Re: [Boost-users] mpl::transform question

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2006-06-19 13:10:20

David,

Thanks a lot. First example worked with Visual Studio 8. I also got your
book today, what a lucky day! Thanks for sharing your valuable thoughts with
the whole world.

Many Kind Regards,
Ovanes.

-----Original Message-----
From: David Abrahams [mailto:dave_at_[hidden]] 
Sent: Monday, June 19, 2006 15:10
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] mpl::transform question

"Ovanes Markarian" <om_boost_at_[hidden]> writes:

> Hello all,
>
> somehow I have extreme difficulties to use mpl. Please do not 
> understand this as any kind of blaming. I find the library really 
> great and the possiblities it offers are tremendous. Currently I have 
> some sort of problem, where I stuck at:
>
> I have an mpl::vector instance of form (mpl namespace was left out for 
> better readability):

> class A;
> class B;
> class C;
>
> typedef vector<
>  pair<int_<0>, A>,
>  pair<int_<1>, B>,
>  pair<int_<2>, C> >   types_vector;
>
> Now I would like to use the mpl::transform algorithm to transform this 
> vector into the mpl::map.  I try to do it in the following way:
>
>   typedef mpl::transform<type_vector, mpl::insert<mpl::map<>,
>   mpl::_1 >::type type_map;
>
> And I also understand that _1 has no embedded typedefs for first and 
> second, what a pair should have, so I am stuck here.

Well, transform doesn't look like the right algorithm to use in this way.
What you're doing would produce a new vector of 3 single-element maps.  You
want to insert an element into a map, and then insert an element into the
result of that insertion, etc.  That's the pattern of the fold<> algorithm:

    fold<
        type_vector
      , map<>
      , insert<_,_>
    >::type

copy is also probably an appropriate algorithm, but you'd need to use an
Inserter.

http://www.boost.org/libs/mpl/doc/refmanual/copy.html
http://www.boost.org/libs/mpl/doc/refmanual/inserter.html
http://www.boost.org/libs/mpl/doc/refmanual/inserters-inserter.html

    copy<
        type_vector
      , inserter< map<>, insert<_,_> > 
    >::type

Also, I'm afraid the MPL associative containers in 1.33.1 had some serious
bugs that could prevent you from succeeding with this.  I suggest if the
above doesn't work, you first replace map<> with map0 in the examples, and
then if that fails, check out the RC_1_34_0 branch from our CVS repository
and try again.

--
Dave Abrahams
Boost Consulting
www.boost-consulting.com