Re: asynchronous task communication

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

"Simon Wright" <simon@pushface.org> wrote in message 
news:lyvcb7v0lk.fsf@pushface.org...
> Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:
>
>> On 01/08/2013 11:18 AM, Simon Wright wrote:
>>> Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:
>>>
>>>> If you have an item in your (unbounded) array, indexed by Positive, at
>>>> index 3, it will still be at index 3 if you assign to components at
>>>> other indices, or extend the array by adding new components beyond the
>>>> current end of the array. However, if you insert a new component
>>>> before the existing item at index 3, then all the components after the
>>>> new component will shift up, and the existing item will then be at
>>>> index 4.
>>>>
>>>> One would think that a Cursor would behave similarly.
>>>
>>> Not so! The Cursor becomes ambiguous[1], and using it is a bounded
>>> error. GNAT GPL 2012 implements Cursor using an Index, & behaves the way
>>> you'd expect.
>>
>> To my mind, that is Cursor behaving similarly. Just as the index 3 is
>> no longer the index of the item in question, so the Cursor should not
>> be relied on.
>
> In AdaCore's implementation, yes. But another implementation might
> validly raise PE. To my mind, that'd be preferable.

That's what Janus/Ada will do (whenever I get the containers written). It's 
actually fairly cheap to check, and it's silly to allow a bug to go 
undetected if it can be detected without too much expense. It's not possible 
to check indexes, of course, so this will be an advantage to using cursors. 
(But usually you use a vector rather than a list because you need 
computability of indexes, which of course you can't do with cursors.)

                                   Randy.