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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Dynamic allocation of unconstrained types
Date: Wed, 30 Sep 2009 10:50:05 -0400
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Date: 2009-09-30T10:50:05-04:00
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Maciej Sobczak <see.my.homepage@gmail.com> writes:

> procedure Test is
>
>    package P is
>       type T (<>) is limited private;
>       function Create return T;
>    private
>       type T is limited record
>          I : Integer;
>       end record;
>    end P;
>
>    package body P is
>       function Create return T is
>       begin
>          return T'(I => 123);
>       end Create;
>    end P;
>
>    S : access P.T;

I try to avoid the use of anonymous access types.
They cause too many surprises.

> begin
>    S := new P.T'(P.Create);   -- ??? (this is line 22)
> end Test;
>
> GNAT says:
>
> test.adb:22:19: uninitialized unconstrained allocation not allowed
> test.adb:22:19: qualified expression required

Looks like a bug in the compiler.

> Interestingly, it works with Strings.
> Why doesn't GNAT recognize it as a qualified expression?
>
> I would like to allocate dynamically something that has a constructor
> function. There is no other way to create the object than with that
> function and presumably it should be possible to use it with dynamic
> allocation.
> How can I do it?
>
> BTW - when preparing this example I tried first with empty (null)
> record, but got stuck with proper way to return an instance of T. I
> remember there was some older discussion about it, but for some reason
> I cannot find it and the following:
>
> return T'(others => <>);

    return (null record);

or 

    return T'(null record);

The "others => <>" should work, too.

> is rejected as well.
> What is the proper way to create null aggregates?

- Bob