The C++ SDK will soon be deprecated. You should migrate to the new C SDK.
https://extensions.sketchup.com/en/developer_center/sketchup_sdk
In the root folder of the SDK archive, there is a file “migration_guide.html” that you should read.
A deprecated C++ example “skptoxml” plugin is included with the SDK in path:
“.../deprecated/SkpReader/Examples/skptoxml”
Examples for the new C API: