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

8 Jul 2004

      "David B. Held" <dheld@codelogicconsulting.com> wrote in message
news:cckeh3$fd0$1@sea.gmane.org...
...
Jonathan Turkanis wrote:
...
[...]
Could you pull the definition of nested outside of sp
template<typename P1, typename P2, typename T>
    struct sp_nested { };
then use a metafunction within sp:
template<typename P1, typename P2>
    struct sp {
         template<typename T>
         struct nested {
             typedef sp_nested<P1, P2, T> type;
         };
    };
[...]
For syntax reasons, this is an undesirable solution.
I 'm not sure I see the syntax problem; if you mean that it's more
cumbersome to use a metafunction, I agree.
...
Although,
you could get pretty close by modifying it a little:
template<typename P1, typename P2>
      struct sp {
           template<typename T>
           struct nested : sp_nested<P1, P2, T>
           { };
      };
Then specializing less< ... > for sp_nested wouldn't affect the
implementation of less<nested>.

<snip>
...
The problem is that sp_nested<> would be an implementation detail
that I would not want to expose to the user.  So I would not want
the user to have to mention it in order to instantiate less<>.
I don't see the distinction between

   template <class P1, class P2, typename T>
   struct less<sp<P1, P2>::nested<T> >
        : binary_function<...>
   {
     bool operator()(...);
   };

and

   template <class P1, class P2, typename T>
   struct less<sp_nested<P1, P2, T> >
        : binary_function<...>
   {
     bool operator()(...);
   };

with respect to exposing implementation details.

Jonathan