Object Oriented Programming in C

One thing I would like to say to the developers of
HTMLTidy is "thanks for the lesson in C programming." 
There are some very cool techniques used in the Tidy
source (esp. in libtidy) that I have learned a lot
from.  I love the use of opaque, "impl" types, and
other object-flavored things.  HTMLTidy is great stuff
to use, but it's also been very educational to me.  Do
these techniques come from a book somewhere, or are
they things that have just evolved over time?  In
other words, is there a book that I can read that will
help me really learn this, or do I have to just read
through the source and learn it?  These techniques
make C so much less painful.


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