On Saturday, May 21, 2016 17:15:16 Jack Applegame via Digitalmars-d-learn wrote: > On Friday, 20 May 2016 at 20:46:18 UTC, Jonathan M Davis wrote: > > Casting away const and mutating is undefined behavior in D. No > > D program should ever do it. > > Really? Mutating immutable is UB too, but look at > std.typecons.Rebindable.
Rebindable is in kind of a weird grey area. It involves a union, not casting, and it's only ever mutating the class reference, not the object itself. Certainly, if it mutated the object, it would be undefined behavior, but it's just mutating the reference - but then again, the type system itself doesn't really differentiate between the two. So, I suspect that what it comes down to is that Rebindable is doing something that you really shouldn't, but because it's using a union rather than a cast, and it's so limited in what it's mutating, its behavior is effectively defined in the sense that the compiler has no leeway, but's probably still technically undefined. I don't know what Walter would say though. Certainly, if casting were involved, it would be undefined behavior without question, and it's not the sort of thing that you should be doing in your own code. So, Rebindable may actually be breaking the rules, but it's so useful that we really have no other choice as long as there isn't a solution in the actual language, and it's so limited in how it goes about it, that there's really no way that it won't work even if it's not technically valid. Personally, I think that what Rebindable has to do internally to work is the biggest strike against it, but I doubt that it's enough to convince Walter that we need a proper in-language solution given how much of a horrid pain it supposedly is to sort it out in the compiler. - Jonathan M Davis
