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NNTP-Posting-Date: Fri, 31 Dec 2004 15:30:53 -0600
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
References: <mailman.19.1104491242.527.comp.lang.ada@ada-france.org>
Subject: Re: Return_By_Reference or Return_By_Copy (GNAT bug?)
Date: Fri, 31 Dec 2004 15:32:31 -0600
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Date: 2004-12-31T15:32:31-06:00
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"Duncan Sands" <baldrick@free.fr> wrote in message
news:mailman.19.1104491242.527.comp.lang.ada@ada-france.org...
> Consider the following version of the Rosen trick.  Should
> A_Type be returned by reference or by copy?  GNAT 3.15p says:
> by reference; more recent versions of GNAT say: by copy.  My
> understanding is that it should be by reference, because it
> has a component R_Type that is a return_by_reference type
> (R_Type is return_by_reference because the full view is limited
> private).

A_Type certainly is return-by-reference.

But take care: Ada 2005 flushes the entire return-by-reference garbage in
favor of build-in-place limited functions. Such functions only can return
aggregates or build-in-place functions (or be built component-by-component
in an extended return statement). To do what you want in Ada 2005 would
require using an anonymous access function:

    function Get_an_A return access A_Type;

Note that this makes the reference return explicit (no magical rules to page
through).

This probably is the biggest incompatibility in Ada 2005 (although it often
is just changing a runtime check to compile-time). We tried various halfway
solutions, but eventually decided that this was so screwed up that we were
better off just doing it over correctly. (Build-in-place functions can be
used to initialize objects, so we now can write meaningful limited
constants, and don't have to stand on our heads to initialize things.)

Moral: Avoid functions returning limited types in Ada 95 code, since they'll
likely have to be changed in Ada 2005 anyway.

                           Randy.