🚀 go-pugleaf

satrajit@iscs.nus.sg (Satrajit Sujit Ghosh) wrote:
>	If a polymorphic constructor can be created as demonstrated below, 
>is RTTI really necessary, considering that it does not really provide a 
>great deal of information.

I don't understand particularly how your sample code makes RTTI less
useful.  One place RTTI is handy is if you have an interaction between
two classes, where knowing the exact type of each class can lead to
optimizations.

For example, you might have a generic bitmap class, which returns an
RGB value for each pixel, and you want to copy one bitmap to another.
A derived class from your bitmap class might be a class that
represents a 256 color bitmap, with an overloaded copy operator.  If
you leave the bitmap you're copying as the generic type, then you need
to obtain the RGB value for a pixel, find the closest match in your
palette, and assign the pixel that palette index.  But if you know the
bitmap you're copying from is also a 256 color bitmap, you can build a
conversion table between matching palette indices, and do a much
faster copy.*

One problem, though, is that you may not care about the actual class
of an object, but just whether it is derived from (or is) of a
particular class, and RTTI doesn't help you there.

Andrew Bell
abell@mindspring.com

* For real quality, you want to dither your data, and thus this
algorithm wouldn't hold.





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