On 11/15/15 9:54 AM, Andrei Alexandrescu wrote:
On 11/15/2015 09:34 AM, Dicebot wrote:
On Sunday, 15 November 2015 at 14:23:05 UTC, Jonathan M Davis wrote:
As I mentioned, he's okay with changing the language to make the
casts well defined. -- Andrei
Well, that's a big change, since it pretty much means that D's const
isn't physical const anymore, and Walter has been _very_ insistent
about that in the past - to the point that he's argued that C++'s
const is outright useless because it isn't physical const. If casting
away const and mutating is well-defined behavior, then we basically
have C++'s const except that it's transitive ...
Casting away _const_ is already legal if programmer can himself
guarantee underlying object has mutable origin (i.e. not available via
immutable reference), by the very definition of const. It is casting
away immutable and mutating that is strictly UB.
Correct. I'm not sure whether that's clarified in the language
documentation yet. -- Andrei
I argued this way, and eventually lost. I don't think it's feasible to
have a const that can be cast away, and have optimizations based on
const or pure.
See this discussion:
forum.dlang.org/post/riiehqozpkyluhhif...@forum.dlang.org
One thing, however, is that if you can mark an island of space within an
object as ALWAYS mutable, the compiler can know this and avoid
optimizing away access to those pieces. It would have to be clearly
marked as such, so the optimizations on non-marked data could still happen.
I think it could be done, because logical const is possible via a global
lookup table. Any time you go through a cast, however, this could easily
break down. I hate to say it, but you would likely need another modifier
like "tainted const" or something in order for that to work.
-Steve
