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-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,1038b3dc09d106c2 X-Google-Attributes: gid103376,public X-Google-ArrivalTime: 2001-09-04 15:33:03 PST Path: archiver1.google.com!newsfeed.google.com!newsfeed.stanford.edu!cyclone.bc.net!arclight.uoregon.edu!wn4feed!worldnet.att.net!207.115.63.142!newscon02.news.prodigy.com!newsmst01.news.prodigy.com!prodigy.com!postmaster.news.prodigy.com!newssvr11.news.prodigy.com.POSTED!not-for-mail From: "Pat Rogers" Newsgroups: comp.lang.ada References: <3B9547D9.BE2CC282@avercom.net> Subject: Re: Dispatching Example X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 5.50.4522.1200 X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4522.1200 Message-ID: NNTP-Posting-Host: 208.191.184.67 X-Complaints-To: abuse@prodigy.net X-Trace: newssvr11.news.prodigy.com 999641937 ST000 208.191.184.67 (Tue, 04 Sep 2001 18:18:57 EDT) NNTP-Posting-Date: Tue, 04 Sep 2001 18:18:57 EDT Organization: Prodigy Internet http://www.prodigy.com X-UserInfo1: TSU[@I_A\S@USVT^ORHL_IXBUSXHQD\MNPWZKB]MPXHZUSAANVUEAE[YETZPIWWI[FCIZA^NBFXZ_D[BFNTCNVPDTNTKHWXKB@X^B_OCJLPZ@ET_O[G\XSG@E\G[ZKVLBL^CJINM@I_KVIOR\T_M_AW_M[_BWU_HFA_]@A_A^SGFAUDE_DFTMQPFWVW[QPJN Date: Tue, 04 Sep 2001 22:18:57 GMT Xref: archiver1.google.com comp.lang.ada:12727 Date: 2001-09-04T22:18:57+00:00 List-Id: Yet AARM 3.9.2{19a} sez: "Ramification: This includes the cases of a tag-indeterminate procedure call, and a tag-indeterminate function_call that is used to initialize a class-wide formal parameter or class-wide object. " Hence I would have thought (and indeed, have taught) that: C : T1'Class := Is_Item; is a call to the function returning a value of type T1. "Tucker Taft" wrote in message news:3B9547D9.BE2CC282@avercom.net... > Ed Colbert wrote: > > > > Hi All, > > > > Below is a simplified example from Richard Riehle's & my course on Ada. > > GNAT v3.13p says that the call Is_Item is an mbiguous expression, and > > ObjectAda v 7.2 says the statement is illegal based on LRM 5.2(6). Both > > Richard and I thought that the expression is tag indeterminate, LRM > > 3.9.2(6), and would static resolved, LRM 3.9.2(19). Are Richard and I > > missing something or are both compilers wrong? > > The call on Is_Item is ambiguous, even though one of the > possible interpretations (the one returning type T1) violates > 5.2(6). This is one of those cases where the overload > resolution rules are less picky than the legality rules, > which can be a bit confusing. > > ObjectAda is not being very helpful by telling you that one > of the interpretations is illegal, without first telling > you that the expression is considered ambiguous by the > overloading rules. > > In any case, if only the Is_Item that returned type T1 were > directly visible, the expression would not be ambiguous, and > the call would be legal. It would involve a "dispatching > on result" where the tag of "C" would determine which body > of Is_Item were called. > > In general the overload resolution and legality rules relating > to classwide assignment are confusing at best. We tried other > rules, but they had worse problems, so I suppose the current > rules are the best of a set of unpleasant alternatives. > ObjectAda's error message was unfortunately only adding to > the confusion. > > -Tucker Taft stt@avercom.net > > > > package Dispatching_Examples > > is > > type T1 is tagged private; > > function Is_Item return T1; > > > > type T2 is new T1 with private; > > function Is_Item return T2; > > > > private > > -- full definition of T1 and T2 > > type T1 is tagged null record; > > type T2 is new T1 with null record; > > end Dispatching_Examples; > > > > with Dispatching_Examples; > > use Dispatching_Examples; > > procedure Dispatching_Examples_Driver is > > A, B : T1; -- simple variables > > Q, R : T2; -- simple variables > > C : T1'Class := A; -- class-wide variable > > D : T2'Class := R ; -- class-wide variable > > begin > > C := Is_Item ; -- static resolution? > > end Dispatching_Examples_Driver; > > -- > -Tucker Taft stt@avercom.net http://www.avercom.net > Chief Technology Officer, AverCom Corporation (A Titan Company) > Bedford, MA USA (AverCom was formerly the Commercial Division of AverStar: > http://www.averstar.com/~stt)