Whats the difference in sharing a QGLContext and sharing a QGLWidget

[Wali]

Hi,
In the QGLWidget constructor

QGLWidget::QGLWidget ( QGLContext * context, QWidget * parent = 0, const QGLWidget * shareWidget = 0, Qt::WindowFlags f = 0 )

QGLWidget::QGLWidget ( QGLContext * context, QWidget * parent = 0, const QGLWidget * shareWidget = 0, Qt::WindowFlags f = 0 )

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How is sharing a QGLContext different from sharing a QGLWidget? It seems to me that passing in a context is the same as sharing a widget. The shared widget and the newly constructed QGLWidget will be sharing the same context any way. So, why is there the option to pass in a context?

Thanks