Re: [boost] [type_erasure] default implementation of concept signatures

Subject: Re: [boost] [type_erasure] default implementation of concept signatures
From: Matt Calabrese (rivorus_at_[hidden])
Date: 2011-05-28 17:00:53

On Sat, May 28, 2011 at 4:16 PM, Steven Watanabe <watanabesj_at_[hidden]>wrote:

> AMDG
>
> On 05/28/2011 12:42 PM, Pyry Jahkola wrote:
> > On 2011-05-28 21:59:57 +0300, Matt Calabrese said:
> >
> >> On Sat, May 28, 2011 at 2:55 PM, Steven Watanabe
> >> <watanabesj_at_[hidden]>wrote:
> >>
> >>> I don't have the concepts proposal handy, but
> >>> doesn't LessThanComparable have an axiom that
> >>> requires that (a < b) is equivalent to (b > a)?
> >>>
> >>
> >> Yes.
> >>
> >> axiom Consistency(T a, T b) {
> >>     (a > b) == (b < a);
> >>     (a <= b) == !(b < a);
> >>     (a >= b) == !(a < b);
> >>   }
> >
> > For what it's worth, e.g. NaN's are an exception to that rule...
> >
>
> It's not an exception.  The axiom only applies to
> LessThanComparable types.  Technically, double
> isn't LessThanComparable, because it violates this
> rule.
>

Sorry, this is sort of on a tangent, but this is, imo, one of the problems
with what would have been 0x concepts. According to N2914,
LessThanComparable is an auto concept, meaning that as long as a type has an
operation that matches the pseudo-signature bool operator< ( T const&, T
const& ), then that type is LessThanComparable, even though the compiler
can't really "check" the axioms. So in hypothetical 0x concept-land,
checking if double models LessThanComparable would yield true.

Most auto concepts that have axioms are scary.

-- 
-Matt Calabrese