> -----Original Message-----
> From: Martin Sebor [mailto:[EMAIL PROTECTED]
> Sent: Monday, June 02, 2008 2:14 PM
> To: dev@stdcxx.apache.org
> Subject: Re: type_traits progress
>
> Eric Lemings wrote:
> >
> >
> >> -----Original Message-----
> >> From: Travis Vitek [mailto:[EMAIL PROTECTED]
> >> Sent: Friday, May 30, 2008 6:28 PM
> >> To: dev@stdcxx.apache.org
> >> Subject: RE: type_traits progress
> >>
> >>
> >>
> >> Martin Sebor wrote:
> >>> Travis Vitek wrote:
> >>> [...]
> >>>>> Right. That could be another wrinkle. Our traits won't
> >>>>> work with generic code that takes integral_constant<T, V>
> >>>>> by reference.
> >>>> I don't really see the motivation, but it is obvious that
> >>>> the committee thought it was important for the standard
> >>>> traits to do so, so we should probably follow suit in our
> >>>> internal implementation.
> >> Can you think of a reason why this 'feature' would be important?
> >>
> >>>> If we did decide to do this then we would probably want
> >> our own write
> >>>> __rw_integral_constant and use that internally to avoid namespace
> >>>> pollution? Then I'd assume we'd want something like the following
> >>>> example for is_const...
> >>> Yes, I think this is close to what we want. The only
> thing that bugs
> >>> me about it is...
> >>>
> >>>> template <class T, T v>
> >>>> struct __rw_integral_constant
> >>>> {
> >>>> static const T value = v;
> >>>> typedef T value_type;
> >>>> typedef integral_constant<T,v> type;
> >>> ...this backward dependency on integral_constant, but I
> don't see how
> >>> to break it without template typedefs. I don't think there's
> >> a compiler
> >>> out there that supports them yet.
> >> Actually, this was originally a typo on my part, but I do see
> >> where this
> >> is going. I haven't read about template typedefs, but it seems that
> >> there would be a serious problem caused by the cyclic dependency.
> >
> > Yeah it looks like a typo to me too. Should it be:
> >
> > typedef __rw_integral_constant<T,v> type;
> >
> > In all cases I've seen, `type' refers to type of self for identity
> > properties. This typedef would not hold up the identity property.
>
> IIUC, for every specialization X of integral_constant, this
> must hold:
>
> is_same<X, typename X::type>::value == true
>
> With integral_constant derived from __rw_integral_constant
> the condition would fail.
Not if it defined its own `type'.
template <class T, T v>
struct integral_constant: __rw_integral_constant<T, v>
{
typedef integral_constant<T, v> type;
};
That's the way I usually see it done.
Brad.
