WIth Delphi 5 and later, it is my understanding it is better to use

FreeAndNil(objMyObject);

Thoughts and comments?

Hmmm...sure, that's a good rule of thumb and thanks for adding the info here (I added it to the flashcard above too). Bottom line.

However, to further the discussion I guess it depends on the situation. It is a routine and not a class method so using Free "feels" more OOP to me so I tend to use it with simple OOP examples when giving demos. ((I know, that's a pretty weak excuse but I think it helps with new coders.)) You do have to be careful to only pass TObjects to this routine too. My own rule of thumb with larger applications is that I use Free when doing simple controlled tasks and FreeAndNil with more complicated things.

Also, when do we stop writing extra checks? You could be even more complete and do something like the following:

If Assigned(MyObject) then
  FreeAndNil(MyObject)
Else
  MyObject := nil;

If by some chance MyObject was not assigned to begin with, then FreeAndNil() will never get called and therefore MyObject will never get set to nil.

Should Be NilAndFree

Finally a minor point but because we're getting into it a bit, I thought it might be important because FreeAndNil actually does the opposite. You can run into problems accessing the object variable while it's being destroyed.

For reference, here are the three VCL code routines in discussion:

In System Unit:

//TObject.Free

procedure TObject.Free;
begin
  if Self <> nil then
    Destroy;
end; 

//TObject.Destroy

destructor TObject.Destroy;
begin
end;

In SysUtils unit:

//FreeAndNil

procedure FreeAndNil(var Obj);

var 

  Temp: TObject;

begin 

  Temp := TObject(Obj); 

  Pointer(Obj) := nil; 

  Temp.Free;

end;

Whether to use Free, or FreeAndNil, is an interesting topic.  It depends on the circumstances.

My rule of thumb is that if an object is local (that is, it's reference variable will soon go out of scope) just use Free.  But if the object is more "global," use FreeAndNil.

A classic example is a form that will create and show another form.  This is often done within the scope of a button click handler, or an ActionItem's Execute method:

procudure MyForm.Button1Click(Sender : TObject);

var 

  StudentForm : TStudentForm;

begin

  StudentForm := TStudentForm.Create(Self);

  StudentForm.ShowModal;

  StudentForm.Free;

end;

In the above snippet, the local variable, StudentForm, isn't accessible outside of the click handler, and it will go out of scope fairly soon.  In this case, I think Free is sufficient.

However, if I'd made my reference variable more "global," say as a private member, FStudentForm, of TMyForm, any and every method of that class has access to it. In that case, I'll use FreeAndNil:

procedure MyForm.Button1Click(Sender : TObject);

begin

  if (FStudentForm = nil) then

    FStudentForm := TStudentForm.Create(Self);

  FStudentForm.ShowModal;

  FreeAndNil(FStudentForm);

end;

Just to play it safe, you'll probably want to set FStudentForm to nil in TMyForm's OnCreate event.

Of course, it does no harm to use FreeAndNil wherever it's legal (As Mike says, the argument to FreeAndNil must be a TObject, or descendant thereof).

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