connect(textEdit, &QPlainTextEdit::copyAvailable, copyAct, &QAction::setEnabled);

#ifndef QT_NO_CLIPBOARD
    cutAct->setEnabled(false);
//! [23] //! [24]
    copyAct->setEnabled(false);
    //信号定义 void copyAvailable(bool b);
    //槽原型： void setEnabled(bool);
    connect(textEdit, &QPlainTextEdit::copyAvailable, cutAct, &QAction::setEnabled);
    connect(textEdit, &QPlainTextEdit::copyAvailable, copyAct, &QAction::setEnabled);
#endif // !QT_NO_CLIPBOARD

 

    QMenu *helpMenu = menuBar()->addMenu(tr("&Help"));
    QAction *aboutAct = helpMenu->addAction(tr("&About"), this, &MainWindow::about);
    aboutAct->setStatusTip(tr("Show the application's About box"));

//! [22]

    QAction *aboutQtAct = helpMenu->addAction(tr("About &Qt"), qApp, &QApplication::aboutQt);
    aboutQtAct->setStatusTip(tr("Show the Qt library's About box"));
//! [22]