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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: Dynamically tagged expression not allowed. Why?
Date: Thu, 03 Jun 2010 04:36:12 +0200
Organization: Ada At Home
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Date: 2010-06-03T04:36:12+02:00
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Le Wed, 02 Jun 2010 01:36:17 +0200, Randy Brukardt <randy@rrsoftware.com=
>  =

a =C3=A9crit:
> You have to explicitly use a
> type conversion here:
>
>     Conn : Class_Type :=3D Class_Type (New_Class_Instance);
>
This one is not legal, as Class_Type is limited and conversion are not  =

allowed on limited type due to their copy based semantic.
[ARM 2005 4.6] says about type conversions
=E2=80=9CNeither the target type nor the operand type shall be limited=E2=
=80=9D

> to specify that you want to truncate the returned object, or make the =
 =

> object
> classwide:
>
>    Conn : Class_Type'Class :=3D New_Class_Instance;
That is a beautiful one (to my eyes)


> But notice that while both of these are legal, they'll both raise
> Program_Error because an access-before-elaboration error (the body of
> New_Class_Instance hasn't been elaborated at the point of this call).
Not necessarily, as he can move the body of the function returning  =

Class_Type before the declaration of the variable initialized from this =
 =

function.

Instead of


    function New_Class_Instance return Class_Type'Class;

    Conn : Class_Type :=3D New_Class_Instance;

    function New_Class_Instance return Class_Type'Class is
    begin
       -- Just a stub
       return New_Class_Instance;
    end New_Class_Instance;


Simply do


    function New_Class_Instance return Class_Type'Class;

    function New_Class_Instance return Class_Type'Class is
    begin
       -- Just a stub
       return New_Class_Instance;
    end New_Class_Instance;

    Conn : Class_Type :=3D New_Class_Instance;

and everything will go fine.

-- =

There is even better than a pragma Assert: a SPARK --# check.
--# check C and WhoKnowWhat and YouKnowWho;
--# assert Ada;
--  i.e. forget about previous premises which leads to conclusion
--  and start with new conclusion as premise.