[fpc-pascal] fcl-passrc errors

[Michael Van Canneyt]

On Mon, 14 Oct 2019, Mattias Gaertner via fpc-pascal wrote:

> On Mon, 14 Oct 2019 07:37:38 +0200
> Sven Barth via fpc-pascal <fpc-pascal at lists.freepascal.org> wrote:
>
>> [...]
>> Because the iterator returns a Pointer and not whatever you think it 
>> might return. If you know it's something else, you cast that inside
>> the loop.
>
> I agree with Sven.
>
> What about the other way round?
>
> For example this is allowed: TControl(aButton):=aControl;

That's a good way to shoot yourself in the foot. 
If aControl is a TEdit, for example, this will also pass.

> This works also when passing something to a "var" argument.

That's like giving someone 2 guns to shoot himself in the feet
simultaneously.

No, seriously:

That's a really bad kludge because for good reasons, you can't pass descendents
as a var argument, and unfortunately the typecast is sometimes used to work around this.

This is also an excellent way to shoot yourself in the foot.

Consider the following:

Procedure DoSomething(var A : TControl);

Var
  N : String;

begin
   if Assigned(a) then
      N:=A.Name;
   A:=TEdit.Create(Nil);
   A.Name:=N;
end;

var
   B : TButton;

begin
   DoSomething(TControl(B));
   // And now B contains a TEdit, despite the declaration
end;

The typecast is something you should not do in this case.

/* Cultural note: this is the reason FreeAndNil has an untyped argument :( */

>
> It would be consistent to allow it as the for-in-loop variable.

As far as I know, you're (rightly) a proponent of not allowing people op shoot themselves in the foot ?

The typecast is something that should be discouraged, not encouraged.

So no, I don't think we should allow this.

Michael.