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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: How to return an empty aggregate
Date: Sun, 6 Apr 2008 12:53:11 -0700 (PDT)
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Date: 2008-04-06T12:53:11-07:00
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On 6 Kwi, 00:36, george.p...@gmail.com wrote:

> > I would create the derived (concrete) log that is *empty* and does
> > nothing and pass it as Log'Class to wherever it is expected.
> > It cannot be simpler than that.
>
> So you derive concrete from not-empty type?

I derive empty concrete from the interface:

type Log is interface;
procedure Put (L : in Log; S : in String) is abstract;

-- ...

type Null_Log is new Log with null record;
procedure Put (L : in Null_Log; S : in String) is
begin
  -- some pragma here to shut up warnings
  -- about unused parameters
  null;
end Put;

--
Maciej Sobczak *www.msobczak.com*www.inspirel.com