----- Original Message -----
Subject: [delphi-en] FreeAndNil should be called NilAndFree>
> Well who would have believed that FreeAndNil has a bug in it!
>
> The following code shows the essence of the problem.
>
> type
> TA = Class
> destructor Destroy;
> end;
>
> TB = Class
> destructor Destroy;
> procedure SomeMethod;
> end;
>
> var
> A : TA;
> B : TB;
>
> destructor TB.Destroy;
> begin
> FreeAndNil(A);
> inherited;
> end;
>
> procedure TB.SomeMethod;
> begin
> { Some code that references other fields/methods in TB - The exception
> occurs at this point };
> end;
>
> destructor TA.Destroy;
> begin
> B.SomeMethod; { If using FreeAndNil, then B is already nil at this
> point. SomeMethod is still called, but with a self of 0 }
> inherited;
> end;
>
> begin
> A : TA.Create;
> B : TB.Create;
>
> FreeAndNil(B); <- This fails in TB.SomeMethod. Because B is set to nil
> before it's freed.
>
> B.Free; <- This works.
> B := nil;
> end;
>
> If you check the VCL source code (in Delphi 7 at least) then it really
> does Nil before it Frees.
You have forgotten to put override on the Destructor so Free is not
correctly freeing up memory.
David Bolton
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