[boost] Re: [mpl] easy access to inherit_linearly elements

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-01-17 16:47:06

On 01/16/2005 03:11 PM, Larry Evans wrote:
 > On 01/15/2005 05:26 PM, David Abrahams wrote:
[snip]
 >  >
 >  > Finally, the kind of access being done is completely different from
 >  > what's done with tuple.
 >
 > I've not looked *real* close at tuple, but I assumed boost::tuple had
 > the following type of access:
[snip]
 > Could you elaborate on how the kinds of element access in boost::tuple
 > and that of tuple_type in get_ith_head_test.cpp are different?

Maybe you meant the way element access was being done.  I've
just looked at boost/tuple/detail/tuple_basic.hpp.  I see two
templates which may be candidates for how tuple implements get.  The
first is:

   template< int N >
struct get_class {
   template<class RET, class HT, class TT >
   inline static RET get(const cons<HT, TT>& t)
   {
#if BOOST_WORKAROUND(__IBMCPP__,==600)
     // vacpp 6.0 is not very consistent regarding the member template 
keyword
     // Here it generates an error when the template keyword is used.
     return get_class<N-1>::get<RET>(t.tail);
#else
     return get_class<N-1>::BOOST_NESTED_TEMPLATE get<RET>(t.tail);
#endif
   }
  ...

and the 2nd, which looks closer to what could be the implementation
is prefixed with comment:

// -cons type accessors ----------------------------------------
// typename tuples::element<N,T>::type gets the type of the
// Nth element ot T, first element is at index 0
// -------------------------------------------------------

but the code for that looks very similar to get_ith_tail.  The
difference is that get_ith_tail uses specialization to split apart the
Head and Tail parts and recurses on the Tail, whereas tuples:element
uses the nested typedef for tail_type to recurse on the tail.

Do you mean this is completely different, or is there some other code
I should look at to see how tuple accesses its elements?