Re: [boost] Re: Partially specializing less<> with nested template class

From: Brian McNamara (lorgon_at_[hidden])
Date: 2004-07-08 14:27:03

On Thu, Jul 08, 2004 at 01:44:28PM -0500, Larry Evans wrote:
> On 07/08/2004 12:58 PM, David B. Held wrote:
> >This is what I would like to do:
> >
> >template <class P1, class P2>
> >struct sp
> >{
> >    template <typename T>
> >    struct nested { };
> >};
> >
> >template <typename T>
> >struct less;
> >
> >template <class P1, class P2, typename T>
> >struct less<sp<P1, P2>::nested<T> >
> >    : binary_function<...>
> >{
> >    bool operator()(...);
> >};
> >
> >However, it doesn't work.  From my research, it seems that the
> >problem is that T is in a non-deduced context, but I don't see
> >why that would be an issue, since this is how I expect it to get
> >instantiated:
> >
> >less<sp<p1, p2>::nested<int> > comp;
> >
> >Thus, I don't see any deduction as being necessary, but obviously
> >I don't exactly understand how to get what I want.
> 
> I very much sympathize with you; however, I have no answers :(

Depending on how much control you have over various parts, you maybe can
kludge it along the lines of:

   struct i_am_a_nested_thingy {};
   
   template <class P1, class P2>
   struct sp {
       template <typename T>
       struct nested : public i_am_a_nested_thingy { 
          typedef P1 NestedP1;
          typedef P2 NestedP2;
       };
   };
   
   template <typename T>
   struct less;
   
   template <typename T>
   struct less_helper;
   
   template <typename T>
   struct less_helper<T,true> : binary_function<...> {
       typedef typename T::NestedP1 P1;
       typedef typename T::NestedP2 P2;
       bool operator()(...);
   };
   
   template <typename T>
   struct less_helper<T,false> {
      // default impl (if there is one)
   };
   
   template <class P1, class P2, typename T>
   struct less<T> : public
      less_helper<T,is_base_and_derived<i_am_a_nested_thingy,T>::value> { };
   
   ...  less<sp<p1, p2>::nested<int> > comp; ...

-- 
-Brian McNamara (lorgon_at_[hidden])