RE: [boost] Re: compile time member detection

From: Brock Peabody (brock.peabody_at_[hidden])
Date: 2003-11-14 09:17:41

> -----Original Message-----
> From: boost-bounces_at_[hidden] [mailto:boost-bounces_at_[hidden]]
> On Behalf Of Brian McNamara

> "Me too."
> 
> In my opinion, "named conformance" is the right way to design most
> interfaces, whereas "structural conformance" (as above) is the wrong
> way.  What you really want to say is
> 
>    template <typename T> T* clone(const T& t) {
>       return mpl::if_<
>          is_clonable<T>,
>          use_clone_function,
>          use_copy_constructor
>       >::type::clone(t);
>    }

After thinking about it some more, I find myself agreeing with you and Dave.

Is there a good use then for member detection, other than types?   

> where is_clonable is a metafunction which tests not only the structural
> conformance (as suggested above), but also named conformance.  Some
> human should have to explicitly say that T is indeed a clonable (e.g. it
> has the right semantics to match the concept).  This can be done in a
> variety of ways, the most popular probably being via specialization:

I like to use inheritance over specialization because the trait is then
passed down to children automatically:

  struct clonable {};

   template <typename T> T* clone(const T& t) {
      return mpl::if_<
         is_base_and_derived<clonable,T>,
         use_clone_function,
         use_copy_constructor
      >::type::clone(t);
   }

Another possibility would be to use member type detection:

   struct Foo { typedef library::clonable clonable; ...};

A class would be clonable if it had a typedef named clonable of type
library::clonable.  This would be inheritable too, but you could more easily
'shut it off' in derived classes.

Brock