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

From: John Femiani (JOHN.FEMIANI_at_[hidden])
Date: 2008-03-26 00:14:56

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