Re: [Boost-users] Range library suggestion

Subject: Re: [Boost-users] Range library suggestion
From: John M. Dlugosz (mpbecey7gu_at_[hidden])
Date: 2013-01-24 06:22:56

On 1/24/2013 3:59 AM, boost-users-request-at-lists.boost.org |Boost/Allow to home| wrote:
>
>> >I just added my second metafunction of the form is_range_of_X<T> for use
>> >with enable_if, and although the first one took some fiddleing to pass
>> >both compilers I was using, the second was a clone of the first with the
>> >name and type changed. So it really ought to be a general purpose
>> >is_range_of<Whatever, T> metafunction.
> I assume it it's simply something like
> is_same<typename mpl::eval_if< is_range<T>, range_value<T>, void>::type,
> Whatever>

I ended up with:

    template <typename Flag, typename T>
    struct is_range_of_MYTHING2 : boost::mpl::false_ { };

    template <typename T>
    struct is_range_of_MYTHING2<boost::mpl::true_, T> {
        typedef typename boost::range_value<T>::type val_type; // chokes here if T is not
    a range
        typedef typename is_convertible<val_type, MYTHING>::type type;
        };
    template <typename T>
    struct is_range_of_MYTHING : is_range_of_MYTHING2<typename
    boost::has_range_iterator<T>::type, T>
        { };

I'm familiar enough with type_traits, but not with all of Boost MPL.  Is eval_if a 
lazy-evaluation for short circuiting the AND of the two tests?

---John