[Boost-users] [mpl] Sorting types by index

From: Scott Meyers (usenet_at_[hidden])
Date: 2008-03-25 23:51:24

I have an mpl::vector of all possible types I care about.  It's called AllTypes. 
    Given a sequence containing some subset of types in AllTypes, I want to 
order the sequence so that the types occur in the same relative order as in 
AllTypes.    So if AllTypes is

   A, B, C, D, E

and I'm given

   B, E, A

I want to produce

   A, B, E

Fundamentally, I'm sorting things based on their index in AllTypes.  Well, 
that's what I want to be doing.  What I'm really doing is getting compilation 
errors referring to a missing tag in my metafunction, which is called IndexOf. 
The code I'm using is below.  I'd appreciate it if people would take a look and 
help me figure out why it's not working.

Thanks,

Scott

#include <boost/mpl/sort.hpp>
#include <boost/mpl/distance.hpp>
#include <boost/mpl/find.hpp>

using namespace boost;

// return the index of type T in sequence S
template<typename S, typename T>
struct IndexOf {
   enum { value = mpl::distance<typename mpl::begin<S>::type,
                                typename mpl::find<S, T>::type
                               >::value };
};

int main()
{
   using mpl::_1;
   using mpl::_2;

   typedef mpl::vector<char, short, int, long, float, double> AllTypes;

   typedef mpl::vector<long, char, short> Unsorted;

   typedef mpl::sort<Unsorted,                            // this call to
                     mpl::less<IndexOf<AllTypes,_1>,      // sort doesn't
                               IndexOf<AllTypes,_2> >     // compile
                    >::type
           Sorted;
}