On Wednesday, September 02, 2015 02:04:58 Sergei Degtiarev via Digitalmars-d-learn wrote: > I can't understand how cast coexist with immutability. Consider > the code: > immutable immutable(int)[4] buf; > auto x=buf[0]; > auto p=buf.ptr; > > auto i=cast(int[]) buf; > i[]=1; > > assert(buf.ptr == p); > assert(buf[0] != x); > > I've just modified immutable data, is it normal? > Another, ever simpler, code: > immutable a=0; > auto b=cast(int*) &a; > *b=1; > assert(&a == b); > assert(a != *b); > Last two assertions both succeed and really puzzle me. What is > pointer in D at all?
You can cast away immutable, just like you can cast away const, but in either case, mutating the data after casting it to mutable is undefined behavior. The compiler is free to assume that you won't do that, and in the case of immutable, if the data is actually in ROM, it could crash your program. The two main places that casting away const or immutable would be used at this point would be to pass it to a C function which isn't properly annotated (but which you know won't mutate the data) or when you cast to immutable to pass something via std.concurrency and then to cast back to mutable on the receiving thread, since we don't currently have a way to indicate ownership in a manner that would allow us to pass mutable data across threads. But if you do that, you need to be sure that you just passed across the only reference to that data, or you're going to have bugs, because anything which isn't immutable or shared is considered thread-local by the compiler. So, there are cases where it makes sense if you're very careful and know what you're doing, but those cases are few and far between. But D allows the cast if nothing else, because it's a systems language and will let you shoot yourself in the foot if you really try to. And when you cast, you're telling the compiler that you know better than it does. So, you'd better be right, or you're going to blow your foot off (figuratively speaking). Regardless, casting away const or immutable and then mutating is undefined behavior. So, don't do it. - Jonathan M Davis
