Accidentally killing immutable is too easy in D (Was: cast()x - a valid expression?)

[Jonathan M Davis]

On 2011-06-02 15:13, Steven Schveighoffer wrote:
> On Thu, 02 Jun 2011 17:52:41 -0400, Jonathan M Davis <jmdavisProg at gmx.com>
> 
> wrote:
> > On 2011-06-02 09:02, Bruno Medeiros wrote:
> >> On 01/06/2011 22:12, Jonathan M Davis wrote:
> >> >> It's nice in a way, but it's all based on the way cast handles
> >> >> modifiers.
> >> >> 
> >> >> > And I've always been a bit unconfortable with how that's handled
> >> 
> >> (In
> >> 
> >> >> > fact, I was just thinking about this yesterday). Specifically, it
> >> >> > seems extremely bad that it's so incredibly easy to accidentaly
> >> 
> >> cast
> >> 
> >> >> > away things like const and immutable:
> >> >> > 
> >> >> > For example, if I know I have an array of uint's, and I want to
> >> 
> >> deal
> >> 
> >> >> > with the individual bytes, it's perfectly safe and sensible to cast
> >> >> > it to a ubyte[] (a long as you factor in endianness, of course).
> >> 
> >> So,
> >> 
> >> >> > you do "cast(ubyte[])myArray". But, OOPS!!: If myArray happened to
> >> >> > be immutable, then merely trying to cast the type has inadvertantly
> >> >> > cast-away immutable. Not good! Casting away const/immutable really,
> >> >> > really should have to be explict.
> >> >> > 
> >> >> > Of course, you can probably use some fancy helper templates to make
> >> >> > sure you preserve all modifiers. But needing to do so is just
> >> 
> >> asking
> >> 
> >> >> > for mistakes: it seems like a huge violation of "make the right way
> >> >> > easy, and the wrong way hard".
> >> > 
> >> > You really shouldn't be casting much anyway. It's the sort of feature
> >> > where you're only supposed to use it when you know what you're doing
> >> > when you use it. And if there's really any possibility that you're
> >> > dealing with immutable, perhaps you should be casting to const rather
> >> > than mutable. Personally, I find how C++ created multiple types of
> >> 
> >> cast
> >> 
> >> > (include const_cast)_highly_ annoying and generally useless, and
> >> > I'm_very_ glad that D didn't do anything of the sort.
> >> > 
> >> > - Jonathan M Davis
> >> 
> >> "you're only supposed to use it when you know what you're doing when you
> >> use it"
> >> Well, that's kinda the requirement for any feature, isn't it?... :P
> >> Well, kinda, I do know what you mean. Yes, one needs to properly learn
> >> how cast() works, but that doesn't mean it would not be better for the
> >> feature to be designed in a way that is easier to learn (and just as
> >> powerful to use), or actually, it doesn't mean that even if you do know
> >> how cast() works, that you are not still likely to make a mistake when
> >> using it.
> > 
> > Really, you should only be using casts when you need a cast, and you
> > should be
> > careful when you use them. So, you have to know what you're doing and
> > why in
> > that particular instance and not use them willy-nilly. In most code,
> > casts
> > should be quite rare. If you're doing a lot of low-level stuff, then they
> > might be more common, but in general, they're a sledgehammer that
> > shouldn't be
> > used except when you really need them.
> 
> Casting is the recommended way to determine if something is actually a
> derived type:
> 
> interface I {}
> 
> class C : I {}
> 
> void foo(I i) // change this to const(I) and you have a huge const bug!
> {
> if(C c = cast(C)i)
> {
> // optimized branch
> }
> else
> {
> // default branch
> }
> }
> 
> so it's not so easy to say you should "never" cast. BTW, dcollections
> uses this quite a bit to support operations between multiple types of
> collections.
> 
> It would be nice if at least dynamic cast (with the added rules to prevent
> casting away const/immutable/shared) was a different syntax from cast,
> since it's a completely safe usage of casting (as long as you manually
> forward modifiers). It's as safe as to! is when used with the right
> context.

No. It's not the case that you should _never_ cast, but in most programs, it 
should still be rare. And yes, casting is the proper way to determine whether 
an object's type, is derived from a particular class, but it's generally 
considered bad OO programming to have to ask that sort of question very often. 
There are a few places that you're forced to (such as opEquals), but in 
general, it's a symptom of a bad design. Code should generally be written in 
such a manner that you either know what it's type is or don't care. You 
shouldn't generally have to ask it. Obviously, there are exceptions, but they 
should be exceptions rather than the norm. As such, casts should be fairly 
rare.

Personally, I find the added complexity of C++'s special casts to _far_ 
outweight what benefit they give you. Yes, having something like Java's 
instanceof instead of having to cast to check whether an object's type is 
derived from a particular type would arguably be an improvement, but even it 
shouldn't be used all that often, and I definitely don't think that creating a 
set of cast operators for different types of cast would be a good idea at all.

- Jonathan M Davis