Re: Question about out parameters of unconstrained array type.

[Phil Thornley <phil.jpthornley@gmail.com>]

In article <20120226132532.7fb8ec1b@t60>, mail@senier.net says...
> 
> On Sun, 26 Feb 2012 09:14:53 -0000
> Phil Thornley <phil.jpthornley@gmail.com> wrote:
> 
> > For your code above, if the cost of including the initialization of the 
> > array is too high then the easiest way to remove the the error is 
> > probably to add another parameter that is the length of the Description 
> > string. Using Alexander's code:
> 
> Can anybody comment on the legality of using attributes of an out parameter of
> an unconstrained (array) type in Ada? A brief skim through LRM could not
> enlighten me...
> 
> If using 'Length (and other attributes) elsewhere than in a loop constraint
> actually is legal Ada, than this seems like an unnecessary limitation of SPARK
> that should be relaxed. For now, I would prefer accepting the resulting flow
> error for several reasons:
> 
> * Default initialization may hide subsequent flow errors that would
>   been spotted by the flow analysis otherwise (the coarse grained treatment
>   of array makes that less important, though)
> 
> * As Phil said, (additional) default initialization may impose a significant
>   performance overhead
> 
> * Passing in the upper bound of the array as a parameter IMHO makes the code
>   less intuitive and readable than it could be - just to silence the flow
>   analysis ;-(
> 

I fully agree with all of this.

It looks as if the statement in Section 4.1 of the LRM:
"... the name of an unconstrained array object (formal parameter) shall 
only appear in the following contexts:
   1  as the prefix of an attribute reference;
   2  ..."
needs some qualification.

(I can't find anything in the release notes to suggest that this is 
already seem as a deficiency in the language or tool.)

Cheers,

Phil