access selected value of QcomboBox has problem

[isan]

I set a QcomboBox in gui widget and ,I add item

for(int i = 1; i < 31; i++)
        {
            ui->combo->addItem(QString::number(i));
        }

for(int i = 1; i < 31; i++)
        {
            ui->combo->addItem(QString::number(i));
        }

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and in QComboBox slot I want to get selected value by

int index =ui->combo->itemData( ui->combo->currentText());

int index =ui->combo->itemData( ui->combo->currentText());

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but have error :316: error: no matching function for call to 'QComboBox::itemData(QString)'

if I use currentIndex instead of currentText return 0 when print it; addItem get Qstring ,

void QComboBox::addItem(const QString & text, const QVariant & userData = QVariant())

void QComboBox::addItem(const QString & text, const QVariant & userData = QVariant())

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and ItemData work with currentIndex,

I use insertItem and it has sae error ,so how can set value or text and get slected value??