Re: Dynamically tagged expression not allowed. Why?

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Wed, 02 Jun 2010 01:36:17 +0200, Randy Brukardt <randy@rrsoftware.com>  
a écrit:
> You have to explicitly use a
> type conversion here:
>
>     Conn : Class_Type := 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
“Neither the target type nor the operand type shall be limited”

> to specify that you want to truncate the returned object, or make the  
> object
> classwide:
>
>    Conn : Class_Type'Class := 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 := 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 := 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.