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=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,8e7f9bcdc1a5b9cd,start X-Google-Attributes: gid103376,public From: Corey Minyard Subject: Seemingly wierd conversion for in out parameter Date: 1998/02/20 Message-ID: #1/1 X-Deja-AN: 326904326 Sender: minyard@wf-rch.cirr.com Organization: Wonderforce Research Newsgroups: comp.lang.ada Date: 1998-02-20T00:00:00+00:00 List-Id: I've been playing with some techniques with derived types and I ran across a rather unusual things. The following program: with Text_IO; use Text_IO; with Ada.Tags; use Ada.Tags; ------------------------------------------------------------------------ procedure Tester is type Base is abstract tagged null record; type Base_Class is access all Base'Class; type Der1 is new Base with null record; type Der1_Class is access all Der1'Class; type Der2 is new Base with null record; type Der2_Class is access all Der2'Class; procedure Process (Var : in out Base_Class) is begin Var := new Der2; end Process; V1 : Der1_Class := new Der1; -- This will be a compile error. -- U1 : Der1_Class := new Der2; begin Put_Line("Base tag = " & Expanded_Name(Base'Tag)); Put_Line("Der1 tag = " & Expanded_Name(Der1'Tag)); Put_Line("Der2 tag = " & Expanded_Name(Der2'Tag)); Put_Line("V1 tag = " & Expanded_Name(V1'Tag)); Process(Base_Class(V1)); Put_Line("V1 tag = " & Expanded_Name(V1'Tag)); end Tester; ------------------------------------------------------------------------ will compile and produce the following output: ------------------------------------------------------------------------ Base tag = TESTER.BASE Der1 tag = TESTER.DER1 Der2 tag = TESTER.DER2 V1 tag = TESTER.DER1 V1 tag = TESTER.DER2 ------------------------------------------------------------------------ As you can see, the variable V1 should not be able to reference a tagged type of Der2, but this program does that without any errors under GNAT (the only compiler I have right now). I would think that the compiler wouldn't allow a class conversion passed to an in out parameter, but it seems to. I'm certainly no language lawyer, but I would image that doing a cast would make something not a "variable" any more (RM 6.4.1(5)), thus this would not be legal. Is GNAT wrong, or is this legal, or is this a grey area? Thanks, -- Corey Minyard Internet: minyard@acm.org Work: minyard@nortel.ca UUCP: minyard@wf-rch.cirr.com