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Date: Tue, 27 Mar 2018 13:26:46 -0700 (PDT)
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Subject: Overriding procedure as null
From: gautier_niouzes@hotmail.com
Injection-Date: Tue, 27 Mar 2018 20:26:46 +0000
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Xref: reader02.eternal-september.org comp.lang.ada:51217
Date: 2018-03-27T13:26:46-07:00
List-Id: <comp.lang.ada>

Hello,

I've just come across a little "legal" case that was easy to solve practically, but I'm curious about what is "right" Ada.

Consider the following program:

  with Ada.Streams;

  procedure test_overriding is

    type Search_stream is new Ada.Streams.Root_Stream_Type with null record;

    overriding procedure Read
      (Stream : in out Search_stream;
       Item   :    out Ada.Streams.Stream_Element_Array;
       Last   :    out Ada.Streams.Stream_Element_Offset);

    overriding procedure Write
     (Stream : in out Search_stream;
      Item   : in     Ada.Streams.Stream_Element_Array);

    --  Implementation of Read & Write:

    overriding procedure Read
      (Stream : in out Search_stream;
       Item   :    out Ada.Streams.Stream_Element_Array;
       Last   :    out Ada.Streams.Stream_Element_Offset) is null;  --  Unused

    overriding procedure Write
     (Stream : in out Search_stream;
      Item   : in     Ada.Streams.Stream_Element_Array) is
    begin
      null;  --  Something done in real case.
    end;

  begin
    null;
  end;

GNAT (GPL 2017/Win32) compiles seamlessly.
ObjectAda (9.2) issues, for the 2nd "Read":
  test_overriding.adb: Error: line 18 col 24 LRM:8.3(26), Illegal to override declaration in same region, prior declaration is Read at line 7

It looks like OA considers the 2nd occurrence as a specification, not a body - with "is null", it could be both actually.
Who's right ?

To make both compilers happy I just put the "Read(...) is null" at the first place and remove the 2nd one, so it is only a theory/law problem.

More interestingly, if I completely remove the Read procedure, OA still compiles fine and GNAT issues an error  (rightfully I think):
test_overriding.adb:5:10: type must be declared abstract or "read" overridden
test_overriding.adb:5:10: "read" has been inherited from subprogram at a-stream.ads:57

_________________________ 
Gautier's Ada programming 
http://www.openhub.net/accounts/gautier_bd