On Thu, Oct 9, 2008 at 4:17 PM, Simonson, Lucanus J <lucanus.j.simonson@intel.com> wrote:
I don't see any value in carrying the individual coordinate data types through the interface into the algorithms.
They're already present at compile-time. No extra work required.
Well, yes and no. If I want to have a
template <int axis> coordinate_type get(const point_type& point);
accessor function I need to go through a lot of work to rewrite it as
template <int axis> typename coordinate_type<axis>::type get(const point_type& point);
and the user needs to go through more work to provide both
template<int axis> struct coordinate_type<axis> { typedef int type; }
and
struct general_coordinate_type { typedef int type; }
and will generally be puzzled about why the geometry library is so complicated to use.
Well, get is already written for Boost.Tuple and Boost.Fusion sequences. This works with built-ins, arrays, and user-defined types. More to the point, if you accept a type that conforms to a Fusion sequence, you have the types at compile time, and I exaggerated a little when I said "no extra work" - for user defined types the user *will* need to adapt his struct using a Fusion macro, but this is a one-time thing, and I think it's a one-liner.
You will have to cast at some point to use the value, why is it more expensive to cast up front rather than at the last possible moment. Assuming the value is going to be used more than once by the algorithm it will be cast more than once. Better to factor that to the front. A smart compiler would actually do that for you. Futhermore, there has to be a teomporary at the interface because as I've just discussed with Brandon, there is the clear need to make things like polar points conform to Cartesian interfaces, which is incompatible with direct access to the data members. Since we already have to make a temporary we should cast at that point instead of having yet another temoporary and cast later on.
The return value temporary is present in both cases, unless the compiler elides that. I was talking about parameters. What if the function takes 5 parameters? I have to make 5 unnecessary temporaries just to call it. --Michael Fawcett