[Boost-users] Re: [MPL] replace_if question

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-23 12:52:10

David Greene wrote:
> 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

You should be seeing a syntax error right here.

> 		    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:

> 
> What's the right way to do this?

Use an algorithm like transform that _computes_ the new values rather
than simply replacing them with a constant value:

template <class Seq>
struct replaceit
  : transform<
        Seq
      , if_<
            is_base_and_derived<pair_base,_>
          , key_type<map<>,_>
          , _
        >
    >
{};

> 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.

Why?  That seems like a pretty darned good approach to me.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com