From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM autolearn=unavailable autolearn_force=no version=3.4.4 X-Received: by 2002:ac8:47cc:: with SMTP id d12mr27021601qtr.246.1582206423405; Thu, 20 Feb 2020 05:47:03 -0800 (PST) X-Received: by 2002:a54:4106:: with SMTP id l6mr2019354oic.76.1582206423252; Thu, 20 Feb 2020 05:47:03 -0800 (PST) Path: eternal-september.org!reader01.eternal-september.org!feeder.eternal-september.org!news.gegeweb.eu!gegeweb.org!usenet-fr.net!proxad.net!feeder1-2.proxad.net!209.85.160.216.MISMATCH!news-out.google.com!nntp.google.com!postnews.google.com!google-groups.googlegroups.com!not-for-mail Newsgroups: comp.lang.ada Date: Thu, 20 Feb 2020 05:47:02 -0800 (PST) In-Reply-To: <3c675051-911e-44b2-9eba-c4712892e6dd@googlegroups.com> Complaints-To: groups-abuse@google.com Injection-Info: google-groups.googlegroups.com; posting-host=85.203.169.79; posting-account=X2aVgQoAAAC9R0AqSW-isDhmlPhqd7-0 NNTP-Posting-Host: 85.203.169.79 References: <3c675051-911e-44b2-9eba-c4712892e6dd@googlegroups.com> User-Agent: G2/1.0 MIME-Version: 1.0 Message-ID: Subject: Re: Different aliasing rules for containers? From: "Martin B. B." Injection-Date: Thu, 20 Feb 2020 13:47:03 +0000 Content-Type: text/plain; charset="UTF-8" Xref: reader01.eternal-september.org comp.lang.ada:58096 Date: 2020-02-20T05:47:02-08:00 List-Id: On Thursday, 20 February 2020 14:36:38 UTC+1, joak...@kth.se wrote: > Den torsdag 20 februari 2020 kl. 13:24:37 UTC+1 skrev Martin B. B.: > > The following code: > > > > with Ada.Containers.Vectors; > > with Ada.Text_IO; > > > > procedure Test is > > > > package IO renames Ada.Text_IO; > > > > type Obj is record > > Id : Integer; > > end record; > > > > package Obj_Vectors is new Ada.Containers.Vectors > > (Index_Type => Positive, > > Element_Type => Obj); > > > > use type Obj_Vectors.Vector; > > > > Objects : Obj_Vectors.Vector := Obj'(Id => 0) & Obj'(Id => 1) & Obj'(Id => 2); > > > > begin > > for O of Objects loop > > O.Id := 999; > > end loop; > > > > for O of Objects loop > > IO.Put_Line (Integer'Image (O.Id)); > > end loop; > > end Test; > > > > Using GNAT Community 2019 (20190517-83) produces the following output: > > 999 > > 999 > > 999 > > > > When looking at the Ada.Containers.Vectors package specification, I see the following: > > type Vector is tagged private > > with > > Constant_Indexing => Constant_Reference, > > Variable_Indexing => Reference, > > Default_Iterator => Iterate, > > Iterator_Element => Element_Type; > > > > However, both the `Rerefence` and `Constant_Reference` functions takes aliased parameters: > > function Reference > > (Container : aliased in out Vector; > > Position : Cursor) return Reference_Type; > > > > My question is then: How is the code above possible then, when my `Objects` vector is not aliased? Shouldn't the compiler complain that I haven't declared `Objects` as being aliased? > > > > Even creating my own procedure that takes a `V : aliased Obj_Vectors.Vector` still produces no aliasing errors when passing it a non-aliased vector. > > > > This code: > > with Ada.Text_IO; > > > > procedure Test is > > > > package IO renames Ada.Text_IO; > > > > procedure Takes_Aliased_Integer (X : aliased Integer); > > > > procedure Takes_Aliased_Integer (X : aliased Integer) is > > begin > > null; > > end Takes_Aliased_Integer; > > > > My_Int : Integer := 123; > > > > begin > > Takes_Aliased_Integer (My_Int); > > end Test; > > > > However, correctly(?) generates the following compiler error: "actual for aliased formal "X" must be aliased object". > > > > So, what exactly is going on here? Am I missing something? > > Hi Martin, > > Could it be because Obj_Vectors.Vector is a tagged type and is therefore always passed by reference in subprogram calls and that implies it is aliased? > > Best regards, > Joakim Ah, it seems you are correct! I was not aware of this difference between tagged and non-tagged types. It seems to be what is mentioned here in the reference manual: "Finally, a formal parameter or generic formal object of a tagged type is defined to be aliased" (http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-3-10.html#p9) Thank you for the help :)