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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Empty Bounded Containers
Date: Fri, 23 May 2014 07:00:06 +0100
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Date: 2014-05-23T07:00:06+01:00
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sbelmont700@gmail.com writes:

> In the sundry Ada distributions I have around, the "empty" bounded
> containers are fully defined as having a capacity of zero, e.g.
>
> Empty_Vector : constant Vector := (Capacity => 0, others => <>);
>
> which is not intuitively what I would assume.  For instance, the
> following line:
>
> x : BC.Vector (Capacity => 42) := BC.Empty_Vector;
>
> implies an container with a capacity of 42 initialized to a length of
> zero, but it of course throws a constraint error.  In fact, there
> really doesn't seem to be any valid use of the Empty_Vector (et al),
> since the actual capacity is hidden in the private part.  The LRM just
> says it should be the same between the two without addressing the
> discriminant, which isn't much help.
>
> Moreover, the wording seems to legally contradict itself, since by
> stating "If an object of type Vector is not otherwise initialized, it
> is initialized to the same value as Empty_Vector" it would seem to
> imply that
>
> x : BC.Vector (Capacity => 42);
>
> ought to use the values from the Empty_Container, causing the same
> error, leaving no actual way to ever have any bounded containers at
> all.

It is initialized to the same value in the sense that your second 'x'
above "=" BC.Empty_Vector.

(Am I the only one left who thinks of the Ada 95 Booch Components on
seeing 'BC'? some initial confusion here!)