I'm trying to run example from Qt documentation about QTreeView.
I copied two classes:
treeitem.h

#ifndef TREEITEM_H
#define TREEITEM_H

#include <QtCore>

class TreeItem
{
public:
    explicit TreeItem(const QVector<QVariant> &data, TreeItem *parentItem = nullptr);
    ~TreeItem();

    void appendChild(TreeItem *child);

    TreeItem *child(int row);
    int childCount() const;
    int columnCount() const;
    QVariant data(int column) const;
    int row() const;
    TreeItem *parentItem();

private:
    QVector<TreeItem*> m_childItems;
    QVector<QVariant> m_itemData;
    TreeItem *m_parentItem;
};

#endif // TREEITEM_H

treeitem.cpp

#include "treeitem.h"

TreeItem::TreeItem(const QVector<QVariant> &data, TreeItem *parent)
    : m_itemData(data), m_parentItem(parent)
{

}

TreeItem::~TreeItem()
{
    qDeleteAll(m_childItems);
}

void TreeItem::appendChild(TreeItem *item)
{
    m_childItems.append(item);
}

TreeItem *TreeItem::child(int row)
{
    if (row < 0 || row >= m_childItems.size())
        return nullptr;
    return m_childItems.at(row);
}

int TreeItem::childCount() const
{
    return m_childItems.count();
}

int TreeItem::row() const
{
    if (m_parentItem)
        return m_parentItem->m_childItems.indexOf(const_cast<TreeItem*>(this));

    return 0;
}

And the seond class:
treemodel.h

#ifndef TREEMODEL_H
#define TREEMODEL_H

#include <QtCore>
#include <treeitem.h>


class TreeModel : public QAbstractItemModel
{
    Q_OBJECT

public:
    explicit TreeModel(const QString &data, QObject *parent = nullptr);
    ~TreeModel();

    QVariant data(const QModelIndex &index, int role) const override;
    Qt::ItemFlags flags(const QModelIndex &index) const override;
    QVariant headerData(int section, Qt::Orientation orientation,
                        int role = Qt::DisplayRole) const override;
    QModelIndex index(int row, int column,
                      const QModelIndex &parent = QModelIndex()) const override;
    QModelIndex parent(const QModelIndex &index) const override;
    int rowCount(const QModelIndex &parent = QModelIndex()) const override;
    int columnCount(const QModelIndex &parent = QModelIndex()) const override;

private:
    void setupModelData(const QStringList &lines, TreeItem *parent);

    TreeItem *rootItem;
};

#endif // TREEMODEL_H

treemodel.cpp

#include "treemodel.h"

TreeModel::TreeModel(const QString &data, QObject *parent)
    : QAbstractItemModel(parent)
{
    rootItem = new TreeItem({tr("Title"), tr("Summary")});
    setupModelData(data.split('\n'), rootItem);
}

TreeModel::~TreeModel()
{
    delete rootItem;
}

QModelIndex TreeModel::index(int row, int column, const QModelIndex &parent) const
{
    if (!hasIndex(row, column, parent))
        return QModelIndex();

    TreeItem *parentItem;

    if (!parent.isValid())
        parentItem = rootItem;
    else
        parentItem = static_cast<TreeItem*>(parent.internalPointer());

    TreeItem *childItem = parentItem->child(row);
    if (childItem)
        return createIndex(row, column, childItem);
    return QModelIndex();
}

QModelIndex TreeModel::parent(const QModelIndex &index) const
{
    if (!index.isValid())
        return QModelIndex();

    TreeItem *childItem = static_cast<TreeItem*>(index.internalPointer());
    TreeItem *parentItem = childItem->parentItem();

    if (parentItem == rootItem)
        return QModelIndex();

    return createIndex(parentItem->row(), 0, parentItem);
}

int TreeModel::rowCount(const QModelIndex &parent) const
{
    TreeItem *parentItem;
    if (parent.column() > 0)
        return 0;

    if (!parent.isValid())
        parentItem = rootItem;
    else
        parentItem = static_cast<TreeItem*>(parent.internalPointer());

    return parentItem->childCount();
}

int TreeModel::columnCount(const QModelIndex &parent) const
{
    if (parent.isValid())
        return static_cast<TreeItem*>(parent.internalPointer())->columnCount();
    return rootItem->columnCount();
}

QVariant TreeModel::data(const QModelIndex &index, int role) const
{
    if (!index.isValid())
        return QVariant();

    if (role != Qt::DisplayRole)
        return QVariant();

    TreeItem *item = static_cast<TreeItem*>(index.internalPointer());

    return item->data(index.column());
}

Qt::ItemFlags TreeModel::flags(const QModelIndex &index) const
{
    if (!index.isValid())
        return Qt::NoItemFlags;

    return QAbstractItemModel::flags(index);
}

QVariant TreeModel::headerData(int section, Qt::Orientation orientation,
                               int role) const
{
    if (orientation == Qt::Horizontal && role == Qt::DisplayRole)
        return rootItem->data(section);

    return QVariant();
}

If I add those file to the Qt project then I get the following errors when the app is building:

How to launch this example?