Computer Vision with OpenCV 3 and Qt5

User Interface Compiler (uic)

Whenever a Qt application with a user interface is built, a Qt internal tool called uic is executed to process and convert the *.ui files into classes and source code usable in C++ code. In our case, mainwindow.h is converted to the ui_mainwindow.h file, which again, you can find in the Build folder. You may have already noticed this, but let's mention that your mainwindow.cpp file already included this header file. Check the topmost part of the file, and you'll find the following two include lines:

    #include "mainwindow.h" 
    #include "ui_mainwindow.h" 

You already knew what and where the mainwindow.h file is (in your Project folder), and you just learned that ui_mainwindow.h is, in fact, a generated source file ...