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

26 Mar 2008


      Scott Meyers wrote:
...
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.
<snip>
...
// 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;
}
Try this:

template<typename S, typename T> 
struct IndexOf: mpl::distance<typename mpl::begin<S>::type,
                                 typename mpl::find<S, T>::type
                               >
{};

-- John Femiani