[fpc-devel] Redefine FillChar() to use out parameter instead

Alexander Klenin klenin at gmail.com
Thu Nov 19 11:21:03 CET 2009

I still do not fully understand.

On Thu, Nov 19, 2009 at 19:20, Martin <lazarus at mfriebe.de> wrote:

> Jonas Maebe wrote:

[skipped example]

> Well in this case, the code is actually positively affected by the "out"
> param (because it avoids the mem leak)

Jonas, can you confirm that your example is incorrect one,
and Martin's example below is actually what you meant?

> But if you ever handled ref-counted data by hand.....
> type TStringArray = Array of string;
> procedure InsertOne(a: TStringArray);
> var l : integer;
> begin
>  l := length(a)
>  SetLength(a, l + 1)
>  Move(a[0], a[1], l*SizeOf(String));
>  // now a[0] and a[1] point to the same string
>  // but the string has still a ref-count of 1
>  // so assigning a[0] := nil => would release the memory used by a[1]
>  FillChar(a[0], SizeOf(String), 0);
> end;
> the above works, because Filchar has a var param. otherwise it would
> finalize the string, and a[1] would point to released memory....

Ugh, now this is what I consider to be very ugly.
I did actually wrote exactly this kind of code for Delphi, but it never even
occurred to me to rely on that undocumented distinction between
out and var parameters. I wrote something like
   Integer(a[0]) := 0;

So I'd say that this the kind of broken code not worth supporting.

Alexander S. Klenin

More information about the fpc-devel mailing list