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From: Shark8 <onewingedshark@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: GNAT - return by anonymous access
Date: Mon, 19 Dec 2011 16:14:34 -0800 (PST)
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Date: 2011-12-19T16:14:34-08:00
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On Dec 19, 5:46=A0pm, "Randy Brukardt" <ra...@rrsoftware.com> wrote:
> "Simon Belmont" <sbelmont...@gmail.com> wrote in message
>
> news:784c67eb-a542-41b0-b23d-fca1234e56b2@n10g2000vbg.googlegroups.com...
>
>
>
> > This ostensibly simple program prints strange results...
> ...
> > =A0 type test_type (p_obj : access Integer) is limited null record;
> ...
> > =A0 function get return test_type is
> > =A0 begin
> > =A0 =A0 =A0return test_type'(p_obj =3D> new Integer'(42));
> > =A0 end get;
>
> You are creating a coextension here. The *language rules* for coextension=
s
> are full of bugs; we're fixing them virtually every ARG meeting. There is
> little hope for a compiler to get these right in the face of that, beyond
> which doing so is quite expensive.
>
> IMHO, any use of an allocator for an anonymous access type is asking for
> trouble. There is no way to free the memory for such things, and the
> lifetime can easily be extended in various ways, so if the compiler tries
> using a properly local pool for the allocator you end up with a dangling
> poiner. Moreover, they appear to promise something that they cannot deliv=
er
> (heap-based allocation).
>
> Ada 2012 has a restriction No_Anonymous_Allocators; I recommend that all =
Ada
> programs include and obey this restriction. (Janus/Ada does this by defau=
lt,
> because we never attempted to use the local pools that supposedly are
> required, and it is better to reject a program rather than implement it
> "wrong".)
>
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 Randy.

Hm, so then are local pools useless? Or do they have their place?
I read some of the rational[e/ization] for them and it seems
reasonable, but I've not actually used them.