Question

I want to pass an object A to a second object B, have B do some processing and finally release A in case it’s not needed anymore. A watered down version is given below.

program Project6;
{$APPTYPE CONSOLE}
uses
  SysUtils;
type
  TMyObject = class(TObject)
  public
    FField1:  string;
    FField2:  string;
  end;
  TBigObject = class(TObject)
  public
    FMyObject:  TMyObject;
    procedure Bind(var MyObject:  TMyObject);
    procedure Free();
  end;
procedure TBigObject.Bind(var MyObject: TMyObject);
begin
  FMyObject := MyObject;
end;
procedure TBigObject.Free;
begin
  FreeAndNil(FMyObject);
  Destroy();
end;
var
  MyObject:   TMyObject;
  BigObject:  TBigObject;
begin
  try
    MyObject := TMyObject.Create();
    BigObject := TBigObject.Create();
    BigObject.Bind(MyObject);
    BigObject.Free();
    if (Assigned(MyObject)) then begin
      WriteLn('Set MyObject free!');
      MyObject.Free();
    end;
    ReadLn;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

(Never mind the awful design.) Now, what I don’t understand is why FreeAndNil actually does free MyObject, yet Assigned(MyObject) is evaluated to true (giving an AV at MyObject.Free()).

Could someone please help enlighten me?

Answer 1

The reason is simple, you nil one reference but not the other. Consider this example:

var
  Obj1, Obj2: TObject;
begin
  Obj1 := TObject.Create;
  Obj2 := Obj1;
  FreeAndNil(Obj1);
  // Obj1 is released and nil, Obj2 is non-nil but now points to undefined memory
  // ie. accessing it will cause access violations
end;