It's just a example. Could you tell me why they use a macro to declare a
abstract base class,
instead of dircetly using '=0' ?
I don't conceal what I have thought, they define the macro is only for
design, overengineering,
Just like "format" here.

For, if I did not know this MACRO, it's hard to understand what's the
meaning of 'int foo(...) PURE;',
but it's very clear of 'int foo(...) = 0;', even if I am a beginner of
C++, isn't it?

PURE is only one of examples in codes of, as you said, polymorphic design.
It's hard to understand,
hard to study those technologies when there are a large number of these
MACROs, essentially,
they can be a little easier.

Well, this is another topic.