[Boost-users] [MPL] replace_if question

From: David Greene (greened_at_[hidden])
Date: 2004-12-23 11:02:12

Well, my mpl adventure continues.

I'm trying to place pair members of a sequence with their keys:

#include <boost/type_traits.hpp>
#include <boost/mpl/pair.hpp>
#include <boost/mpl/key_type.hpp>
#include <boost/mpl/map.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/replace_if.hpp>
#include <boost/mpl/int.hpp>

#include <iostream>
#include <typeinfo>

using namespace boost;
using namespace boost::mpl;

struct pair_base {};

template<typename Key, typename Value>
struct pair_type : public pair<Key, Value>, private pair_base {};

template<typename Sequence>
struct replaceit {
                typedef typename replace_if<
                        Sequence,
                        is_base_and_derived<pair_base, _>,
                        key_type<map<>, _>
		>::type type;
};

int main(void)
{
        replaceit<
                vector<
                     charm
                    pair_type<int, int_<32> >,
                    pair_type<long, int_<64> >
                     double
		>
	>::type toprint;

        std::cout << typeid(toprint).name() << std::endl;

        return(0);
};

I know this is incorrect because the pairs will be replaced by
the key_type metafunction itself.

If I add ::type to the end of key_type<map<>, _> the compiler
complains that no ::first member exists:

boost-1_32/boost/mpl/pair.hpp: In instantiation of 
`boost::mpl::first<mpl_::_>':
replace.cc:25:   instantiated from 
`boost::mpl::key_type_impl<boost::mpl::aux::map_tag>::apply<boost::mpl::map<mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, mpl_::_>'
replace.cc:25:   instantiated from 
`boost::mpl::key_type<boost::mpl::map<mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, 
mpl_::na, mpl_::na, mpl_::na>, mpl_::_>'
replace.cc:25:   instantiated from here
boost-1_32/boost/mpl/pair.hpp:43: no
    type named `first' in `struct mpl_::_'

What's the right way to do this?

Furthermore, is there a better way to identify pairs in
replace_if than deriving from a special class?  All I can
think of is writing a predicate class and specializing it
for mpl::pair<U, V>.  Seems like there should be an easier
way.

Thanks!

                               -Dave