Add a signal to QAbstractSpinBox that informs about invalid input

#include <QtGui>

class Dialog : public QDialog
{
    Q_OBJECT
public:
    Dialog()
        : isValid(true)
    {
        spinBox = new QSpinBox;
        spinBox->setMinimum(32);
        spinBox->setMaximum(1024);

        QHBoxLayout *hbox = new QHBoxLayout;
        hbox->addWidget(spinBox);
        setLayout(hbox);

        connect(spinBox, SIGNAL(valueChanged(int)), this, SLOT(mySlot(int)));
        
        if (QLineEdit *lineEdit = qFindChild<QLineEdit*>(spinBox)) {
            connect(lineEdit, SIGNAL(textEdited(QString)), this, SLOT(mySlot(QString)));
        }
    }

protected:
    void closeEvent(QCloseEvent *e)
    {
        if (!isValid)
            e->ignore();
    }

private slots:
    void mySlot(int value)
    {
        qDebug() << value;
    }
    void mySlot(const QString &value)
    {
        int pos = 0;
        QString copyValue = value;
        QAbstractSpinBox *aSpinBox = spinBox;
        isValid = aSpinBox->validate(copyValue, pos) == QValidator::Acceptable;
    }

private:
    QSpinBox *spinBox;
    bool isValid;
};

#include "main.moc"

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    Dialog dialog;
    dialog.show();

    return app.exec();
}