I m new in Qt, my doubt is here that,why we use this in ui header in qt?
Hi and welcome to devnet,
Please take the time to read the chapter about creating GUI with Designer, it's all explained there.
why we use this in ui header in qt?
The short answer is this: The UI files make it easy to automatically generate C++ code for your GUI.
I believe @SGaist is talking about the chapter at http://doc.qt.io/qt-5/designer-using-a-ui-file.html