Overriding procedure as null

Overriding procedure as null

[gautier_niouzes]

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

Re: Overriding procedure as null

[Randy Brukardt]

<gautier_niouzes@hotmail.com> wrote in message 
news:77e909f6-9f24-4638-ba6a-1872e5a46580@googlegroups.com...
> 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.
>...
>    overriding procedure Write

You're not supposed to repeat "overriding" on bodies. It should be thought 
of like "generic" or aspects, which are not part of the profile. It's not 
illegal, but it's bad practice.

>     (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 ?

A body freezes everything, and nothing can be overridden after. We had some 
issues with these rules, so it's possible that both are right (Ada 2005 and 
Ada 2012 may differ on these rules).

We've got an ARG meeting on Monday, so I don't have time to investigate 
further, sorry.

                 Randy.