How to break const

[deadalnix]

Le 19/06/2012 14:30, Artur Skawina a écrit :
>> Due to D concept of weak purity, this doesn't ensure the required what we need here.
>
> Actually, it does - if it can be proved that the delegate can't alter the object
> via the context pointer (eg because whatever it points to is not mutable) then
> even D's "pure" is enough. Because the delegate would need to be passed a mutable
> ref to be able to alter the object, which then could hardly be constructed as
> "breaking" constness.
>
> But such a limit (const/immutable context) would be a problem for the cases where
> the delegate needs to alter some state (like export the result of some operation),
> but does _not_ modify the object it's embedded in. Note that the current object may
> very well be reachable (and mutable) from the delegate - but at some point you have
> to trust the programmer. Sure, fixing this hole would be great, but /how/ - w/o
> incurring unacceptable collateral damage?
>

This isn't a problem as long as the delegate isn't a member of the 
object. If it is, transitivity is broken, which is something you don't want.

Relying on the trust on the programmer is a dumb idea. Human do mistake, 
way more than computers. The basic behavior MUST be a safe one.

Transitivity has been introduced for good reason and language already 
provide a way to break it via cast. So it is unacceptable to rely on 
programmer on that point.

>> It is possible to get the error when trying to call the delegate instead of preventing to make it const, as I said in the post you quote. It is probably a better solution.
>
> Any delegate?
>

No, any delegate that have type that isn't covariant with the expected 
delegate type.