Re: [boost] mpl help needed - doing the impossible?

From: Manfred Doudar (manfred.doudar_at_[hidden])
Date: 2005-11-16 08:31:38

Martin Bonner wrote:

>>Manfred Doudar wrote:
>>
>>    
>>
>>>/* Now, how do I get T out when Rank == 1, ... compiler complains
>>>  about seeing a MyArray/blitz::Array of <T, Rank = 0>
>>>  (, which is not defined for blitz Arrays)
>>>*/
>>>template <typename T, int Rank>
>>>typename boost::mpl::if_
>>>                        < typename boost::mpl::equal_to
>>>                          < boost::mpl::int_<Rank>
>>>                          , boost::mpl::int_<1>
>>>                          >::type
>>>                        , T
>>>                        , blitz::Array<T,
>>>                                
>>>boost::mpl::int_<Rank>::prior::value> 
>>>
>>>
>>>      
>>>
>>                            ^
>>                            Oops typo, should be:
>>                                MyArray<T,
>>                                 
>>boost::mpl::int_<Rank>::prior::value> 
>>
>>... but nonetheless, the problem remains ...
>>
>>    
>>
>>>                        >::type
>>>MyArray<T, Rank> :: operator[] (const int& index) throw(); {
>>>  // Slicing
>>>      
>>>
>
>How about a helper traits class?
>
>template <typename T, int Rank>
>struct SliceType
>{
>  typedef typename MyArray<T, Rank-1> type;
>};
>
>template <typename T>
>struct SliceType<T,1>
>{
>   typedef T type;
>};
>
>template <typename T, int Rank>
>typename SliceType<T,Rank>::type
>   MyArray<T,Rank> :: operator[] (const int& index) throw();
>
>  
>

Now I'm sure that ought to have worked (even after modifying
as you've suggested) ... but the problem _still persists_ ,
not per say on the definition of return type, but seemingly
inside the _template body_, where the *static_cast* is done
to map the final result accordingly ... please see below:

Partial Code Listing:
--------------------

template <typename T, int Rank>
typename SliceType<T, boost::mpl::int_<Rank>::value>::type
MyArray<T, Rank> :: operator[] (const int& index) throw()
{
  // Slicing

  blitz::TinyVector<boost::variant<blitz::Range,
                                   blitz::nilArraySection>, 2> plist;
  this->getParamList(plist);

  return

         // ***********************************************
         // ** Complier complains about this cast seeing
         // ** MyArray/blitz::Array <T, 0>
         // ** when Rank == 1
         // ** ... see partial compiler error msg below
         // ***********************************************
         static_cast< typename SliceType<T,    // ** [ Line 697 ] **
                      boost::mpl::int_<Rank>::value>::type
                    > (

 ((* static_cast<const blitz::Array<T,
        boost::mpl::int_<Rank>::value> *> (this))

 /* the parameter list */
 (index,
     *boost::get<
                   typename boost::mpl::if_<
                      typename boost::mpl::less<
                            boost::mpl::int_<1>
                          , boost::mpl::int_<Rank>
                      >::type
                      , blitz::Range
                      , blitz::nilArraySection
                   >::type
                >

      (&plist(boost::mpl::if_<
                  typename boost::mpl::less<
                       boost::mpl::int_<1>
                     , boost::mpl::int_<Rank>
                  >::type
                  , boost::mpl::int_<0>
                  , boost::mpl::int_<1>
              >::type::value
              )
       )

     /*
         , ... we can extend the parameter list with Boost.PP
     */
 );

}

(Partial) Compiler Error Message:
---------------------------------

In member function `SliceType<T,
   boost::mpl::int_<Rank>::value>::type MyArray<T, Rank>::operator[](const
   S&) [with S = int, T = float, int Rank = 1]':
arr.cc:197:   instantiated from here
arr.cc:697: invalid static_cast from type `blitz::Array<float, 0>' to type `
   float'

<snip .. more messages>

/usr/local/include/blitz/array/storage.h:60: no match for call to `(
   blitz::TinyVector<int, 0>) (int&)'

<snip .. more messages>

Somewhere in main(..)
---------------------

blitz::Array<float, 2> b2(2, 2);    // 2D array (2 x 2)

b2 =  1.1, -2.2,
     -9.9,  8.8;

MyArray<float, 2> a2(b2);

std::cout << a2[1] ;      // this works
std::cout << a2[1][1] ;   // ** [Line 197] ** the problem is here

Any and All help is Appreciated,
I know it's got to be something simple, I just can't see it yet.

With Thanks,
-- 
Manfred Doudar
MetOcean Engineers
www.metoceanengineers.com