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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: How come Ada isn't more popular?
Newsgroups: comp.lang.ada
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Date: Fri, 9 Feb 2007 15:02:59 +0100
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Date: 2007-02-09T15:02:59+01:00
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On Fri, 09 Feb 2007 07:50:43 -0500, Robert A Duff wrote:

> Maciej Sobczak <no.spam@no.spam.com> writes:
> 
>> The ideal solution is called "constructor", everything else is just
>> patchwork. Returning limited types is a compromise that can be
>> satisfying in most cases - that's why it's good that Ada 2005 provides
>> at least this feature.
> 
> I presume by "constructor", you mean the C++ style feature, right?
> 
> Why is that superior to just using functions as constructors?
> Ada has:
> 
>     type T(<>) is ...
> 
> to indicate that clients MUST call some constructor function when
> creating objects.  And it now allows constructor functions (and
> aggregates) for limited types.

Because it does not compose upon inheritance and aggregation.

When inherited the constructing function need to be overridden, even if
null record was the only thing added. Further one can barely publicly
derive anything from T, which breaks encapsulation.

When aggregated:

   type Outer  is record
      Inner : T .= Constructor_T (...);
   end 

there is no way to evaluate the parameters of T's constructing function
from ones of Outer.

IMO, the constructor concept is cleaner and more general than constructing
functions, which are as little functions as ":=" in X : T := 123; is an
assignment.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de