Re: Ada2012 Invariants and obaque types

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Tue, 21 Jun 2011 14:08:15 +0200, Dmitry A. Kazakov  
<mailbox@dmitry-kazakov.de> a écrit:

> On Tue, 21 Jun 2011 01:53:31 -0700 (PDT), Martin wrote:
>
>> package P1 is
>>    type T1 is tagged private
>>       with Invariant => Is_Valid (T1);
>
> Unrelated to Ada, but in theory, an invariant is a private implementation
> dependent thing. An invariant is trivially true in all public views of  
> the object,
I understand the point and had the same first feeling too. While that's OK  
in theory, in practice the user may wish methods to check for validity  
rules defined for a type. Then, Is_Valid is abstract enough. And  
after-all, you already have 'Valid attribute in Ada. This is useful for  
designs relying on defensive programming and which disallow use of  
exceptions.

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