Re: "out" and "in out"

["Nick Roberts" <nick.roberts@acm.org>]

On Mon, 26 Jul 2004 19:58:22 +1000, zork <zork@nospam.com> wrote:

> ...
> By chance I created a small program as follows:
>
> ===========
> s : string := "CAT";
>
> procedure modify ( s1 : out string ) is
> begin
>    s1(2) := 'U';
> end modify;
>
> ..
>
> put ( modify(s) );
> ===========
>
> now I get as a result "CUT", and i dont understand why i get
> this result.

> Doesnt the "out" specify that its initial value isnt passed
> in via the parameter?

Yes, but it doesn't forbid the compiler to (effectively) pass
the value in. In practice, this is likely to happen if the
parameter is of a composite type (an array or record), because
the compiler will probably choose to pass it by reference.

> But it seems to be passed in the above. In fact the "out"
> is acting like an "in out".

Yes. This is permitted. But your program would be wrong to
rely upon it.

If you were to try:

===========
n : integer := 456;

procedure modify ( n1 : out integer ) is
begin
    n1 := n1+44;
end modify;

...

modify(n);
put(n);
===========

I think you might not get 500 printed out (I don't, running
this on GNAT 3.15p on Win XP (and GNAT gives a warning)).

-- 
Nick Roberts