On Monday, September 10, 2012 02:05:08 Namespace wrote: > I had never problems with that in C++. > If I have members which are const because they are assigned only > one time and needs no other assignment, why should I declare this > member not as const? > > In the example I know exactly that I assign only one time a name > to this struct, so why I should not declare it as const? > > Other example: you have a unique birthday date. This is const you > cannot change it like a name or a telephone number. So if you > have a Person struct which holds any data of a single people, you > won't declare the date as const? I would. And I had expected that > this is normal behaviour.
Once something is const, you _cannot_ change it. C++ lets you cast away const and mutate things, meaning that they aren't really const. D doesn't allow that. Casting away const and mutating a variable is undefined behavior. D has no mutable keyword. D's const is also transitive, meaning that once something is const, everything within it is const (e.g. once you declare a container const, _every_ element inside it is const as well). So, any and all operations which would involve mutating a const variable are illegal - including mutating something which a const variable refers to, since _everything_ it refers to is const. With structs, this has the effect that once one member variable is const, you can never again reassign the whole thing. You can assign to its non-const members but not the whole thing. In this particular case, you're dealing with an AA. Depending on the AA's implementation, it should be possible to initialize tests[4] and then never assign to it again, but that's problematic due to rehashing (hash table's _need_ to be able to move objects around), and the current AA implementation is fairly poor anyway. So, it ends up default-initializing the value and _then_ assigning it, which doesn't work if you can't reassign the value. It shouldn't be doing that. It's a bug, and it causes problems with stuff like exceptions. For instance, if you have aa[key] = func(); and func throws, then aa ends up with a default-initialized value at key: http://d.puremagic.com/issues/show_bug.cgi?id=3825 druntime's AA implementation needs a fair bit of work. H.S. Teoh is working on a better implementation, but who knows when we'll have it. We just have to put up with buggy corner cases until then unfortunately (though at least AA's work just fine most of the time). I don't know if this particular use case will ever work though, because it makes it so that the struct can never be reassigned, and I'm not sure that it's at all reasonable for an AA to never be able to reassign its values. Yes, initialization should work (unlike now), but rehashing needs to move values around, and hash tables have to be able to rehash when they get full enough, meaning that being able to reassign elements will probably be required. - Jonathan M Davis