before this.

I ran into the exact same issue this week, and also can't understand why this is not allowed any more. It *is* allowed in Delphi (at least D7). Luckily I could change my code to TStringList to work around the problem.

So yes, if anybody could please explain what was the logic behind this crippling change?

The other issue I have is passing a property value via a var or const parameter. I used this a lot in my code too, and it worked for years without problems. This again, is supported in Delphi too. I have no idea why this was changed in FPC either. The work around looks damn stupid too. Declare a local variable and assign the property to it, then pass the local variable. In the end it has the same endresult, but just inconveniences to the developer with these unnecessary local variables.

Works for me:

program a;
{$mode objfpc}{$H+}
uses
  Classes;
procedure foo(AClass: TStrings);
begin
end;
var
  sl: TStringList;
begin
  foo(sl);
end.

ik wrote

Since 2.6.0, when you write something like this: procedure foo(AClass : TStrings); foo(MyStringList); // Will return an error that TStrings is expected ...

Are you certain there is no "var" or "out" in front of that parameter?

I can not use TStringList as the parameter without casting it to TStrings. But I do not understand why this limitation added. Can you please explain it to me ?

In case of a var/out parameters: http://wiki.freepascal.org/User_Changes_2.6.0#Passing_derived_classes_to_var-_and_out-parameters

That page is also linked from the announcement mail and the readme.

Jonas

Graeme Geldenhuys wrote

Luckily I could change my code to TStringList to work around the problem.

One can type-cast to get around the issue, so then method declarations don't need to be changed:

foo(TStrings(MyStringList));

It's not allowed in Delphi in case of a var/out parameter. It's allowed in both FPC and Delphi in case of a value/const parameter.

The other issue I have is passing a property value via a var or const parameter. I used this a lot in my code too, and it worked for years without problems. This again, is supported in Delphi too.

No, it is not allowed by Delphi (at least not passing a property to a var-parameter; passing it to a const parameter is allowed by both FPC and Delphi).

Jonas

That would be strange indeed, because then most events (onclick handlers etc) would not compile anymore, since they are in the format procedure(Sender: TObject) of object...

Bart

Bart wrote

That would be strange indeed, because then most events (onclick handlers etc) would not compile anymore, since they are in the format procedure(Sender: TObject) of object...

It is not like that, he forgot adding var or out to the parameter. The example which you are mentioning has no var.

Indeed. I checked my code and Delphi 5-7 compatibility. The problem I had, wast that I specified those parameters as var.

Sorry for the late response, but why does out and var limit this issue ? I understand that it does, but not the reason for it.

The reason is mentioned in the link I sent you:
http://wiki.freepascal.org/User_Changes_2.6.0#Passing_derived_classes_to_var-_and_out-parameters

Regards,
Sven

I read it, but I do not understand it.

TStrings is a class. TStringList is a class inherited from TStrings. How does it prevent the type checking ?
As I remember, if I placed there "TFom" based class, it will not like it and report and error. That's why I do not understand it I guess.

Emagine the following code:

procedure Foo(var aList: TStrings);
begin
   aList.Free;
   aList := TStrings.Create;
end;
var
   l: TStringList;
begin
   l := TStringList.Create;
   Foo(l);
   l.Sort;
end.

When you run this code you will have a big problem. Your "l" variable is a TStringList, but after calling Foo your variable holds a TStrings instance (which is lower in the hierarchy than TStringList)! If you now call Sort you will get an access violation if you're lucky or some strange error if not, because Sort is introduced by TStringList. Thus you call into "nothing" when executing this on a TStrings.

Is the reason why this is forbidden more clear now?

Regards,
Sven

‏Yes, thank you

The l.Sort is the important line, that is missing in the wiki example.

Maybe this example can replace the example in the wiki?

Mattias

I have extended the example in the wiki a bit in the hope to clarify the seriousness of this issue.

Regards,
Sven