D's greatest mistakes

[Andrei Alexandrescu]

On 11/29/10 10:12 AM, Steven Schveighoffer wrote:
> On Mon, 29 Nov 2010 10:54:04 -0500, Nick Sabalausky <a at a.a> wrote:
>
>> "Steven Schveighoffer" <schveiguy at yahoo.com> wrote in message
>> news:op.vmxqi4kmeav7ka at steve-laptop...
>>> On Sun, 28 Nov 2010 23:19:44 -0500, Jack <jt at overlook.biz> wrote:
>>>
>>>> The post "C#'s greatest mistakes" prompts/begs this post. Have at it,
>>>> pick up the ball and run with it, don't be shy. I expect Walter and
>>>> Andrei to answer (if Walter and Andrei so dare!) after others' posts
>>>> have
>>>> stopped or stagnated into that cesspool of threaded discussion that is
>>>> "the subthread" or "tangential thread" (which surely needs a rock
>>>> anthem).
>>>
>>> As I understand this is a troll post, but still provoked some good
>>> discussion, I'll throw in my biggest problem with D:
>>>
>>> lack of tail-const/immutable for classes.
>>>
>>
>> I've heard this before, but I'm a little unclear. Can you give an
>> example?
>
>
> This is a tail-const pointer:
>
> const(int)* ptr;
>
> I can reassign ptr at will:
>
> ptr = &x;
> ptr = &y;
>
> but I can't change what ptr points to:
>
> *ptr = 5; // error
>
> Because classes are references, just like pointers, I would like a way
> to have a tail-const class reference. But it's not possible:
>
> const(C) myref;
>
> myref = a; // error, cannot change const myref
>
> You can have a tail-const pointer to a class reference:
>
> C myref;
> const(C) *myrefptr = &myref;
>
> But a class is on the heap, and the reference 'myref' is on the stack,
> so myrefptr is only usable locally. In order to keep the 'infinite'
> lifetime property, I have to create a class reference on the heap, and
> then return a pointer to *that*. Just try to create a class reference on
> the heap.
>
> Not only that, but I now have to waste 16 bytes of heap space just to
> have a tail-const class reference, *just because* the syntax is
> incomplete. I find this unacceptable.
>
> -Steve

Syntax is the main issue in implementing this feature. Due to the 
implicit nature of reference semantics for classes, there was no syntax 
to distinguish between the head and the tail when qualifying.

FWIW I just thought of this syntax. It might work but it's not intuitive:

const()C tailConst;

Ultimately I believe we need to make Rebindable palatable. That would 
have the nice side effect of enabling other proxy types.


Andrei