#include <QtWidgets>

class RangedSlider : public QSlider {
    int mRangeMin, mRangeMax;
public:
    explicit RangedSlider(Qt::Orientation orientation, QWidget* parent= nullptr):
        QSlider(orientation, parent),mRangeMin(minimum()),mRangeMax(maximum()) {}
    explicit RangedSlider(QWidget* parent=nullptr):RangedSlider(Qt::Vertical, parent){}

    void sliderChange(SliderChange change) override {
        if(change == QAbstractSlider::SliderValueChange) {
            fixValue();
        }
        QSlider::sliderChange(change);
    }

    void setRangeMin(int rangeMin) { this->mRangeMin = rangeMin; fixValue(); }
    void setRangeMax(int rangeMax) { this->mRangeMax = rangeMax; fixValue(); }

    int rangeMin() const { return mRangeMin; }
    int rangeMax() const { return mRangeMax; }
private:
    void fixValue() {
        int correctValue = value();
        if(value() < mRangeMin)
            correctValue = mRangeMin;
        if(value() > mRangeMax)
            correctValue = mRangeMax;
        if(correctValue!= value()){
            //this will emit value changed with the correct value
            setValue(correctValue);
            //a hack to stop QSlider from emiting valueChanged with the
            //old value
            blockSignals(true);
            QObject signalSource;
            //unblock signals for this object as soon as the event loop is
            //reached again
            QObject::connect(&signalSource, &QObject::destroyed, this, [=] {
                blockSignals(false);
            }, Qt::QueuedConnection);
        }
    }
};

int main(int argc, char* argv[]) {
    QApplication a(argc, argv);
    QWidget widget;
    QVBoxLayout layout(&widget);
    RangedSlider slider;
    QObject::connect(&slider, &QSlider::valueChanged, [&slider](int value){
        qDebug() << value << " " << slider.value();
    });
    slider.setRangeMin(10);
    slider.setRangeMax(80);
    layout.addWidget(&slider);
    widget.show();

    return a.exec();
}