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