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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
 =?utf-8?Q?=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: Parameterless generic instance in place of opaque type instance
Date: Sun, 28 Jul 2013 09:02:22 +0200
Organization: Ada @ Home
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Xref: news.eternal-september.org comp.lang.ada:16571
Date: 2013-07-28T09:02:22+02:00
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Le Sun, 28 Jul 2013 08:36:36 +0200, Yannick Duch=C3=AAne (Hibou57)  =

<yannick_duchene@yahoo.fr> a =C3=A9crit:
> The Ada 95 Q&S Guide mention this pattern, and seems to have no more t=
o  =

> say about it, not more than what is obvious:
> http://www.adaic.org/resources/add_content/docs/95style/html/sec_8/8-3=
-5.html
> They call this pattern, =E2=80=9CGeneric Abstract Data Object=E2=80=9D=
.

An idea, and there may have missed another interesting variation,  =

something with mixed properties from a GADO and an ADT.

Say a GADO still defines an ADT, with an unknown discriminant, so that i=
t  =

cannot be instantiated without initialization. What the private declares=
,  =

is just an access type to a deferred type (defined in the body). The bod=
y  =

defines an instance of the type it fully defines. The package offers a  =

single function returning an (opaque) access to this unique and  =

per=E2=80=91paquage instance, instance. The function could be named  =

`Instance_Reference`, to clearly show it's always the same behind, howev=
er  =

a new distinct one per generic instance (not strictly a singleton, a  =

per=E2=80=91package singleton).

This would provide some of the benefit of an ADT and some of a GADO,  =

together. The same as an ADT regarding the ability to pass it to a  =

sub=E2=80=91program, while avoiding the heap allocation (which I surely =
don't  =

want) otherwise required with an access to a deferred type (or else an  =

allocation in a static pool, =E2=80=A6 not so nice for many reasons) and=
 avoiding  =

to pollute the private part of the package (I don't enjoy when the priva=
te  =

part monopolizes 85% of a package's text).

This could be named, say =E2=80=9CGeneric Abstract Singleton Data Type=E2=
=80=9D, or  =

something like that :-D

What's you feeling on that variation?

-- =

=E2=80=9CSyntactic sugar causes cancer of the semi-colons.=E2=80=9D [1]
=E2=80=9CStructured Programming supports the law of the excluded muddle.=
=E2=80=9D [1]
[1]: Epigrams on Programming =E2=80=94 Alan J. =E2=80=94 P. Yale Univers=
ity