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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Private extension of a synchronized interface
Date: Sun, 17 Feb 2019 22:36:38 +0000
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Date: 2019-02-17T22:36:38+00:00
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Jere <jhb.chat@gmail.com> writes:

>   -- The following line gives the error:
>   -- call to abstract procedure must be dispatching
>   
>   --v.p1;

If you replace this with the dispatching call

   Example.An_Interface'Class (v).p1;

it compiles & works just fine.

I'm far from being an expert on this, because I've never found the need
to use 'interface' at all, but when you say

      type An_Interface is synchronized interface;
      procedure p1(Self : in out An_Interface) is abstract;

      type Instance is synchronized new An_Interface with private;

you know that Instance provides a procedure p1, but you don't know
whether it's a task or protected entry or a protected procedure (or
even, to my surprise, a plain procedure with that profile, see Ada 2005
Rationale).

Of course the compiler knows, so you'd have thought that your v.p1 call
would be OK (and it _is_ OK if the full declaration of Example.Instance
is public).

But the actual problem comes from ARM 3.9.3(7)[1],

   A call on an abstract subprogram shall be a dispatching call;
   nondispatching calls to an abstract subprogram are not allowed.

... which has been the case since Ada 95!

[1] http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-3-9-3.html#p7