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template<class T>
class Document : public T {
....
};

class mywidget : public Document<QPlainTextEdit>  {
...
};

MainWindow::MainWindow()
{
	connect(qApp, SIGNAL(focusChanged(QWidget*,QWidget*))
			, this, SLOT(focusChanged(QWidget*,QWidget*)));
}

void checkActionForWidget(QWidget *wdg, QAction *action)
{
	QString signature = QString(action->objectName()).remove("Action");
	signature += "()";
	QByteArray methodName = QMetaObject::normalizedSignature(signature.toLatin1());
	int methodId;
	if ((methodId = wdg->metaObject()->indexOfMethod(methodName)) != -1) {
		action->setEnabled(true);
	} else {
		action->setEnabled(false);
	}
}

void MainWindow::focusChanged(QWidget* old, QWidget* now)
{
	Q_UNUSED(old);
	if (!now)
		return ;

	//QMetaMethod method;
	checkActionForWidget(now, ui->copyAction);
	checkActionForWidget(now, ui->cutAction);
	checkActionForWidget(now, ui->pasteAction);
}

void MainWindow::actionTriggered()
{
	QAction *action = qobject_cast<QAction *>(sender());

	QWidget *now = qApp->focusWidget();
	QMetaMethod method;
	QString signature = QString(action->objectName()).remove("Action");
	signature += "()";
	QByteArray methodName = QMetaObject::normalizedSignature(signature.toLatin1());
	int methodId;
	if ((methodId = now->metaObject()->indexOfMethod(methodName)) != -1) {
		method = now->metaObject()->method(methodId);
		method.invoke(now);
	}
}