Documentation doesn't show example code:
http://qt-project.org/doc/qt-4.8/qthread.html

After this sentence, no example code is visible:
"Another way to make code run in a separate thread, is to subclass QThread and reimplement run(). For example:"
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qthread.cpp should be changed from
\snippet code/src_corelib_thread_qthread.cpp reimpl-run
to
\snippet doc/src/snippets/code/src_corelib_thread_qthread.cpp reimpl-run

BTW:
remove unused 'QThread workerThread' in 'Worker' class in:
doc/src/snippets/code/src_corelib_thread_qthread.cpp