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 autolearn=ham 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!news3.google.com!news2.volia.net!border1.nntp.ams.giganews.com!nntp.giganews.com!fi.sn.net!newsfeed2.fi.sn.net!feeder2.news.jippii.net!feeder1.news.jippii.net!nntp.inet.fi!inet.fi!newsfeed1.nokia.com!news1.nokia.com!news2.nokia.com.POSTED!not-for-mail From: rick H Subject: Re: Instantiating private types with discriminants? Newsgroups: comp.lang.ada References: <1147252198.138173.203910@j73g2000cwa.googlegroups.com> Organization: home User-Agent: tin/1.8.1-20060215 ("Mealasta") (UNIX) (CYGWIN_NT-5.1/1.5.19(0.150/4/2) (i686)) Message-ID: Date: Wed, 10 May 2006 13:44:00 GMT NNTP-Posting-Host: 172.26.210.110 X-Complaints-To: newsmaster@nokia.com X-Trace: news2.nokia.com 1147268640 172.26.210.110 (Wed, 10 May 2006 16:44:00 EET DST) NNTP-Posting-Date: Wed, 10 May 2006 16:44:00 EET DST Xref: g2news2.google.com comp.lang.ada:4175 Date: 2006-05-10T13:44:00+00:00 List-Id: 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. What I had to do is to create two new types - one to access Type_A and one to access Type_B. Then I can call Put without dereferencing Var_A, but at the expense of having to perform a type-conversion, e.g. Put (Type_B_Ptr(Var_A)); Obviously this gets me back to where I was before - having to check the type prior to converting that type to either Type_A_Ptr or to Type_B_Ptr... if Var_A.all in Type_A then Put (Type_A_Ptr (Var_A)); elsif Var_A.all in Type_B then Put (Type_A_Ptr (Var_A)); end if; Rick