Hi,
Please try this build command:
$ g++ -Wall -std=c++11 a.cpp -o test $(pkg-config --cflags --libs gstreamer-app-1.0)
Since appsink is after flvmux, you may save it to test.flv.
For fetching buffers in appsink, may refer to this sample:
Gstreamer decode live video stream with the delay difference between gst-launch-1.0 command and appsink callback - #6 by DaneLLL