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Subject: Re: [boost] review request: addition to type_traits library of has_operator_xxx
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-11-17 15:26:27

Jeffrey Lee Hellrung, Jr. wrote:
> On 11/17/2010 6:58 AM, Stewart, Robert wrote:
> > Jeffrey Lee Hellrung, Jr. wrote:
> >>
> >> One thing that I've either found useful is to have an additional
> >> template parameter in addition to the parameter types and
> >> the return
> >> type, which is a Boost.MPL metapredicate intended to be
> >> applied to the actual result of the operation. The trait then
> >> evaluates to true only if the actual result type satisfies
> >> the given Boost.MPL metapredicate.
> >
> > Perhaps checking for convertible-to-return-type could be
> > the default predicate rather than adding another template parameter?
> How do you envision the interface then? Are you suggesting that the
> convertible-to-return-type type and the Boost.MPL metapredicate occupy
> the same template parameter?


> To be clear, I'm suggesting something like
> template< class T0, class T1 = T0, class Result = void,
> class ResultCond = boost::mpl::always< boost::true_type > >
> struct is_less_than_comparable;

I'm thinking something like this:

   class T
   , class U = T
   , class Pred = boost::type_traits::result_converts_to<void>
struct is_less_than_comparable;

boost::type_traits::result_converts_to<void> should inherit from boost::true_type, of course.

> It seems to me that, by far, the common case is checking
> convertibility to some given type, so I think it makes sense to make
> the common case syntactically simple.

is_less_than_comparable<int, int, bool> would become is_less_than_comparable<int, int, result_converts_to<bool> > which is more verbose but also more obvious and indicative of the additional power.

> If you want to avoid the extra template parameter, maybe you can wrap
> the predicate in something which could be detected in the Result
> template parameter, e.g.,

That would be ideal, but I'm not sure there's a satisfying means to do so without imposing too much on the acceptable predicates.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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