https://*.com/questions/50578661/how-to-implement-vertical-tabs-in-qt#

#include <QApplication>
#include <QStyleOptionTab>
#include <QStylePainter>
#include <QTabBar>
#include <QTabWidget>

class TabBar: public QTabBar{
public:
    QSize tabSizeHint(int index) const{
        QSize s = QTabBar::tabSizeHint(index);
        s.transpose();
        return s;
    }
protected:
    void paintEvent(QPaintEvent * /*event*/){
        QStylePainter painter(this);
        QStyleOptionTab opt;

        for(int i = 0;i < count();i++)
        {
            initStyleOption(&opt,i);
            painter.drawControl(QStyle::CE_TabBarTabShape, opt);
            painter.save();

            QSize s = opt.rect.size();
            s.transpose();
            QRect r(QPoint(), s);
            r.moveCenter(opt.rect.center());
            opt.rect = r;

            QPoint c = tabRect(i).center();
            painter.translate(c);
            painter.rotate(90);
            painter.translate(-c);
            painter.drawControl(QStyle::CE_TabBarTabLabel,opt);
            painter.restore();
        }
    }
};

class TabWidget : public QTabWidget
{
public:
    TabWidget(QWidget *parent=0):QTabWidget(parent){
        setTabBar(new TabBar);
        setTabPosition(QTabWidget::West);
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    TabWidget w;
    w.addTab(new QWidget, "tab1");
    w.addTab(new QWidget, "tab2");
    w.addTab(new QWidget, "tab3");
    w.show();

    return a.exec();
}