ComboBox of CheckBoxes?

You can try this with QListView:

QStringList values = QStringList << "check 1" << "check 2" << "check 3" << "check 4";

QStandardItemModel model = new QStandardItemModel;
for (int i = 0; i < values.count(); i++)
{
    QStandardItem *item = new QStandardItem();
    item->setText(values[i]);
    item->setCheckable(true);
    item->setCheckState(Qt::Unchecked);
    model->setItem(i, item);
}

ui->list->setModel(model);

Have you set a check state as well as making them checkable?

In my example below, this line is critical:

item->setData(Qt::Unchecked, Qt::CheckStateRole);

If it is omitted the check boxes won't render as there isn't a valid check-state to render.

The example shows check boxes in a combobox, list and table, as I couldn't get it to work at first either, so I tried different views.

test.cpp

#include <QtGui>

int main(int argc, char** argv)
{
    QApplication app(argc, argv);

    QStandardItemModel model(3, 1); // 3 rows, 1 col
    for (int r = 0; r < 3; ++r)
    {
        QStandardItem* item = new QStandardItem(QString("Item %0").arg(r));

        item->setFlags(Qt::ItemIsUserCheckable | Qt::ItemIsEnabled);
        item->setData(Qt::Unchecked, Qt::CheckStateRole);

        model.setItem(r, 0, item);
    }

    QComboBox* combo = new QComboBox();
    combo->setModel(&model);

    QListView* list = new QListView();
    list->setModel(&model);

    QTableView* table = new QTableView();
    table->setModel(&model);

    QWidget container;
    QVBoxLayout* containerLayout = new QVBoxLayout();
    container.setLayout(containerLayout);
    containerLayout->addWidget(combo);
    containerLayout->addWidget(list);
    containerLayout->addWidget(table);

    container.show();

    return app.exec();
}

test.pro

QT=core gui
SOURCES=test.cpp

I have a little addition.

If one compiles the skyhisi's code then the combobox on Mac OS X doesn't look as combobox with native checkboxes. You can see it on the screenshot.

Tested with qt-4.8.5 and 5.1.1.

It seems like Qt draws these controls by itself. Our team has found the following workaround by pure accident. You can subclass QStyledItemDelegate and reimplement paint() this way:

void SubclassOfQStyledItemDelegate::paint(QPainter * painter_, const QStyleOptionViewItem & option_, const QModelIndex & index_) const
{
    QStyleOptionViewItem & refToNonConstOption = const_cast<QStyleOptionViewItem &>(option_);
    refToNonConstOption.showDecorationSelected = false;
    //refToNonConstOption.state &= ~QStyle::State_HasFocus & ~QStyle::State_MouseOver;

    QStyledItemDelegate::paint(painter_, refToNonConstOption, index_);
}

You can then set this delegate to the combo box by adding the following lines to skyhisi's code:

SubclassOfQStyledItemDelegate *delegate = new SubclassOfQStyledItemDelegate(this);
combo->setItemDelegate(delegate);

The comboBox installed with this delegate looks the following way:

On Windows there may be a different issue: text of the checkBoxes has sticked background or dotted border around an item:

To change this appearance one can add the following line to the overridden paint just before the line QStyledItemDelegate::paint(painter_, refToNonConstOption, index_) (in the code sample this line was commented):

refToNonConstOption.state &= ~QStyle::State_HasFocus & ~QStyle::State_MouseOver;

Result: