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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: record extension aggregate for returned type legal?
Date: Wed, 13 Oct 2004 09:52:46 +0200
Organization: AdaCL
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Date: 2004-10-13T09:52:46+02:00
List-Id: <comp.lang.ada>

Georg Bauhaus wrote:

> Martin Krischik <krischik@users.sourceforge.net> wrote:
> : T' (spoken T kick) tells the compiler that the next expression is of
> : type T. i.E: Positive'(1) means that you want a Positive and not an
> : Integer. If you won't say anything the compiler will choose automaticly.
> 
> In this case T is specific, and `make` is not a primitive operation
> of T (because make is in child package B). So does the compiler
> have a choice at all?
> 
> LRM 4.3.2, quoted in one compiler's error message, says,
> 
>   4. The expected type for an extension_aggregate shall be a single
>      nonlimited type that is a record extension. If the ancestor_part
>      is an expression, it is expected to be of any nonlimited tagged
>      type.
> 
> Do I understand this phrase? The expected type in the return expression
> for "function make return T" is the specific tagged type T.
> In "return (n => 0)", there is no extension aggregate.
> In "return (T with n => 0)", there is an extension aggregate,
> the ancestor part being T.

But T in your example is a root class and therefore has no ancestor. T needs
an standart aggregate and not an extension aggregate.

> T is not limited.
> T is not an extension (a record extension?).
> Does T have an ancestor (does T maybe count as an ancestor of T here?)

Consider the other constructor pattern:

type P is abstract tagged private;
type C is new P with private;

function Create returns P'Class;

function Create
returns
    P'Class
is
begin
    return C'(P with ...);
end Create

With Regards.

Martin

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