Reading Qt 4.4 Tutorial 1

• The QApplication object must be created before any GUI-related features of Qt are used.

•     QPushButton hello("Hello world!");

  A push button is created. Because we don't specify a parent window (as second argument to the QPushButton constructor), the button will be a window of its own, with its own window frame and title bar.
• The button's size is determined by its default size hint. We could call QWidget::move() to assign a specific screen position to the widget, but instead we let the windowing system choose a position.

•     hello.show();

  A widget is never visible when you create it. You must call QWidget::show() to make it visible.

•    return app.exec();

  This is where main() passes control to Qt
  

  int QApplication::exec ()

  Enters the main event loop and waits until exit() is called, then returns the value that was set to exit() (which is 0 if exit() is called via quit()).

• 基本把教程上的COPY下来了，细节注意到了却影响到整体阅读速度，在博客上修改这破格式花了不少时间。