I need to write GUI applications for cross platform in C++ but since most of GUI libraries for C++ are bit tedious and I am quite familiar with C#/.NET I found out code Vala codes with GTK is quite interesting and bit easy compare with other ways.so how would I mix up VAlA with C++. I meant use VALA for front end and code rest of parts in C++.
You can use QT if it's meeting your requirements. Qt is a cross-platform application and UI framework with APIs for C++ programming as well. http://developer.qt.nokia.com/doc/qt-4.8/gettingstartedqt.html