Re: [Boost-users] [mpl] can't transform a map

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2007-06-06 02:55:50

Aleksey Gurtovoy wrote:
> Yuval Ronen writes:
>> I have the following code:
>>
>> #include <utility>
>> #include <boost/mpl/map.hpp>
>> #include <boost/mpl/transform.hpp>
>>
>> template <typename MplPair>
>> struct make_ptr_pair1
>> {
>>      typedef std::pair<typename MplPair::first *, typename MplPair::second *> type;
>                ^^^
>                mpl
> 

I'm really puzzled about this. Why can't I have std::pair here? The 
whole purpose of the transform operation is to create a type-sequence of 
std::pairs. This sequence can be used, for example, with 
make_variant_over to create a variant of std::pairs. Can't I do that?

>> };
>>
>> template <typename First, typename Second>
>> struct make_ptr_pair2
>> {
>>      typedef std::pair<First *, Second *> type;
>> };
>>
>> int main(int argc, char *argv[])
>> {
>>      namespace mpl = boost::mpl;
>>      typedef mpl::map< mpl::pair<int, double> > my_map;
>>      //typedef mpl::transform< my_map, make_ptr_pair1<mpl::_1> >::type ptr_pair_seq1;
>>      //typedef mpl::transform< my_map, make_ptr_pair2<mpl::_1, mpl::_2> >::type ptr_pair_seq2;
>>      return 0;
>> }
>>
>> If I uncomment any of the two mpl::transform lines in main(), the code 
>> doesn't compile (VC8). The point is to get a sequence of std::pairs out 
>> of the content of the mpl::map. Is there anything I'm doing wrong? Can 
>> mpl::transform be used for mpl::map? 
> 
> It can be, but you need to specify the inserter manually, because the
> default (front_inserter, as per
> http://boost.org/libs/mpl/doc/refmanual/reversible-algorithm.html)
> is not suitable for accociated sequences:
> 
> ...
> #include <boost/mpl/inserter.hpp>
> #include <boost/mpl/insert.hpp>
> 
> ...
> 
> int main(int argc, char *argv[])
> {
>      typedef mpl::map< mpl::pair<int, double> > my_map;
>      typedef mpl::inserter< mpl::map0<>, mpl::insert<mpl::_1,mpl::_2> > map_inserter;
>      typedef mpl::transform< my_map, make_ptr_pair1<mpl::_1>, map_inserter >::type ptr_pair_seq1;
>      //                                                     ^^^^^^^^^^^^^^
>      return 0;
> }
> 
> 
>> Maybe I can do it with fusion, but the above code seems to be
>> something that's supposed to be working, isn't it?
> 
> Yes, ideally, your original code should just work. It's a design
> bug. I've added the issue to the library's TODO list
> (http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?MPL_TODO_List).
> 
> Thanks for the report,

Thanks you!