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 10.25.86.130 with SMTP id k124mr829243lfb.6.1466302461623; Sat, 18 Jun 2016 19:14:21 -0700 (PDT) X-Received: by 10.157.60.49 with SMTP id q46mr296177otc.0.1466302461528; Sat, 18 Jun 2016 19:14:21 -0700 (PDT) Path: eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!feeder.eternal-september.org!goblin1!goblin.stu.neva.ru!oe3no4536501lbb.1!news-out.google.com!di11ni29010lbb.1!nntp.google.com!oe3no4536500lbb.1!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail Newsgroups: comp.lang.ada Date: Sat, 18 Jun 2016 19:14:21 -0700 (PDT) In-Reply-To: <66c14298-c62d-4f4b-b0c0-e969454f9334@googlegroups.com> Complaints-To: groups-abuse@google.com Injection-Info: glegroupsg2000goo.googlegroups.com; posting-host=71.171.109.232; posting-account=vk6_JwoAAABkMyHO1YfdP69Hm3CpbdGR NNTP-Posting-Host: 71.171.109.232 References: <66c14298-c62d-4f4b-b0c0-e969454f9334@googlegroups.com> User-Agent: G2/1.0 MIME-Version: 1.0 Message-ID: <2e39857a-7121-476b-807a-d2bff1e598f4@googlegroups.com> Subject: Re: Generic Embedded List Nodes From: Jeremiah Injection-Date: Sun, 19 Jun 2016 02:14:21 +0000 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Xref: news.eternal-september.org comp.lang.ada:30801 Date: 2016-06-18T19:14:21-07:00 List-Id: On Saturday, June 18, 2016 at 6:52:06 PM UTC-4, Warren wrote: > Getting back to Ada after a hiatus, I currently have a need to build a=20 > generic package to implement "embedded list node" lists. The advantage=20 > is high performance in a server setting to avoid underlying malloc/free > calls. >=20 > The idea is that the list node resides within the structure/tagged type, > and acts as a doubly linked list node when in a list. The Emb_Node can > be used as a list head when itself (or as part of another structure). Thi= s > kind of thing is done in the Linux kernel, for example. > =20 > The Emb_Node and its operations are trivial. The problem occurs when > you traverse a linked list of Emb_Nodes (or its derived type). With > a given node, I need to then access the object that _contains_ it. In > C/C++ you do some offset calculation from the node address back to > the owning struct/class. >=20 > My idea (in Ada) was to save some kind of access value in a type > derived from Emb_List. But that is where the trouble starts. >=20 > package Emb_List is > =20 > type Emb_Node is tagged private; > =20 > procedure Insert_Head(Head: access Emb_Node; Node: access Emb_Node); > procedure Unlink(Node: access Emb_Node); >=20 > private >=20 > type Emb_Node is tagged > record > Next: access Emb_Node; -- Ptr to next node in list (if any) > Prev: access Emb_Node; -- Ptr to prev node in list (if any) > end record; > =20 > end Emb_List; > =20 > The generic extension (below) was intended to hold the reference to the > containing object, which is where the trouble starts. I was hoping this > would work, but running into "missing full declaration for private > extension". >=20 > generic > type Object_Type is tagged private; > package Emb_List.Nodes is >=20 > type Node_Type(Obj: access Object_Type) is new Emb_Node; > =20 > function Object(Node: Node_Type) return Object_Type; > =20 > end Emb_List.Nodes; Change Node_Type to: type Node_Type(Obj: access Object_Type) is new Emb_Node with null record; You need an extension, even if it is a null record. > The other end of the challenge will be to have the Node_Type save the > owning object access value at the time it is created: >=20 > type Something_New_Type is tagged > record > Stuff: Natural; > Timeout_Node: Node_Type(some access to this record); > ... > end record; > =20 > I am trying to avoid using 'Address for this, since there is likely a > way to do this "right". >=20 > Warren. Well, I'm only a novice, but the only way I have gotten something like that= was to use incomplete types: type Something_New_Type; package SNT_Pkg is new Emb_List.Nodes(Object_Type =3D> Something_New_Typ= e); use SNT_Pkg; =20 type Something_New_Type is tagged limited record Stuff: Natural; Timeout_Node: Node_Type(Something_New_Type'Access); end record;=20 This means, however, that you would need to change Emp_List.Nodes to use in= complete types: generic type Object_Type; package Emb_List.Nodes is =20 type Node_Type(Obj: access Object_Type) is new Emb_Node with null record= ; =20 end Emb_List.Nodes;=20 However, you would need to rethink your function to return the Object as yo= u cannot return a complete type when using incomplete types. You might be = stuck with just using the access value instead or perhaps with some other d= esign changes you can come up with a better solution. Again, I am still learning, so maybe some of the more experienced users can= comment.