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=-0.3 required=5.0 tests=BAYES_00, REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4 X-Google-Thread: 103376,dad94612ff745427 X-Google-Attributes: gid103376,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news4.google.com!news2.volia.net!newsfeed01.sul.t-online.de!t-online.de!news.belwue.de!newsfeed.arcor.de!news.arcor.de!not-for-mail From: "Dmitry A. Kazakov" Subject: Re: Instantiating private types with discriminants? Newsgroups: comp.lang.ada User-Agent: 40tude_Dialog/2.0.15.1 MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Reply-To: mailbox@dmitry-kazakov.de Organization: cbb software GmbH References: <1147252198.138173.203910@j73g2000cwa.googlegroups.com> Date: Wed, 10 May 2006 16:41:07 +0200 Message-ID: NNTP-Posting-Date: 10 May 2006 16:41:11 MEST NNTP-Posting-Host: 4809bfb5.newsread4.arcor-online.net X-Trace: DXC=9?V8o X-Complaints-To: usenet-abuse@arcor.de Xref: g2news2.google.com comp.lang.ada:4177 Date: 2006-05-10T16:41:11+02:00 List-Id: On Wed, 10 May 2006 13:44:00 GMT, rick H wrote: > Ludovic Brenta wrote: >> rick H writes: >>> Dmitry A. Kazakov wrote: >>> > BTW, You should probably make Put a primitive operation of General_T. Then >>> > the whole could look like simply: >>> > >>> > Put (Var_A.all); -- Do what have to be done >>> >>> I'm still a bit new to Ada terminology - does "making Put a primitive >>> operation" mean doing this kind of thing with a generic package... >>> type Fred is new Integer; >>> package Fred_IO is new Ada.Text_IO.Integer_IO (Fred); >>> use Fred_IO; >>> If that's right - how would I do this in my example? I'm puzzled >>> because Var_A.all.Data will resolve to either an Integer or a Float at >>> run-time. Have I got to declare two new packages - one against >>> Integer_IO and one for Float_IO and let the dispatcher get it right >>> at run-time? >> >> A "primitive operation" of a type T is a subprogram that: >> >> - takes a parameter of type T or "access T", or returns a result of >> type T or "access T" >> >> - is declared in the same package specification as T (same declarative >> region, really) >> >> If these conditions are met, then the operation is primitive. If, in >> addition, T is tagged, then a call to a primitive operation dispatches >> dynamically at run time according to the exact type T. Essentially, the >> compiler does the tag comparison and dispatching for you. So you could >> write: >> >> package P is >> type T is tagged null record; >> procedure Put (Item : in T); -- primitive operation >> >> type Type_A is new T with Data : Integer; end record; >> procedure Put (Item : in Type_A); -- primitive operation >> >> type Type_B is new T with Data : Float; end record; >> procedure Put (Item : in Type_B); -- also primitive >> end P; >> >> package body P is >> procedure Put (Item : in T) is >> begin >> null; >> end Put; -- we could also have declared this procedure abstract >> >> procedure Put (Item : in Type_A) is >> begin >> Ada.Integer_Text_IO.Put (Item.Data); >> end Put; >> >> procedure Put (Item : in Type_B) is >> begin >> Ada.Float_Text_IO.Put (Item.Data); >> end Put; >> end P; >> >> with P; >> procedure Simple_Case is >> Var_A : P.T'Class := ...; -- class-wide type: could be T, Type_A or >> Type_B >> begin >> P.Put (Var_A); -- dynamic dispatch to suitable version of Put >> end Simple_Case; >> > Thanks, Ludovic. Bearing in mind that, in my original example, Var_A is > declared as an *access* to the class-wide type, I tried declaring the > various Put procedures as you showed, and then simply dereferencing > Var_A when calling Put: > Put (Var_A.all); > However, the compiler (gnat) complained that I cannot use a class-wide > argument in this case. You should have tried it in the same body. Do it as Ludovic did, using packages: ---------- Simple_Case_Types.ads package Simple_Case_Types is type General_T is abstract tagged null record; procedure Put (Item : in General_T) is abstract; type Type_A is new General_T with record Data : Integer; end record; procedure Put (Item : in Type_A); type Type_B is new General_T with record Data : Float; end record; procedure Put (Item : in Type_B); end Simple_Case_Types; ---------- Simple_Case_Types.adb with Ada.Integer_Text_IO; with Ada.Float_Text_IO; package body Simple_Case_Types is procedure Put (Item : in Type_A) is begin Ada.Integer_Text_IO.Put (Item.Data); end Put; procedure Put (Item : in Type_B) is begin Ada.Float_Text_IO.Put (Item.Data); end Put; end Simple_Case_Types; ----------- Simple_Case.adb with Simple_Case_Types; use Simple_Case_Types; procedure Simple_Case is type Access_T is access General_T'Class; Var_A : Access_T; -- could be "new Type_A" or a "new Type_B" begin Var_A := new Type_B' (Data => 100.0); Put (Var_A.all); end Simple_Case; This will compile as expected. BTW, I should use pointers only when needed. Differently to C++, in Ada you can create class-wide objects on the stack. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de