通过设置listview实现。

class MultiColumnComboBoxItemDelegate;
class MultiColumnComboBoxListView;

class MultiColumnComboBox : public QComboBox
{
    Q_OBJECT
public:
    explicit MultiColumnComboBox(QWidget *parent = nullptr);
    ~MultiColumnComboBox();
protected:
       void showPopup() override;;
private:
    MultiColumnComboBoxItemDelegate *m_Delegate = nullptr;
    MultiColumnComboBoxListView *m_ListView = nullptr;
};



class MultiColumnComboBoxListView: public QListView
{
    Q_OBJECT
public:
    explicit MultiColumnComboBoxListView(QWidget *parent = nullptr);
    ~MultiColumnComboBoxListView();
};



class MultiColumnComboBoxItemDelegate: public QStyledItemDelegate
{
    Q_OBJECT
public:
    MultiColumnComboBoxItemDelegate(QObject* parent = nullptr);
    ~MultiColumnComboBoxItemDelegate();
    QSize sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const override;
private:

    int m_width;
    friend class MultiColumnComboBox;
};

MultiColumnComboBox::MultiColumnComboBox(QWidget *parent)
    : QComboBox(parent)
    , m_Delegate(new MultiColumnComboBoxItemDelegate(this))
    , m_ListView(new MultiColumnComboBoxListView(this))
{
    setSizeAdjustPolicy(QComboBox::AdjustToContentsOnFirstShow);

    setView(m_ListView);

    setItemDelegate(m_Delegate);

    setMaxVisibleItems(6);

    setFocusPolicy(Qt::ClickFocus);
}

MultiColumnComboBox::~MultiColumnComboBox()
{
    if(nullptr != m_Delegate)
    {
        delete m_Delegate;
        m_Delegate = nullptr;
    }
    if(nullptr != m_ListView)
    {
        delete m_ListView;
        m_ListView = nullptr;
    }
}

void MultiColumnComboBox::showPopup()
{
    int rowHeight = view()->sizeHintForRow(0);
    int rowNum = model()->rowCount();
    int showRows = 0;
    for(int curRow = 0; curRow<rowNum; curRow++)
    {
        if(!m_ListView->isRowHidden(curRow))
        {
            showRows++;
        }
    }
    int maxrow = showRows;
    if(maxrow > maxVisibleItems())
    {
        int curCloumn = ceil(double(maxrow) / double(maxVisibleItems()));
        m_Delegate->m_width = this->width();
        view()->setMinimumWidth(curCloumn * this->width() + 5);
        maxrow = ceil(double(maxrow) / double(curCloumn));
        if(maxrow < maxVisibleItems())
        {
            setMaxVisibleItems(maxrow);
        }
    }
    else
    {
        m_Delegate->m_width = this->width();
        view()->setMinimumWidth(this->width() + 5);
    }
    QComboBox::showPopup();
    view()->setMinimumHeight(rowHeight * maxrow + 5);
}




MultiColumnComboBoxListView::MultiColumnComboBoxListView(QWidget *parent)
    :QListView(parent)
{
    setSpacing(0);
    setResizeMode(QListView::Adjust);
    setFlow(QListView::TopToBottom);
    setWrapping(true);
}

MultiColumnComboBoxListView::~MultiColumnComboBoxListView()
{

}


MultiColumnComboBoxItemDelegate::MultiColumnComboBoxItemDelegate(QObject *parent)
    :QStyledItemDelegate(parent)
{
}

MultiColumnComboBoxItemDelegate::~MultiColumnComboBoxItemDelegate()
{

}


QSize MultiColumnComboBoxItemDelegate::sizeHint(const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    auto originalSize = QStyledItemDelegate::sizeHint(option, index);
    originalSize.setWidth(m_width);
    return originalSize;
}