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

From: Larry Evans (cppljevans_at_[hidden])
Date: 2004-07-09 10:46:49

On 07/08/2004 05:40 PM, Jonathan Turkanis wrote:
[snip]
>> Got it. One last try -- a variation on Brian's:
> 
>     struct nested_base { };
> 
>     template <class P1, class P2>
>     struct sp
>     {
>          template <typename T>
>          struct nested : nested_base {
>              typedef P1 p1;
>              typedef P1 p2;
>              typedef T t;
>          };
>     };
> 
>     template <typename T, typename Enabler = void>
>     struct less { };
> 
>     template<typename T>
>     struct enable_if_nested
>         : boost::enable_if<
>               boost::is_base_and_derived<nested_base, T>
>           >
>         { };
> 
>     template <typename T>
>     struct less< T, typename enable_if_nested<T>::type >
>     { ... };
> 

There's another related problem which David might face, templated
CTOR's.  As I understand (from previous emails from David), P1 and P2
are policies and the T is the referent in a smart pointer.  For any
S which is a subtype of T, then the nested class would probably
have:

    template <typename T>
    struct nested
    {
        template
        < typename S //subtype of T'
        >
        nested(sp<P1,P2>::nested<S>& a_sp)
        ;
    };

Now both P1 and P2 are in a non-deduced context and, AFAICT, there
no workaround using some helper class like the above nested_base
or Brian's i_am_a_nested_thingy. Maybe I'm wrong.  If so, could someone
please outline a solution?

TIA.