how to find the implementation of all classes and their member functions

i was just looking a screenshot application but i directly came across a grabwindow function
which captures your desktop ....but i realised that this way if i will directly used the function and don't
know about its implementation it..will be unfair.....and i can't customise my programs in my own way

please guide how to find the source code of all classes and their implementations.....
please help

You can find implementation of each class in "src" folder of your Qt installation, it's separated into modules - for example grabWindow is in "src/gui/image/qpixmap_win.cpp".
If you are using an IDE like QtCreator or Eclipse, "Ctrl+left click" on class or method name should open the corresponding source file ( if they are parsed by indexer ).