setCentralWidget problems

[tlerner]

Setup: Qt 4.4.3, using visual studio 2005

I have an app ui file (which is empty) and I want to add another centralWidget UI file on top of it (so when I maximize I can have a black background in my app ui and my centralWidget UI can be center screen. I am trying to use setCentralWidget() for this. If I explained things correctly, is this the path I want to follow? I cant even get a button to display in the central widget:

QApp::QApp(QWidget *parent, Qt::WFlags flags)
	: QMainWindow(parent, flags)
{
	ui.setupUi(this);
	CentralWidget cw;
	QPushButton b;
	b.setGeometry(20,20,20,20);
	b.setEnabled(true);//.show();
	setCentralWidget( &b );
}

QApp::QApp(QWidget *parent, Qt::WFlags flags)
	: QMainWindow(parent, flags)
{
	ui.setupUi(this);
	CentralWidget cw;
	QPushButton b;
	b.setGeometry(20,20,20,20);
	b.setEnabled(true);//.show();
	setCentralWidget( &b );
}

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Any help is appreciated. Thank you!