Re: [Boost-users] mpl vector of pair access

Subject: Re: [Boost-users] mpl vector of pair access
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2013-04-18 15:54:05

On Thu, Apr 18, 2013 at 11:47 AM, Olaf Peter <ope-devel_at_[hidden]> wrote:

> Hello,
>
> in these code I try to unpack the 1stand 2nd element, but it doesn't
> compile (here, it complains about »apply« in »struct
> boost::mpl::at_impl<boost::**mpl::non_sequence_tag>«
>

This suggest mpl::pair's are not MPL sequences :(

Is there a better way and how to get it to compile?
>

Try using mpl::vector2 instead of mpl::pair? Or patch Boost.MPL to make
mpl::pair's be MPL sequences?

Thanks,
> Olaf
>
> ---8<---
> #include <boost/mpl/vector.hpp>
> #include <boost/mpl/vector_c.hpp>
> #include <boost/mpl/max_element.hpp>
> #include <boost/mpl/pair.hpp>
> #include <boost/mpl/integral_c.hpp>
> #include <boost/mpl/size_t.hpp>
> #include <boost/mpl/placeholders.hpp>
> #include <boost/mpl/at.hpp>
>
> #include <iostream>
>
> namespace mpl = boost::mpl;
>
> struct code {
>         enum type {
>                 undefined, foo, bar
>         };
> };
>
> typedef mpl::vector<std::size_t,
>         //                    min             max
>         mpl::pair< mpl::size_t<0>, mpl::size_t<0> >, // undefined
>         mpl::pair< mpl::size_t<1>, mpl::size_t<3> >, // foo
>         mpl::pair< mpl::size_t<2>, mpl::size_t<5> >  // bar
> > arity_info;
>
>
> template <code::type code>
> struct min_arity
>         : mpl::at_c<
>                   typename mpl::at_c<arity_info, code>::type, 0
>           >::type
> { };
>
> // same for max_arity at index 1 (pair::second)
>
> int main()
> {
>         std::cout << "min arity = "
>                 << min_arity<code::bar>::value << '\n';
> //      std::cout << "max arity = "
> //              << max_arity<code::bar>::value << '\n';
> }
>
> --->8---
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>