examples\dialogs\extension下面的例子
    moreButton = new QPushButton(tr("&More"));
    moreButton->setCheckable(true);                           ←●
//! [0]
    moreButton->setAutoDefault(false);
    buttonBox = new QDialogButtonBox(Qt::Vertical);
    buttonBox->addButton(findButton, QDialogButtonBox::ActionRole);
    buttonBox->addButton(moreButton, QDialogButtonBox::ActionRole);
//! [1]
//! [2]
    extension = new QWidget;                ←●
    wholeWordsCheckBox = new QCheckBox(tr("&Whole words"));
    backwardCheckBox = new QCheckBox(tr("Search &backward"));
    searchSelectionCheckBox = new QCheckBox(tr("Search se&lection"));
//! [2]
//! [3]
    connect(moreButton, SIGNAL(toggled(bool)), extension, SLOT(setVisible(bool)));   ←●