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Subject: Re: [boost] Geometry and spatial indexes, my opinion
From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2008-10-09 17:03:18

Simonson, Lucanus J wrote:
>> 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.

Steven Watanabe
>This only applies to built in types. For user defined types, it may
>be necessary to
>cast in order to operate on objects of two different types.
>Maybe I'm misunderstanding something, but it seems to me that an
>algorithm that works
>with Cartesian coordinates will tend to be very inefficient if it has
>to and from polar coordinates on the fly at every access.

User defined mixed type arithmetic operators seems like a fairly
unlikely circumstance. I'm not sure introducing complexity of keeping
track of the different data types into the algorithms is warranted by
such a use case.

The interface is more likely to be used between modules rather than
internally. For instance, imagine one legacy module operates on and
produces its output in the form of polar coordinate points. A generic
geometry library module consumes Cartesian coordinate points. Rather
than iterate over polar points and convert them to Cartesian points that
are fed into the generic library they can be converted on the fly
through the generic interface. Rather than use the user point type for
internal computation I use my own point type because I know it is fast.
If the output of the Cartesian algorithm needs to be fed back into the
polar coordinate module it can write out polar coordinates directly.
The interface is providing the ability to refactor the application code
and abstract away data type conversion.

There are other good reasons not to require direct access to a data
type's member data, most notable among them is that people typically
don't provide such access in the public interfaces of their legacy
classes because they learned in school that was a bad thing. For this
reason, there has to be a temporary created by the generic interface and
it might as well cast it to whatever global coordinate type the user
deems best at that time.


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