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From "Eric Lemings" <Eric.Lemi...@roguewave.com>
Subject RE: type_traits progress
Date Mon, 02 Jun 2008 20:38:12 GMT
 

> -----Original Message-----
> From: Martin Sebor [mailto:sebor@roguewave.com] 
> 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:Travis.Vitek@roguewave.com] 
> >> 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.

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