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I'm presuming you are using the installed version of OpenCV. All of the source code has been compiled into the .dll files that were installed. The .h and .hpp files just contain declarations and information about what is in, and how to call the .dll files. That way you and the compiler know what's there to be called.
You can see and download the source code for all of OpenCV HERE.
Incidentally, you should be up one level. You are at opencv2/highgui/highgui.hpp, and should be including opencv2/highgui.hpp
