From: David Abrahams (dave_at_[hidden])
Date: 2006-06-19 09:09:46
"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