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From: Joel de Guzman (joel_at_[hidden])
Date: 2007-10-05 19:22:31

Simonson, Lucanus J wrote:
> Joel wrote:
>> That's very good. I'm not quite sure about inheritance though.
>>> P.S. my point2d data structure is a class encapsulated array of size
> 2.
>>> You can override it with your own, of course, through the template
>>> inheritance generic interface.
>> With the restriction that it has to be indexable by a runtime int?
>> That is a severely limited concept of a point, I would say. And,
>> again, I find the need for inheritance dubious. Inheritance gives
>> you tight coupling. Something that you'd want to avoid when
>> adopting a 3rd party type. It is possible to adopt a 2d-point
>>from library A and library B without ever touching them. That's
>> the very essence of generic programming.
> I put a lot of thought into the implications of inheritance, believe me.
> There is no restrition that it has to be indexable by a runtime int.

Hmm. I was confused by your earlier post, it seems:

> Moreover, the technique you are proposing works fine at compile time,
> but won't work if the axis of the coordinate you are accessing is a
> runtime parameter:
> int my_axis = 0;
> get<my_axis>(pt);
> will throw an error. We specify the axis we access at run time. If we
> specify a constant it should compile away and be light weight. gcc
> 4.2.1 still needs us to play with the compiler flags to get decent
> inlining.
> pt.get(orient.getPerpendicular()); //orientation determined at run time.
> My scanline algorithm actually parameterizes the orientation of the
> scanline at runtime, so you can sweep left to right or bottom to top.


> All this seems like overkill with simple point2d data types but when you
> have polygons that are sequences of vertices it becomes a Rosetta stone
> of type inter-compatibility and a mechanism for getting legacy data in
> and out of algorithms in a generic, zero cost of abstraction way that is
> minimal effort and very productive (not error prone.) When you realize
> that my Polygon concept class uses my Point concept class to interact
> with its vertices the whole picture suddenly becomes clear. Sometimes
> it is useful to have a polygon that is a linked list of points.
> Sometimes it is useful to have a vector, sometime it is useful to have a
> compressed storage for polygon data. The polygon concept class is the
> glue that can really tie everything together and make it all work
> together as smooth as butter.
> By the way if you have a 3D point you can view it as a 2D point (because
> conceptually it is a 2D point with extra semantics.) I do that all the
> time, and use it to refactor my higher dimension code instead of
> templating the dimensionality.
> It might have been better if I called it the Geometry Concepts Library,
> since the term template is too *ahem* generic.

I'll refrain on commenting on actual concept classes before I see
any real code. Seems like an interesting concept; pun intentional,
but I won't really know unless I see it and get to know the
pros and cons of such an approach. I still doubt the inheritance
approach, fwiw.


Joel de Guzman

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