Re: [boost] A design for geometric objects

Subject: Re: [boost] A design for geometric objects
From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2008-11-14 16:57:07

Bruno wrote:
>The "specialized" typedef is a bit weird, as it forces the user to
>sort of say "yes, I've really specialized it". But the goal is reached
>I think: I have never explicitly specialized ellipse_traits<circle>. I
>have only said that if circle_traits<X> exists then ellipse_traits<X>
>exists and ellipse_traits<X>::width() is circle_traits<X>::radius(),
>except if a better specialization of ellipse_traits<X> exists. And the
>compiler correctly understood what I meant when I asked for
>ellipse_traits<circle>::width().
>
>Does anybody know a way to know if a specialization exists without
>using this kind of trick?

I was in the middle of typing up a proposal for what Bruno suggests, but had a different solution to knowing whether a specialization exists.  My solution was simply to register the conceptual type of the object with a meta function, which indicates both that the corresponding traits should also be defined as well as an easy way to tell what conceptual type that object most closely models.

template <typename T1, typename T2>
struct SFINAE_is_same_type {};

template <typename T>
struct SFINAE_is_same_type<T, T> { typedef void type; }

template <typename T>
struct geometry_concept {};

struct circle_concept {};

struct circle_data_type {};

template <>
struct geometry_concept<circle_data_type> { typedef circle_concept type; };

template <class X>
struct ellipse_traits< X,
        typename SFINAE_is_same_type<
                   typename geometry_concept<X>::type, 
                   circle_concept>::type > {
        static void width() { 
                cout << "another way to get at ellipse width" << endl; 
        }
};

I'm not sure which way is better, but I provide the geometry_concept meta-function anyway to use for tag dispatching/SFNAE purposes.

Regards,
Luke