Re: [boost] [iterator] Using zip_iterator with std::pair

Subject: Re: [boost] [iterator] Using zip_iterator with std::pair
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-10-17 07:21:44

On Wed, Oct 17, 2012 at 12:08 AM, "Claas H. Köhler" <claas.koehler_at_[hidden]>wrote:

>
>
> On 17/10/12 04:28, Jeffrey Lee Hellrung, Jr. wrote:
> > On Tue, Oct 16, 2012 at 10:06 AM, "Claas H. Köhler" <
> claas.koehler_at_[hidden]>wrote:
> >
> >>
> >>
> >> On 16/10/12 14:27, Jeffrey Lee Hellrung, Jr. wrote:
> >>> On Tue, Oct 16, 2012 at 4:21 AM, "Claas H. Köhler" <
> claas.koehler_at_[hidden]
> >>> wrote:
> >>>
> >>>>
> >>>>
> >>>> On 16/10/12 12:01, Giovanni Piero Deretta wrote:
> >>>>> On Tue, Oct 16, 2012 at 9:06 AM, "Claas H. Köhler" <
> >> claas.koehler_at_[hidden]>
> >>>> wrote:
> >>>>>> Hi!
> >>>>>>
> >>>>>> I would like to use the boost::zip_iterator with std::pair.
> >>>> Unfortunately this does not work out of
> >>>>>> the box, but there exists a namespace
> >>>>  boost::detail::tuple_impl_specific, which apparently needs to
> >>>>>> be specialised for each tuple type.
> >>>>
> >>>
> >>> Hmmm...I guess it makes sense you might have to specialize something if
> >>> zip_iterator doesn't specifically know about your desired tuple type.
> >>>
> >>>
> >>>>>> I managed to re-implement tuple_meta_transform and
> >>>> tuple_meta_accumulate for std::tuple, which seems
> >>>>>> to be straightforward. However, there are functions of the form
> >>>>>>
> >>>>>> template<typename Tuple, typename Fun>
> >>>>>> complex_result_type tuple_transform(const Tuple&, Fun)
> >>>>>>
> >>>>>> which have to be overloaded as well, if I am not mistaken. However,
> >>>> overloading this with something like
> >>>>>> template<typename... Args, typename Fun>
> >>>>>> complex_result_type tuple_transform(const std::tuple<Args...>&, Fun)
> >>>>>>
> >>>>>> is probably not going to work, since this would be a partial
> template
> >>>> specialisation of a function.
> >>>>>
> >>>>> That's an overload actually and should work fine. Did you try it?
> >>>>>
> >>>>> HTH,
> >>>>
> >>>> Thanks for the hint. I have to admit I did not try it yet. I have just
> >>>> created a similar simple test
> >>>> case, which indeed compiles/works fine. Shame on me for not trying it
> >> out.
> >>>> For me this was always a
> >>>> classical example of template function specialisation. I guess I
> should
> >>>> take a look at template
> >>>> basics again :-)
> >>>>
> >>>> Sorry for that and thanks for the quick response.
> >>>>
> >>>
> >>> I'm glad you got it working, but ideally, I think, zip_iterator should
> >> work
> >>> out of the box for std::pair and std::tuple. Could I trouble you to
> >> provide
> >>> patches based on what you needed to get this to work?
> >>>
> >>> - Jeff
> >>
> >> Unfortunately things are not working yet. As far as I can judge, the
> >> problem is that some of the
> >> functions are called from within the zip_iterator.hpp header file, which
> >> is before I can actually
> >> define the overloaded functions. As a result the overloaded functions
> are
> >> not called.
> >>
> >> Should I get this working, however, I am glad to share the wisdom.
> >>
> >
> > Well, I'd consider it fair game to edit the zip_iterator.hpp header file
> > directly :)
> >
> > - Jeff
> >
> Looks like I found a working solution. I transformed the tuple_xxx
> template functions into template
> structs, for which overloading seemingly works fine. Since I am not
> familiar with boost::mpl I used
> my own versions of the meta transformations based on C++11. For tuple this
> may not be an issue, since
> its a C++11 feature, but for std::pair it could be.
>
> Furthermore I would suggest to implement these functions exclusively for
> boost::tuple and leave the
> general template unimplemented. This may ease specialisations of other
> classes. Using C++11 for
> example the specialisations for std::pair and std::tuple are almost
> identical, but have to be
> specialised individually. Just my 0.02$ .
>
> Shall I post the modified code directly to the mailing list or somewhere
> else?
>

Create a trac ticket, attach any patches, and I'll take a look at it when I
get a chance.

- Jeff