Re: [Boost-users] [mpl] Documentation suggestion

Subject: Re: [Boost-users] [mpl] Documentation suggestion
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-06-22 14:39:25

Hi Jeremiah,

I bet Aleksey would be happy to have a patch...

on Tue Jun 19 2012, Jeremiah Willcock <jewillco-AT-osl.iu.edu> wrote:

> I have noticed that several MPL algorithms do not work when applied to
> views unless an output iterator (inserter) is passed in explicitly.
> The error messages when you hit one of those cases are very
> non-obvious, however.  It might be nice to have something in the
> documentation that talks about this issue and the workarounds for it.
> Here is an example of something that breaks:
>
> #include <boost/mpl/vector_c.hpp>
> #include <boost/mpl/joint_view.hpp>
> #include <boost/mpl/sort.hpp>
> #include <boost/mpl/print.hpp>
>
> typedef boost::mpl::vector_c<int, 2, 0> va;
> typedef boost::mpl::vector_c<int, 1, 3, -1, 5> vb;
> typedef boost::mpl::joint_view<va, vb> vc;
> typedef boost::mpl::sort<vc>::type vd;
>
> The first important part of the error message with GCC 4.7.0 in C++11
> mode is:
>
> .../boost-svn/boost/mpl/clear.hpp: In instantiation of ‘struct
> boost::mpl::clear<boost::mpl::joint_view<boost::mpl::vector_c<int, 2l,
> 0l>, boost::mpl::vector_c<int, 1l, 3l, -1l, 5l> > >’:
> .../boost-svn/boost/mpl/sort.hpp:23:1: required from ‘struct
> boost::mpl::sort<boost::mpl::joint_view<boost::mpl::vector_c<int, 2l,
> 0l>, boost::mpl::vector_c<int, 1l, 3l, -1l, 5l> > >’
> mpl_test.cpp:9:29:   required from here
> .../boost-svn/boost/mpl/clear.hpp:28:8: error: invalid use of
> incomplete type ‘struct
> boost::mpl::clear_impl<boost::mpl::aux::joint_view_tag>::apply<boost::mpl::joint_view<boost::mpl::vector_c<int,
> 2l, 0l>, boost::mpl::vector_c<int, 1l, 3l, -1l, 5l> > >’
>
> which seems to indicate that the default inserter needs a "real"
> container as input to the algorithm in order to know what container to
> use for the result.  Since this limitation is probably unavoidable,
> some kind of note in the documentation might be useful.  Thanks.
>
> -- Jeremiah Willcock
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-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com