Re: [Boost-users] combining mpl sequences

Subject: Re: [Boost-users] combining mpl sequences
From: David Abrahams (dave_at_[hidden])
Date: 2010-09-05 16:16:52

At Sun, 05 Sep 2010 12:10:17 +0200,
Karsten Ahnert wrote:
> 
> Hi,
> 
> I have a templatized mpl-vector
> 
> template< class T > class containers :
>   public mpl::vector<
>     std::vector< T > ,
>     std::list< T > ,
>     std::deque< T >
> > { };
> 
> and a mpl-vector holding some types:
> 
> typedef mpl::vector< float , double , long double >::type my_types;
> 
> Is there a simple way to build a mpl vector which contains the elements
> of the first vector with template arguments of the second vector, hence
> 
> mpl:vector<
>   std::vector< float > ,
>   std::vector< double > ,
>   std::vector< long double > ,
>   std::list< float > ,
>   std::list< double > ,
>   std::list< long double > ,
>   std::deque< float > ,
>   std::deque< double > ,
>   std::deque< long double >
> >
> 
> 
> I can do this manually with mpl::insert_range
> 
> typedef mpl::insert_range<
>   mpl::vector0<> ,
>   mpl::end< mpl::vector0<> >::type ,
>   containers< mpl::at< my_types , mpl::int_<0> >::type >
> >::type first;
> 
> typedef mpl::insert_range<
>   first ,
>   mpl::end< first >::type ,
>   containers< mpl::at< my_types , mpl::int_<1> >::type >
> >::type second;
> 
> typedef mpl::insert_range<
>   second ,
>   mpl::end< second >::type ,
>   containers< mpl::at< my_types , mpl::int_<2> >::type >
> >::type all_containers;
> 
> But this way is not very elegant. Does some has any hints how to do this
> automatically?

You're already most of the way there.  If you can break it down into
three similar steps, you can write a single step as a metafunction:

template <class PrevResult, class MyType>
struct step 
: mpl::insert_range<
    PrevResult ,
    mpl::end< PrevResult >::type ,
    containers< MyType >
  >
{};

Then you can fold that step across your types:

typedef typename 
    mpl::fold< my_types, mpl::vector<>, step<mpl::_,mpl::_> >::type
all_containers;

This produces a sequence with a different ordering than the one in
your "hence" clause, but it's the same ordering as you'd get in your
own all_containers above.  

HTH,

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com