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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Running a preprocessor from GPS?
Date: Sat, 1 Aug 2015 09:53:15 -0700
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Xref: news.eternal-september.org comp.lang.ada:27300
Date: 2015-08-01T09:53:15-07:00
List-Id: <comp.lang.ada>

On 08/01/2015 01:06 AM, EGarrulo wrote:
> 
> I can't see how Ada lets you postpone the creation of a singleton until a time
> of your choosing, without resorting to Get_Instance.  What is the equivalent of
> this code:
>
> if Some_Condition then
>    Singleton := Singleton_Package.Get_Instance;
>    -- Use singleton.
> end if;

The Ada equivalent is

if Some_Condition then
   -- Call operations of Singleton_Package
end if;

You don't have a variable for a singleton; the pkg is the singleton. The need
for a variable in other languages is why this "pattern" is a workaround for the
lack of modules in those languages.

If for some reason your application has a requirement that the storage for the
state of the pkg not be allocated until the first use of the pkg, then the
implementation of the pkg can allocate that storage the 1st time the pkg is
used. The clients of the pkg use it the same way in both cases.

Such a requirement is rare in my experience. What is more common is that the
initialization of the pkg state requires information that is not available
during elaboration; in that case, the pkg has an Initialize operation that is
called when the information is available.

-- 
Jeff Carter
"I don't know why I ever come in here. The
flies get the best of everything."
Never Give a Sucker an Even Break
102