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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: storage error: stack overflow
Date: Tue, 18 Aug 2015 15:56:04 -0500
Organization: JSA Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:27496
Date: 2015-08-18T15:56:04-05:00
List-Id: <comp.lang.ada>

"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:mqvejr$hai$2@dont-email.me...
> On 08/18/2015 07:02 AM, hreba wrote:
>>
>> Huh? As much as I know you cannot declare variables of a type with 
>> unknown
>> storage requirements at compile time, such as class-wide types and arrays 
>> with
>> unknown range.
>
> These are "indefinite types". You can declare an object of an indefinite 
> type as
> long as you supply an initialization expression:
>
> S : String := "Hello";
>
> V : T'Class := Some_Function;

And if you need to change the type of the object after initialization, you 
can use the "Holder" container, which exists for this very purpose:

      package T_Class_Holder is new Ada.Containers.Indefinite_Holders 
(T'Class);

      V : T_Class_Holder.Holder;
  begin
      V.Replace_Element (Some_Function);
      ...
      V.Replace_Element (Some_Other_Function);
      ...
   end;

In Ada 2012, you can also use the reference function to modify the object, 
but you can't change the tag that way. That is:

    V.Reference := Some_Function;

As noted, this will raise Constraint_Error if the tag of V.Reference is 
changed.

I may sound like a broken record (scratched CD?? ;-), but using a Holder 
rather than explicit access types lets the library do the storage management 
rather than the programmer. There are much less likely to be bugs that way.

And of course, you can do this with all of the containers; if you need a Map 
or Tree or Vector [including an array] of T'Class, just instantiate the 
appropriate container and again let someone at your compiler vendor get the 
storage management right, rather than doing it yourself.

                                              Randy.