Why are Delphi objects assigned even after calling .Free?

Question

In Delphi, why does the Assigned() function still return True after I call the destructor?

The below example code will write \"sl is still assigned\" to the console.

Answer 1

If you use sl.Free, the object is freed but the variable sl still points to the now invalid memory.

Use FreeAndNil(sl) to both free the object and clear the pointer.

By the way, if you do:

var
  sl1, sl2: TStringList;
begin
  sl1 := TStringList.Create;
  sl2 := sl1;
  FreeAndNil(sl1);
  // sl2 is still assigned and must be cleared separately (not with FreeAndNil because it points to the already freed object.)
end;




procedure TObject.Free;
asm
    TEST    EAX,EAX
    JE      @@exit              // Jump to exit if pointer is nil.
    MOV     ECX,[EAX]           
    MOV     DL,1
    CALL    dword ptr [ECX].vmtDestroy  // Call cleanup code (and destructor).
@@exit:
end;