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From: hreba <f_hreba@yahoo.com.br>
Newsgroups: comp.lang.ada
Subject: Re: storage error: stack overflow
Date: Wed, 19 Aug 2015 08:56:22 -0300
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Xref: news.eternal-september.org comp.lang.ada:27504
Date: 2015-08-19T08:56:22-03:00
List-Id: <comp.lang.ada>

On 08/19/2015 05:44 AM, Georg Bauhaus wrote:
> On 19.08.15 03:48, hreba wrote:
>> My knowledge of Ada exceeds Ada 95 only by very little but I am pretty
>> sure that in such a case one uses variables of a class-wide type, and
>> as one cannot initialize them one has to declare them as access types.
>
> The assumption that limited objects cannot be initialized like non-limited
> (by sort of copying a value) is no longer true, and it wasn't the whole
> truth
> in Ada 95.
> There are cases when nothing but an access type will do. But due
> to build-in-place, limited tagged objects can be declared and initialized,
> by aggregate or by function call, using just traditional syntax as
> Jeffrey Carter explains. This includes objects of a class-wide type,
>
> Built-in-place (in situ), perhaps simplifying,  means that the limited
> object is declared and passed/used behind the scenes, for initialization.
> For example, the object that is declared becomes the to-be-returned
> object if there is a function call after “:=” in the declaration.
> (See "extended return" of Ada 2005.)
>
> Given
>
> package Classwide is
>
>     type T0 is abstract tagged limited null record;
>
>     type T1 is new T0 with private;
>     function Make return T1;
>
>     function Produce (Variant : Boolean) return T1'Class;
>
> private
>     type T1 is new T0 with
>        record  -- note default values for "default construction"
>           Name : Character := '?';
>        end record;
> end Classwide;
>
> package body Classwide is
>     type T2 is new T1 with
>        record
>           Count : Natural;
>        end record;
>     function Make return T2;
>
>     function Make return T1 is
>     begin
>        return T1'(T0 with Name => '*');
>     end Make;
>
>     function Make return T2 is
>     begin
>        return T2'(T1'(Make) with Count => 0);
>     end Make;
>
>     function Produce (Variant : Boolean) return T1'Class is
>     begin
>        case Variant is
>        when False =>
>           return T1'(T0 with Name => 'T');
>        when True =>
>           return T2'(T0 with Name => 'F', Count => 0);
>        end case;
>     end Produce;
> end Classwide;
>
>
> Then, the type hierarchy above includes limited T0, T1, and invisible T2.
> All of these types can be used to declare or create objects of specific
> types and of class-wide types.
>
> with Classwide;
> procedure Main is
>     use Classwide;
>     X  : T1;
>     C  : T1'Class := Make;
>     CC : T1'Class := Produce (Variant => False);
>     procedure Takes_T1C (Object : in T1'Class) is separate;
>     procedure Mods_T1C (Object : in out T1'Class) is separate;
> begin
>     Takes_T1C (X);
>     Takes_T1C (C);
>     Takes_T1C (CC);
>     Takes_T1C (Make);
>     Takes_T1C (Produce (Variant => True));
>     Mods_T1C (X);
>     Mods_T1C (C);
>     Mods_T1C (CC);
> end Main;
>
> The Ada Rationale will illustrate and explain.
>

Thanks for your explications Jeffrey an Georg. Apparently Barnes: 
"Programming in Ada 95" is obsolete with respect to this topic (not 
regarding correctness, just best practices). What didactic text about 
object oriented programming in Ada would you recommend?

-- 
Frank Hrebabetzky		+55 / 48 / 3235 1106
Florianopolis, Brazil