Delphi compiler record memory leak

Question

I realise this is a contrived example but the following code causes a memory leak in Delphi XE2 (and maybe other versions).

type
  TTest =record
    a :string;
    procedure SetA;
  end;

procedure TTest.SetA;
begin
  a:='Test';
end;

procedure TfrmMain.btGoClick(Sender: TObject);
var
  t :TTest;

  procedure SetA(const Rec: TTest);
  begin
    Memo.Lines.Add(IntToStr(NativeInt(@Rec)));

    Rec.SetA;
  end;

begin
  Memo.Lines.Add(IntToStr(NativeInt(@t)));

  SetA(t);

  if t.a='' then
    Memo.Lines.Add('t.a: (empty)')
  else
    Memo.Lines.Add('t.a: '+t.a);
end;

The source of the memory leak is the setting of a in TTest.SetA, the string is never released.

The output of the above is:

1700728
1700688
t.a: (empty)

From this we can see that even though the record parameter is passed as a const the address of Rec is different from the main variable t.

Now if I change the record type definition to:

  TTest =record
    a :string;
    a1 :string;
    a2 :string;
    a3 :string;
    procedure SetA;
  end;

The output is now:

1700712
1700712
t.a: Test

So the same address of Rec and t, also t.a has been set, also no memory leak.

I have tried to look at the CPU but have a hard time seeing the difference when the dummy a1, a2 and a3 variables are added.

Obviously all this can be avoided by making the parameter to SetA a var parameter, but the reason for this question is to try and understand what the compiler is doing in the two different situations.

So what is going on here?

Delphi compiler record memory leak

0 Answers