On Thu, 10 Feb 2011 14:45:14 -0500, Tomek Sowiński <j...@ask.me> wrote:
Michel Fortin napisał:
> Thanks for doing this. Is it approved by Walter?
Depends on what you mean by "approved".
He commented once on the newsgroup after I posted an earlier version of
the patch, saying I should add tests for type deduction and some other
stuff. This change his something he attempted to do in the past and
failed, I expect him to be skeptical.
It would be much easier if he provided the specific case(s) which broke
his teeth. Then we'll all know where's the problem. If it's soluble,
it'll open the door to tail type modifiers in general, not just in
classes. It's a burning issue e.g. with ranges (mostly struct).
http://d.puremagic.com/issues/show_bug.cgi?id=5377
Look at the attachment to get a feel of what hoops we'll have to jump
through to side-step lack of tail X.
I've worked through this very same problem (a few months back), thinking
that we need a general solution to tail-const. The large issue with
tail-const for structs in the general case is that you cannot control the
type of 'this'. It's always ref. This might seem like a very
inconsequential detail, but I realized that a ref to X does not implicitly
convert to a ref to a tail-const X. This violates a rule of two
indirections, in which case you are not able to implicitly convert the
indirect type, even if the indirect type would implicitly convert outside
the reference.
A simple example, you cannot convert an int** to a const(int)**. Reason
being, then you could change the indirect pointer to point to something
that's immutable, and the original int ** now points to immutable data.
The same is for tail-const structs, because you go through one ref via
'this' and the other ref via the referring member.
What does this all mean? It basically means that you have to define
*separate* functions for tail-const and const, and separate functions for
tail-immutable and immutable. This is untenable.
You might ask "why doesn't this problem occur with tail-const arrays?",
well because you *don't pass them by ref*. With structs we have no choice.
I think what we need is a way to define two different structs as being the
tail-const version of the other, with some compiler help, and then we do
not need to define a new flavor of const functions. We still need to
define these "tail-const" functions, but it comes in a more understandable
form. But importantly, the implicit cast makes a *temporary* copy of the
struct, allowing the cast to work.
-Steve
