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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: How to: communication between multiple tasks using protected
 objects - with no polling?
Date: Wed, 21 Jan 2015 08:11:32 +0000
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Date: 2015-01-21T08:11:32+00:00
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Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> Another approach might be
>
> type Q_ID is (Q1, Q2, ...);
>
> type Q_Item (Q : Q_ID := Q_ID'First) is record
>    case Q is
>    when Q1 =>
>       Item_1 : Q1_Item;
>    when Q2 =>
>       Item_2 : Q2:Item;
>    ...
>    end case;
> end record;
>
> protected Qs is
>    entry Get (Item : out Q_Item);
> private -- Qs
>    Q_1 : Q1_Q;
>    Q_2 : Q2_Q;
>    ...
> end Qs;
>
> protected body Qs is
>    entry Get (Item : out Q_Item) when
>       not Q_1.Is_Empty or not Q_2.Is_Empty or ...
>    is
>       -- Empty declarative part
>    begin -- Get
>       if not Q_1.Is_Empty then
>          Item := (Q => Q1, Item_1 => Q_1.Get);
>       elsif not Q_2.Is_Empty then
>          item := (Q => Q2, Item_2 => Q_2.Get;
>       ...
>       end if;
>    end Get;
> end Qs;

Yet another approach: make Q_Item an abstract tagged type with an
abstract Handle operation. You'd need the actual queue inside the PO to
be of Q_Item'Class (or perhaps an access to such, in which case you'd
need to do your own deallocation).