Invalid pointer operation when try to replace object in list

[RaelB 4]

I create a number of Frames and store them in a Frame Object List (declared as FFrames: TObjectList<TMyFrame>)

The frames are also added to the VCL form.

 

I try to replace a Frame with below code, but it results in an "Invalid Pointer Operation": (I'm replacing the last Frame in the list with a new one)

 

procedure TForm1.btnReplaceClick(Sender: TObject);
var
  NewFrame: TMyFrame;
  OldFrame: TMyFrame;
begin
  if FFrames.Count = 0 then exit;

  NewFrame := GetNewFrame;
  OldFrame := FFrames[FFrames.Count - 1];
  FFrames[FFrames.Count - 1] := NewFrame;
  OldFrame.Free;
end;

What's wrong with the code?

 

Full code follows:

 

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls,
  System.Generics.Collections, MyFrameU;

type
  TForm1 = class(TForm)
    ScrollBox1: TScrollBox;
    btnAdd: TButton;
    btnReplace: TButton;
    procedure btnAddClick(Sender: TObject);
    procedure btnReplaceClick(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
    FFrames: TObjectList<TMyFrame>;
    function GetNewFrame: TMyFrame;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  Counter: Integer;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  FFrames := TObjectList<TMyFrame>.Create;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FFrames.Free;
end;

function TForm1.GetNewFrame: TMyFrame;
begin
  Result := TMyFrame.Create(Self);
  Result.Name := 'MyFrame' + Counter.ToString;
  Result.Parent := ScrollBox1;
  Result.Align  := alLeft;
  Result.SetName(Result.Name);
  Inc(Counter);
end;

procedure TForm1.btnAddClick(Sender: TObject);
var
  Frame: TMyFrame;
begin
  Frame := GetNewFrame;
  FFrames.Add(Frame);
end;

procedure TForm1.btnReplaceClick(Sender: TObject);
var
  NewFrame: TMyFrame;
  OldFrame: TMyFrame;
begin
  if FFrames.Count = 0 then exit;

  NewFrame := GetNewFrame;
  OldFrame := FFrames[FFrames.Count - 1];
  FFrames[FFrames.Count - 1] := NewFrame;
  OldFrame.Free;
end;

end.

 

[Alexander Sviridenkov 357]

By default TObjectList owns contained objects. So create it with False parameter in constructor or use Extract to remove object from list.

[RaelB 4]

Thanks - didn't realise it would affect the operation I was doing.

Edited February 11, 2023 by RaelB

[RaelB 4]

So, if I keep OwnObjects True, I can just call:

  FFrames[FFrames.Count - 1] := GetNewFrame;

I don't need to worry about Freeing the previous object.

[programmerdelphi2k 237]

by doing it this way, you're saying something like this: ob1 now you're obj2... so wouldn't it be more correct for you to "remove obj1" and "add obj2"?  

see procedure "Remove()" and "Add()" from the list.

 if the list is not the "owner" of the object then you will need to release it from memory to avoid memory leak

[RaelB 4]

Quote

you're saying something like this: ob1 now you're obj2...

Well no. We're not saying ob1 is now obj2. The ObjectList is pointing to one object, then pointing to a different object. And when you make that change, it automatically frees the first object. This I didn't know before.. 🙂

[programmerdelphi2k 237]

then, the obj1 not more exists not? it was removed not? (implicitly)

[David Heffernan 2345]

13 hours ago, programmerdelphi2k said:

wouldn't it be more correct for you to "remove obj1" and "add obj2"?  

see procedure "Remove()" and "Add()" from the list

No. 

 

13 hours ago, programmerdelphi2k said:

if the list is not the "owner" of the object then you will need to release it from memory to avoid memory leak

When OwnsObjects is true, as it is here, the list is the owner. 

[David Heffernan 2345]

18 hours ago, RaelB said:

So, if I keep OwnObjects True, I can just call:

  FFrames[FFrames.Count - 1] := GetNewFrame;

I don't need to worry about Freeing the previous object.

Yes. 

[Remy Lebeau 1394]

On 2/11/2023 at 3:10 PM, Alexander Sviridenkov said:

By default TObjectList owns contained objects. So create it with False parameter in constructor or use Extract to remove object from list.

Or, use TList<TMyFrame> instead, which doesn't try to own objects.