Re: MD7 Q's

From: MW (mwdejager_at_hotmail.com)
Date: 10/13/03 

Date: Mon, 13 Oct 2003 11:26:22 +0200

Thanks Rob

Much of your explanations here, have answered and confirmed my findings, on
many a question I've had myself. You answer a few more questions later on
the thread regarding the freeing of objects, and I have still some more
questions.

Say you define a TButton and instatiate it as follows

procedure TForm1.FormMouseDown [..]
var
    Btn: TButton;
begin
    Btn := TButton.Create (Self);
end;

Am I corrent in assuming that this instance of TButton, Btn, only gets freed
once I close Form1? (unless if I call the free/destroy method myself)

Also, which properties of Btn are set by TButton.Create, if any, or do I
need to populate all of its properties?

Now say, I went and instantiated an object of type TStringList.

procedure StoreNames;
var
    Names : TStringList;
begin
    Names := TStringList.Create;
    With Names Do
    Begin
        Clear; { is this necessary }
        Add('Rob');
        Add('name');
        Add('MW');
    End;
    ...
    ...
// ClearList(Names);

    Names.Free;
end

Does the above code (Names.Free) take care of cleaning up properly? I've
read some code where the programmer did cleaning up that freed each of the
strings that were added individually. (commented out ClearList)

procedure ClearList(List: TStrings);
var
  i : integer;
begin
  for i := 0 to List.Count - 1 do
    if List.Objects[i] <> nil then
      List.Objects[i].Free;
  List.Clear;
end;

Is the above procedure really necessary?

Also, what happens to Names, if I do not call Free. Does Delphi not clean
up for a local variable of type TStringList, like it would have for a
variable of type String, when the procedure closes?

Thank you in advance for any answers

Kind regards

Groete
MW

"Rob Kennedy" <.> wrote in message news:vogn97e929l5f6@corp.supernews.com...
> name wrote:
> > --- Self/Parent Question ---
> > procedure TForm1.FormMouseDown [..]
> > var
> > Btn: TButton;
> > begin
> > Btn := TButton.Create (Self); // What is this doing and why is it
important?
>
> It's creating a new instance of the TButton class. It's also telling
> that new TButton instance that it should set its Owner property to refer
> to the current TForm1 instance. (Since the code is running within a
> method of the TForm1 class, Self refers to the TForm1 instance on which
> that method was called.)
>
> Descendents of TComponent all have an Owner property. When a component
> is destroyed, it will also destroy anything that it owns. That can make
> things more convenient since you, the programmer, don't always need to
> keep close tabs on every control you create. Instead, you can assign
> responsibility to some other control.
>
> > Btn.Parent := Self;
> > // Why is setting the buttons parent to it self important?
>
> That's not what it's doing; that would be an invalid operation. It's
> setting the new button's Parent property to refer to the current TForm1
> instance. The button is now a child control of the form.
>
> > //Can it really be its own parent?
>
> No. Luckily, that's not what the code is doing.
>
> > //Wouldent the parent be the form the button is on? (procedure TForm1)
>
> Yes. But not automatically. By default, a control does not have a parent
> at all.
>
> > // Why does parent need to be set?
>
> So the control knows where to display itself.
>
> Every windowed control in the system needs to have a parent. Windows
> without parents are called top-level windows, but they cannot be
> controls such as buttons, edit boxes, and tool bars. If a VCL object's
> Parent property hasn't been set, then it doesn't have a parent, so when
> it comes time for the VCL object to make itself known to the OS by
> allocating itself a window handle, there will be problems since the OS
> won't know what to do with this orphan window.
>
> > "Use the Parent property to get or set the parent of this control. The
> > parent of a control is the control that contains the control. For
example,
> > if an application includes three radio buttons in a group box, the group
box
> > is the parent of the three radio buttons, and the radio buttons are the
> > child controls of the group box." - D7 Help
>
> Uh, yes, thank you. So what?
>
> --
> Rob
>