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

8 Jul 2004

      Jonathan Turkanis wrote:
...
"David B. Held" <dheld@codelogicconsulting.com> wrote in message
news:cckeh3$fd0$1@sea.gmane.org...
[...]
I 'm not sure I see the syntax problem; if you mean that it's
more cumbersome to use a metafunction, I agree.
The user would have to write:

     sp<p1, p2>::nested<t>::type p;

instead of:

     sp<p1, p2>::nested<t> p;
...
[...]
Then specializing less< ... > for sp_nested wouldn't affect the
implementation of less<nested>.
Ah, back to square one.
...
[...]
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.
The difference is that the component is called sp<>::nested<>.
It's not called sp_nested<>, because they might want to typedef
sp<> and instantiate the nested class over various types.  By
also exposing sp_nested<>, I'm asking them to call it by two
different names depending on usage.  It isn't totally evil, but
it's less pleasant:

     typedef sp<p1, p2> my_sp;
     my_sp::nested<foo> p;
     my_sp::nested<bar> q;

     less<my_sp::nested<foo> > comp;

vs.:

     less<sp_nested<p1, p2, foo> > comp;

Somewhat defeats the point of the nice typedef.

Dave