Qt Model/View A simple tree model

Example

QModelIndex does not actually know about it's parent/child indexes, it only contains a row, a column and a pointer, and it is the models responsibility to use this data to provide information an index's relations. The model therefore needs to do a lot of conversions from the void* stored inside the QModelIndex to an internal data type and back.

TreeModel.h:

#pragma once

#include <QAbstractItemModel>

class TreeModel : public QAbstractItemModel
{
    Q_OBJECT
public:
    explicit TreeModel(QObject *parent = nullptr);

    // Reimplementation of QAbstractItemModel methods
    int rowCount(const QModelIndex &index) const override;
    int columnCount(const QModelIndex &index) const override;
    QModelIndex index(const int row, const int column,
        const QModelIndex &parent) const override;
    QModelIndex parent(const QModelIndex &childIndex) const override;
    QVariant data(const QModelIndex &index, const int role) const override;
    bool setData(const QModelIndex &index, const QVariant &value,
        const int role) override;
    Qt::ItemFlags flags(const QModelIndex &index) const override;

    void addRow(const QModelIndex &parent, const QVector<QVariant> &values);
    void removeRow(const QModelIndex &index);

private:
    struct Item
    {
        ~Item();

        // This could individual members, or maybe some other object that
        // contains the data we want to display/edit
        QVector<QVariant> values;

        // It is this information that the model needs to be able to answer
        // questions like "What's the parent QModelIndex of this QModelIndex?"
        QVector<Item *> children;
        Item *parent = nullptr;

        // Convenience method that's used in several places
        int rowInParent() const;
    };
    Item *m_root;
};

TreeModel.cpp:

#include "TreeModel.h"

// Adapt this to own needs
static constexpr int COLUMNS = 3;

TreeModel::Item::~Item()
{
    qDeleteAll(children);
}
int TreeModel::Item::rowInParent() const
{
    if (parent) {
        return parent->children.indexOf(const_cast<Item *>(this));
    } else {
        return 0;
    }
}

TreeModel::TreeModel(QObject *parent)
    : QAbstractItemModel(parent), m_root(new Item) {}

int TreeModel::rowCount(const QModelIndex &parent) const
{
    // Parent being invalid means we ask for how many rows the root of the
    // model has, thus we ask the root item
    // If parent is valid we access the Item from the pointer stored
    // inside the QModelIndex
    return parent.isValid()
        ? static_cast<Item *>(parent.internalPointer())->children.size()
        : m_root->children.size();
}
int TreeModel::columnCount(const QModelIndex &parent) const
{
    return COLUMNS;
}

QModelIndex TreeModel::index(const int row, const int column,
    const QModelIndex &parent) const
{
    // hasIndex checks if the values are in the valid ranges by using
    // rowCount and columnCount
    if (!hasIndex(row, column, parent)) {
        return QModelIndex();
    }

    // In order to create an index we first need to get a pointer to the Item
    // To get started we have either the parent index, which contains a pointer
    // to the parent item, or simply the root item

    Item *parentItem = parent.isValid()
        ? static_cast<Item *>(parent.internalPointer())
        : m_root;

    // We can now simply look up the item we want given the parent and the row
    Item *childItem = parentItem->children.at(row);

    // There is no public constructor in QModelIndex we can use, instead we need
    // to use createIndex, which does a little bit more, like setting the
    // model() in the QModelIndex to the model that calls createIndex
    return createIndex(row, column, childItem);
}
QModelIndex TreeModel::parent(const QModelIndex &childIndex) const
{
    if (!childIndex.isValid()) {
        return QModelIndex();
    }

    // Simply get the parent pointer and create an index for it
    Item *parentItem = static_cast<Item*>(childIndex.internalPointer())->parent;
    return parentItem == m_root
        ? QModelIndex() // the root doesn't have a parent
        : createIndex(parentItem->rowInParent(), 0, parentItem);
}

QVariant TreeModel::data(const QModelIndex &index, const int role) const
{
    // Usually there will be more stuff here, like type conversion from
    // QVariant, handling more roles etc.
    if (!index.isValid() || role != Qt::DisplayRole) {
        return QVariant();
    }
    Item *item = static_cast<Item *>(index.internalPointer());
    return item->values.at(index.column());
}
bool TreeModel::setData(const QModelIndex &index, const QVariant &value,
    const int role)
{
    // As in data there will usually be more stuff here, like type conversion to
    // QVariant, checking values for validity etc.
    if (!index.isValid() || role != Qt::EditRole) {
        return false;
    }
    Item *item = static_cast<Item *>(index.internalPointer());
    item->values[index.column()] = value;
    emit dataChanged(index, index, QVector<int>() << role);
    return true;
}
Qt::ItemFlags TreeModel::flags(const QModelIndex &index) const
{
    if (index.isValid()) {
        return Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsEditable;
    } else {
        return Qt::NoItemFlags;
    }
}

// Simple add/remove functions to illustrate {begin,end}{Insert,Remove}Rows
// usage in a tree model
void TreeModel::addRow(const QModelIndex &parent,
    const QVector<QVariant> &values)
{
    Item *parentItem = parent.isValid()
        ? static_cast<Item *>(parent.internalPointer())
        : m_root;
    beginInsertRows(parent,
        parentItem->children.size(), parentItem->children.size());
    Item *item = new Item;
    item->values = values;
    item->parent = parentItem;
    parentItem->children.append(item);
    endInsertRows();
}
void TreeModel::removeRow(const QModelIndex &index)
{
    if (!index.isValid()) {
        return;
    }
    Item *item = static_cast<Item *>(index.internalPointer());
    Q_ASSERT(item != m_root);
    beginRemoveRows(index.parent(), item->rowInParent(), item->rowInParent());
    item->parent->children.removeOne(item);
    delete item;
    endRemoveRows();
}