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

[Steven Schveighoffer]

On Thu, 02 Jun 2011 18:39:48 -0400, Jonathan M Davis <jmdavisp...@gmx.com>  
wrote:

On 2011-06-02 15:13, Steven Schveighoffer wrote:
On Thu, 02 Jun 2011 17:52:41 -0400, Jonathan M Davis  
<jmdavisp...@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.

Any place you have double-dispatch, you need to use it in the dispatch  
function.

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.

added complexity?  How so?  There are four types of casts:

static_cast -> cast without discarding const or volatile
const_cast -> cast only removing const or volatile
dynamic_cast -> do a runtime cast to see if an object is actually a  
derived type
reinterpret_cast -> force everything, and avoid any cast overloads.

In all of these, only const_cast and reinterpret_cast are unsafe.  Each  
one clearly separates what you wish to do.

this is not complication, this is expressiveness.  Something that D's  
casting does not have.  D's casting conflates multiple things together,  
and because of the "I trust you" nature that casts have, you can easily  
make mistakes without complaint from the compiler.  The fact that a  
dynamic cast is inherently unsafe in D is unnecessary.  I don't know if it  
will change, but I can't say it's better than C++.  It can be made better  
than C++, but I think I'd rather have the four cast types than what D has  
now.

-Steve