building decision trees with mpl

From: Markus Werle (yg-boost-users_at_[hidden])
Date: 2003-06-02 08:03:34

Hi!

The following piece of code works (finally, sigh), 
but I would like to hear some comments on whether and how 
one can do better.

#include "boost/mpl/apply_if.hpp"
#include "boost/mpl/identity.hpp"

#include "boost/mpl/lower_bound.hpp"
#include "boost/mpl/deref.hpp"
#include "boost/mpl/find.hpp"
#include "boost/mpl/vector.hpp"
#include "boost/mpl/pair.hpp"
#include "boost/mpl/bool.hpp"
#include "boost/mpl/select1st.hpp"
#include "boost/mpl/select2nd.hpp"
#include "boost/mpl/lambda.hpp"

#include "boost/type_traits/is_same.hpp"

#include <typeinfo> 
#include <iostream> 

namespace mpl = boost::mpl;

template <int m, int n>
class ComplicatedDecision
{
private:
  template <int i> struct C {
    // typedef probably_very_expensive_template type;
  };

  // order matters
  typedef mpl::vector
  <
    mpl::pair<mpl::bool_< (m == n)  >, C<0> >,
    mpl::pair<mpl::bool_< (m == -n) >, C<1> >,
    mpl::pair<mpl::bool_< (m == 1)  >, C<2> >,
    mpl::pair<mpl::bool_< (m < 0)   >, C<3> >,      
    mpl::pair<mpl::bool_< true      >, C<4> >
  > decision_sequence;
  

  typedef typename mpl::find_if<
    decision_sequence,
    boost::is_same<mpl::select1st<mpl::_1>, 
                   mpl::bool_<true> > > 
  iter_;

  typedef typename iter_::type::type pair_;

public:
  typedef typename mpl::select2nd<pair_>::type type;
};

int main()
{
  std::cerr << typeid(ComplicatedDecision<1,1>::type).name()  << std::endl;
  std::cerr << typeid(ComplicatedDecision<-1,1>::type).name() << std::endl;
  std::cerr << typeid(ComplicatedDecision<1,2>::type).name()  << std::endl;
  std::cerr << typeid(ComplicatedDecision<-1,2>::type).name() << std::endl;
  std::cerr << typeid(ComplicatedDecision<2,3>::type).name()  << std::endl;
}

best regards,

Markus

-- 
Compile time analytic differentiation? 
Yes, at http://daixtrose.sourceforge.net/