postinst is in cpp-doc 4:4.6.3-1ubuntu5.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17  | #! /bin/sh -e
install-info --quiet \
	--section "Development" "Development" \
	--menuentry=cpp \
	--description="The GNU C Preprocessor" \
	/usr/share/info/cpp.info.gz
install-info --quiet \
	--section "Development" "Development" \
	--menuentry=cppinternals \
	--description="Cpplib Internals" \
	/usr/share/info/cppinternals.info.gz
exit 0
 |