Developing a plan for D2.0: Getting everything on the table

[Lars T. Kyllingstad]

Eldar Insafutdinov wrote:
> dsimcha Wrote:
> 
>> == Quote from Jarrett Billingsley (jarrett.billingsley at gmail.com)'s article
>>> On Tue, Jul 14, 2009 at 12:42 PM, Andrei
>>> Alexandrescu<SeeWebsiteForEmail at erdani.org> wrote:
>>>>> - opImplicitCast
>>>> I think alias this should render that unnecesary.
>>> 'alias this' might cover a lot of cases, but this is the pretty big
>>> one that I can think of: consider a Bigint or the like.  You might
>>> want to use such a type transparently in place of any other integer
>>> type, i.e. as an array index.  Something like "a[bi.toSizet()]" looks
>>> pretty awful.  But 'alias this' couldn't work in this case, because
>>> the underlying representation is *not* an integer.  It's probably an
>>> array or somesuch.  opImplicitCast would allow you to transparently
>>> use a Bigint in place of a normal int while still letting you
>>> represent the data any way you want (and letting you check the
>>> validity of the cast at runtime).  Basically any type which represents
>>> its data as something other than what you want to implicitly cast to
>>> would have the same problem.
>> But you can alias this a function, not just a member.  Example:
>>
>> import std.conv;
>>
>> struct Foo {
>>    string num;
>>
>>     uint numToString() {
>>        return to!uint(num);
>>     }
>>
>>     alias numToString this;
>> }
> 
> First of all I remember Walter saying that current limitation of one alias this per class/struct is temporary, it would be nice to hear a confirmation.
> Secondly, what about implicit cast in another way. I want some arbitrary type to be implicitly casted to my type. You can say that I can implement alias this for the former, but what if it is a primitive type. Can constructors be used for this?
> 
> struct A
> {
>     this(int) {
>     ....
>     }
> }
> 
> // int now can be implicitly casted to A. Just a syntax sugar.
> 
> Eldar


I don't think it should be this simple. If I do this:

   class Vector
   {
       this (size_t length) { ... }
   }

I don't want to be able to write this:

   Vector v = 3;

Nor this:

   size_t length(Vector v) { ... }
   assert (length(3) == 3);

I like the idea of being able to specify arbitrary implicit casts, but 
not the syntax. Rather, there should be an opImplicitCastFrom operator 
overload or something.

-Lars