Qt销毁非模态对话框

很多时候需要非模态对话框：当关闭主窗口时，往往非模态对话框依然存在。

bool QWidget::close () 

       Closes this widget. Returns true if the widget was closed; otherwise returns false.

       First it sends the widget a QCloseEvent. The widget is hidden if it accepts the close event. If it ignores the event, nothing happens. The default implementation of QWidget::closeEvent() accepts the close event.

       If the widget has the Qt::WA_DeleteOnClose flag, the widget is also deleted. A close events is delivered to the widget no matter if the widget is visible or not.

       The QApplication::lastWindowClosed() signal is emitted when the last visible primary window (i.e. window with no parent) with the Qt::WA_QuitOnClose attribute set is closed. By default this attribute is set for all widgets except transient windows such
as splash screens, tool windows, and popup menus.

由以上可以通过QPointer，在closeEvent中delete掉非模态对话框，如下：

ParentWidget窗体类中有一个QPointer<ChildWidget> m_pChildWidget;子窗体。

ParentWidget::ParentWidget(QWidget *parent, Qt::WFlags flags)
: QWidget(parent, flags),m_pChildWidget(NULL)
{
ui.setupUi(this);
connect(ui.pushButton,SIGNAL(clicked()),this,SLOT(createWidget()));
}

ParentWidget::~ParentWidget()
{
}

void ParentWidget::createWidget()
{
if(m_pChildWidget)
{
qDebug()<<"m_pChildWidget != NULL";
m_pChildWidget->showNormal();
m_pChildWidget->activateWindow();
}
else
{
qDebug()<<"m_pChildWidget == NULL";
m_pChildWidget = new ChildWidget;
m_pChildWidget->show();
}

}

void ParentWidget::closeEvent( QCloseEvent *ev )
{
qDebug()<<m_pChildWidget;
delete m_pChildWidget;
}

ChildWidget可以设置setAttribute (Qt::WA_DeleteOnClose);属性，当close时就delete widget。QPointer指针在对象被销毁时会自动被设置为NULL。以下是QPointer的Assistant中的介绍：
      The QPointer class is a template class that provides guarded pointers to QObject.
      A guarded pointer, QPointer<T>, behaves like a normal C++ pointer T *, except that it is automatically set to 0 when the referenced object is destroyed (unlike normal C++ pointers, which
become "dangling pointers" in such cases). T must be a subclass of QObject.
      Guarded pointers are useful whenever you need to store a pointer to a QObject that is owned by someone else, and therefore might be destroyed while you still hold a reference to it.
You can safely test the pointer for validity.

QPointer<QLabel> label = new QLabel;
label->setText("&Status:");
...
if (label)
label->show();