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From: "Alejandro R. Mosteo" <alejandro@mosteo.com>
Newsgroups: comp.lang.ada
Subject: Re: Unknown constraints and type composition
Date: Fri, 15 Jun 2018 11:34:53 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:53118
Date: 2018-06-15T11:34:53+02:00
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On 15/06/2018 07:13, J-P. Rosen wrote:
> Le 14/06/2018 à 19:53, Jeffrey R. Carter a écrit :
>> On 06/14/2018 05:37 PM, Alejandro R. Mosteo wrote:
>>> I think I have read somewhere that types with unknown constraints are
>>> a good way of ensuring you (or your users) don't end with
>>> uninitialized values:
>>>
>>> types Whatever (<>) is [limited] private;
>>>
>>> function Create return Whatever;
>>
>> It's one way. There are others that might be better. Unknown
>> discriminants are more for generic formal types, to show that the
>> generic accepts indefinite actual types.
>>
>> One way to deal with this is to make the full type a record with
>> reasonable defaults for all the components. This works for all versions
>> of the language.
>>
> Even better, the default can be a raise expression (or a function that
> raises an exception for pre-2012), so no uninitialized object can be
> created. This is a run-time check, but a decent compiler would warn you
> at compile time.

Interesting. So you can have the definite type, but you're forced to 
provide a initialization, and are warned about it. Seems one trick to 
remember, thanks!