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From: "Alejandro R. Mosteo" <alejandro@mosteo.com>
Newsgroups: comp.lang.ada
Subject: Re: Community Input for the Maintenance and Revision of the Ada
 Programming Language
Date: Thu, 14 Sep 2017 14:30:56 +0200
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Xref: news.eternal-september.org comp.lang.ada:48129
Date: 2017-09-14T14:30:56+02:00
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On 13/09/17 18:34, Jeffrey R. Carter wrote:
> On 09/13/2017 03:07 PM, Alejandro R. Mosteo wrote:
>> On 12/09/17 19:43, Jeffrey R. Carter wrote:
>>> On 09/12/2017 04:36 PM, Egil H H wrote:
>>>> On Tuesday, September 12, 2017 at 3:43:40 PM UTC+2, Dmitry A. 
>>>> Kazakov wrote:
>>>>> So it can do with the custom type:
>>>>>
>>>>> with Ada.Text_IO;  use Ada.Text_IO;
>>>>> procedure Test is
>>>>>      Max : constant := 1000;
>>>>>      subtype Max_Length is Natural range 0..Max;
>>>>>      type Bounded (Size : Max_Length := Max_Length'Last) is record
>>>>>         Length : Max_Length := 0;
>>>>>         Text   : String (1..Size);
>>>>>      end record;
>>>>>
>>>> ...
>>>>>      Y : Bounded (10);
>>>>
>>>> Also, allocating 1000 Characters for a string of max length 10 is 
>>>> hardly an optimal solution...
>>>
>>> If an actual discriminant is provided, the default is not used. This 
>>> only allocates 10 characters.
>>
>> Just going back to my basics, I think it's been years since I used 
>> default discriminants. In such a case, if I tried to later assign a 
>> larger instance to this Y, would it work, given that only 10 chars 
>> were allocated initially?
> 
> No. If you supply a discriminant, the object is constrained to that 
> subtype. Only is you do not supply a discriminant is the object 
> unconstrained.

Thanks (also for the demo), I suspected as much.

Then I guess that an in/out parameter of such a type might be unsafe to 
assign, in the sense that depending on the actual argument it might 
succeed or raise.