The Ultimate 2025 Guide to QTableView & ComboBox Events

We only care about the last case—the *committed* change. If we react too early, we might save incomplete data to our database or trigger unnecessary updates throughout our application. The view's standard signals, like dataChanged from the model, won't fire until the entire editing process is complete, so how do we tap into that exact moment?

Method 1: The currentIndexChanged Signal (The Obvious, But Flawed Approach)

Your first instinct might be to get a pointer to the QComboBox editor and connect to its currentIndexChanged(int index) signal. It seems logical. Let's see why this is usually a trap.

// Inside a custom delegate's createEditor method...
QComboBox *editor = new QComboBox(parent);
// ... populate the editor ...
connect(editor, &QComboBox::currentIndexChanged, this, &MyDelegate::onEditorChanged);
return editor;

The problem? currentIndexChanged fires every single time the highlighted item changes. If a user presses the down arrow three times to get to their desired option, your onEditorChanged slot will be called three times. This is rarely the behavior you want. It's noisy and doesn't represent a final decision.

Method 2: Subclassing QStyledItemDelegate (The Canonical Way)

The correct and robust solution lies in properly implementing a custom delegate. The delegate acts as the bridge, and it has specific methods designed for this exact workflow. The two most important methods for us are setEditorData and setModelData.

The Key Delegate Methods

1. createEditor(...): This is called by the view when the user starts editing an item. Your job is to create the editor widget (our QComboBox) and return it.
2. setEditorData(...): Immediately after the editor is created, this method is called. Its purpose is to take data from the model and use it to set the initial state of the editor. For a combo box, this means setting its current index to match the data in the model.
3. setModelData(...): This is the magic bullet. This method is called only when the editing is finished and the editor is about to be destroyed. This happens when the user selects an item and presses Enter or clicks away. Here, you take the final value from the editor (the QComboBox's current text or index) and use it to update the model. This is the perfect place to know a change has been committed.

By using setModelData, you are guaranteed to be acting on the user's final decision, not on their in-progress exploration of the options.

A Deep Dive into the Delegate's Event Lifecycle

To really solidify the concept, let's compare the events in a table. Imagine we have a combo box with options ["Low", "Medium", "High"].

User Action	currentIndexChanged Fires?	setModelData Called?	Correct Interpretation
Double-clicks cell to open editor	No	No	Editing has begun. createEditor and setEditorData are called.
Presses down-arrow to highlight "Medium"	Yes	No	User is browsing. The change is not yet committed.
Presses down-arrow again to highlight "High"	Yes	No	User is still browsing.
Clicks on "High" or presses Enter	No (if already on "High")	Yes	Editing is finished! The user has committed the value "High".
Opens editor, then clicks outside the table	No (if no change was made)	No (if no change was made)	Editing was cancelled. The model is not updated.

As you can see, setModelData is the only reliable indicator of a completed edit.

Putting It All Together: A Practical C++ Example

Let's build a minimal, working example. We'll create a custom delegate, StatusDelegate, for a column that lets the user choose a status from a combo box.

The Delegate Header (StatusDelegate.h)

#include <QStyledItemDelegate>

class StatusDelegate : public QStyledItemDelegate
{
    Q_OBJECT
public:
    explicit StatusDelegate(QObject *parent = nullptr);

    QWidget *createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const override;
    void setEditorData(QWidget *editor, const QModelIndex &index) const override;
    void setModelData(QWidget *editor, QAbstractItemModel *model, const QModelIndex &index) const override;
    void updateEditorGeometry(QWidget *editor, const QStyleOptionViewItem &option, const QModelIndex &index) const override;
};

The Delegate Implementation (StatusDelegate.cpp)

#include "StatusDelegate.h"
#include <QComboBox>

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

QWidget *StatusDelegate::createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    Q_UNUSED(option);
    Q_UNUSED(index);

    QComboBox *editor = new QComboBox(parent);
    editor->addItems({"Pending", "In Progress", "Completed", "Cancelled"});
    return editor;
}

void StatusDelegate::setEditorData(QWidget *editor, const QModelIndex &index) const
{
    QString value = index.model()->data(index, Qt::EditRole).toString();
    QComboBox *comboBox = static_cast<QComboBox*>(editor);
    comboBox->setCurrentText(value);
}

void StatusDelegate::setModelData(QWidget *editor, QAbstractItemModel *model, const QModelIndex &index) const
{
    QComboBox *comboBox = static_cast<QComboBox*>(editor);
    QString value = comboBox->currentText();

    // This is where the magic happens! The model is updated.
    model->setData(index, value, Qt::EditRole);

    // You could also emit a custom signal here if other parts of your app need to know immediately.
    // emit dataCommitted(index, value);
}

void StatusDelegate::updateEditorGeometry(QWidget *editor, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    Q_UNUSED(index);
    editor->setGeometry(option.rect);
}

Using the Delegate in Your Main Window

// In your main window or widget constructor
QTableView *tableView = new QTableView(this);
QStandardItemModel *model = new QStandardItemModel(4, 2, this);

// ... populate your model with some data ...

// This is the crucial step
StatusDelegate *delegate = new StatusDelegate(this);
tableView->setItemDelegateForColumn(1, delegate); // Use our custom delegate for column 1

tableView->setModel(model);

// Now, you can simply connect to the model's dataChanged signal.
// It will only be emitted after setModelData completes.
connect(model, &QStandardItemModel::dataChanged,
        this, [](const QModelIndex &topLeft, const QModelIndex &bottomRight) {
    qDebug() << "Data changed at row:" << topLeft.row() << "New value:" << topLeft.data().toString();
});

Conclusion: Mastering Your Qt UI Events

Navigating the event system for editors within Qt's item views can be tricky, but it's built on a logical and powerful foundation. While connecting directly to an editor's signal like currentIndexChanged is tempting, it's a path fraught with premature updates and messy logic.

The key takeaway for 2025 and beyond is to embrace the delegate pattern fully. By subclassing QStyledItemDelegate and placing your commit logic inside the setModelData method, you align your code with Qt's intended design. This ensures you are only reacting when a user has made a definitive choice, leading to cleaner, more predictable, and more robust applications. Happy coding!