The docs it says I should include opencv2/core.hpp but it is not there

edit

after you run make install to build the opencv libs , the opencv headers will be in /usr/local/include/opencv4 and the libs will be in /usr/local/lib , so, a cmdline like :

g++ my.cpp -std=c++11 -I/usr/local/include/opencv4 -lopencv_core -lopencv_imgcodecs -lopencv_highgui -lopencv_videoio -lopencv_imgproc -o myprog

will get you started ;)