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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Dynamically tagged expression not allowed. Why?
Date: Tue, 1 Jun 2010 18:36:17 -0500
Organization: Jacob Sparre Andersen
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Date: 2010-06-01T18:36:17-05:00
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"Marc A. Criley" <mcNOSPAM@mckae.com> wrote in message 
news:c2ead$4c0150a0$4ca4a823$11809@API-DIGITAL.COM...
...
> Stripping out a test case, I see that it has nothing to with the C++ 
> binding per se, but it's an Ada issue that's flummoxing me.  Here's the 
> test code:
>
> procedure Dyty_Test is
>
>    type Class_Type is tagged limited record
>       null;
>    end record;
>
>    function New_Class_Instance return Class_Type'Class;
>
>    Conn : Class_Type := New_Class_Instance;

This is illegal because the function returns a class-wide object and you are 
assigning it into an object of a specific type. You have to explicitly use a 
type conversion here:

    Conn : Class_Type := Class_Type (New_Class_Instance);

to specify that you want to truncate the returned object, or make the object 
classwide:

   Conn : Class_Type'Class := New_Class_Instance;

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).

                               Randy.