Re: [Newbie] doubly constrained array, dumb question

["Randy Brukardt" <randy@rrsoftware.com>]

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"J-P. Rosen" <rosen@adalog.fr> wrote in message 
news:p71rvj$vgh$1@gioia.aioe.org...
> Le 26/02/2018 à 17:26, Mehdi Saada a écrit :
>> Hello.
>>
>> I would like an constrained String subtype with narrower bounds that 
>> Positive'Range.
>> How can I do something like:
>> subtype Possible_Length is NATURAL range 1..80;
>> subtype T_Line is String (Possible_Length range <>);
>> ? I know "range <>" isn't included in the definition of 
>> "range_constraint". How can I express the same thing.
>> So that I can get after, a dynamic string with:
>> A: access T_LINE := new T_LINE'("BLABLABLA"); while checking for its 
>> range.
>>
> You can't do that, because a subtype can't be both constrained and
> unconstrainde at the same time...
>
> OTOH, you can define your own string type:
> type Short_String is array (Possible_length range <>) of character;

Or you could use a subtype with a dynamic predicate:

    subtype Short_String is String
       with Dynamic_Predicate => Short_String'First >= Possible_Length'First 
and Short_String'Last <= Possible_Length'Last;

This would be checked any time that you convert a string value into a 
Short_String subtype (explicitly or implicitly), so it probably would give 
the right effect. Note that unlike a real constraint, it wouldn't have any 
effect on other subtypes, so:
     Silly_Object : Short_String (1..100);
would not raise Constraint_Error, but any attempt to assign into it would 
raise Assertion_Error. (Assuming the Assertion_Policy is Check, it isn't for 
GNAT by default.)

                 Randy.