64bit int Spin Box in QT

You can derive QAbstractSpinBox and reimplement at least the virtual functions stepBy, stepEnabled and possibly validate() and fixup() for the input validation.

I don't use fixup function. See code of my Сustom QSpinBox. class QLongLongSpinBox derived from QAbstractSpinBox

Don't forget call

setMaximum(std::numeric_limits<qlonglong>::max());
setMinimum(std::numeric_limits<qlonglong>::min());

after creating QLongLongSpinBox.

see qlonglongspinbox.h file:

#include <QtWidgets/QWidget>
#include <QtWidgets/QAbstractSpinBox>
#include <QtWidgets/QLineEdit>

class QLongLongSpinBoxPrivate;
class Q_WIDGETS_EXPORT QLongLongSpinBox : public QAbstractSpinBox
{
    Q_OBJECT

    Q_PROPERTY(qlonglong minimum READ minimum WRITE setMinimum)
    Q_PROPERTY(qlonglong maximum READ maximum WRITE setMaximum)

    Q_PROPERTY(qlonglong value READ value WRITE setValue NOTIFY valueChanged USER true)


    qlonglong m_minimum;
    qlonglong m_maximum;
    qlonglong m_value;

public:
    explicit QLongLongSpinBox(QWidget *parent = 0)
    {
        connect(lineEdit(), SIGNAL(textEdited(QString)), this, SLOT(onEditFinished()));
    };
    ~QLongLongSpinBox() {};

    qlonglong value() const
    {
        return m_value;
    };

    qlonglong minimum() const
    {
        return m_minimum;
    };

    void setMinimum(qlonglong min)
    {
        m_minimum = min;
    }

    qlonglong maximum() const
    {
        return m_maximum;
    };

    void setMaximum(qlonglong max)
    {
        m_maximum = max;
    }

    void setRange(qlonglong min, qlonglong max)
    {
        setMinimum(min);
        setMaximum(max);
    }

    virtual void stepBy(int steps)
    {
        auto new_value = m_value;
        if (steps < 0 && new_value + steps > new_value) {
            new_value = std::numeric_limits<qlonglong>::min();
        }
        else if (steps > 0 && new_value + steps < new_value) {
            new_value = std::numeric_limits<qlonglong>::max();
        }
        else {
            new_value += steps;
        }

        lineEdit()->setText(textFromValue(new_value));
        setValue(new_value);
    }

protected:
    //bool event(QEvent *event);
    virtual QValidator::State validate(QString &input, int &pos) const
    {
        bool ok;
        qlonglong val = input.toLongLong(&ok);
        if (!ok)
            return QValidator::Invalid;

        if (val < m_minimum || val > m_maximum)
            return QValidator::Invalid;

        return QValidator::Acceptable;
    }

    virtual qlonglong valueFromText(const QString &text) const
    {
        return text.toLongLong();
    }

    virtual QString textFromValue(qlonglong val) const
    {
        return QString::number(val);
    }
    //virtual void fixup(QString &str) const;

    virtual QAbstractSpinBox::StepEnabled stepEnabled() const
    {
        return StepUpEnabled | StepDownEnabled;
    }


public Q_SLOTS:
    void setValue(qlonglong val)
    {
        if (m_value != val) {
            lineEdit()->setText(textFromValue(val));
            m_value = val;
        }
    }

    void onEditFinished()
    {
        QString input = lineEdit()->text();
        int pos = 0;
        if (QValidator::Acceptable == validate(input, pos))
            setValue(valueFromText(input));
        else
            lineEdit()->setText(textFromValue(m_value));
    }

Q_SIGNALS:
    void valueChanged(qlonglong v);

private:
    Q_DISABLE_COPY(QLongLongSpinBox)

    Q_DECLARE_PRIVATE(QLongLongSpinBox)
};