QModelIndex as parent?

Question

In Qt, QModelIndex is used to represent an index to my understanding. Officially :

This class is used as an index into item models derived from QAbstractItemModel . The index is used by item views, delegates, and selection models to locate an item in the model.

But I see it being used to represent a parent object. For instance, if I want to get an index in a QFileSystemModel object, I need a row, column and a parent:

QModelIndex QFileSystemModel::index(int row, int column, const QModelIndex &parent = QModelIndex()) const

I am trying to get a QModelIndex object, but to do that, I need another QModelIndex object? I am merely trying to iterate over the model. I don't have a separate parent object. How do I just create an index from row/column number? I don't understand the role of QModelIndex as a "parent". Shouldn't the model itself know what the parent object is? We passed a pointer to the constructor when creating the model.

Here's a bit of code showing the problem:

#include "MainWindow.hpp"
#include "ui_MainWindow.h"

#include <QFileSystemModel>
#include <QDebug>


MainWindow::MainWindow(QWidget *parent) :
  QMainWindow(parent),
  ui(new Ui::MainWindow)
{
  ui->setupUi(this);

  auto* model = new QFileSystemModel{ui->listView};
  ui->listView->setModel(model);
  ui->listView->setRootIndex(model->setRootPath("C:\\Program Files"));
  connect(ui->pushButton, &QPushButton::clicked, [this] {
    auto* model = static_cast<QFileSystemModel*>(ui->listView->model());
    int row_count = model->rowCount();
    for (int i = 0; i != row_count; ++i) {
      qDebug() << model->fileName(model->index(i, 0)) << '\n';
    }
  });
}

Here I have a QListView object ( *listView ) and a QFileSystemModel object ( *model ). I would like to iterate over the model and do something, like print the names of the files. The output is

C:

No matter which directory the rootpath is. I assume that is because I did't pass anything as the parent.

Answer 1

You're just accessing the children of the root of the QFileSystemModel when you default the parent node to QModelIndex() in the call model->index(i, 0) .

If you also want to list the children of those items, we'll want to iterate them, too:

#include <QApplication>
#include <QDebug>
#include <QFileSystemModel>

void list_files(const QFileSystemModel *model, QModelIndex ix = {},
                QString indent = {})
{
    auto const row_count = model->rowCount(ix);
    for (int i = 0;  i < row_count;  ++i) {
        auto const child = model->index(i, 0, ix);
        qDebug() << qPrintable(indent) << model->fileName(child);
        list_files(model, child, indent + " ");
    }
}

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

    QFileSystemModel model;
    model.setRootPath(".");

    list_files(&model);
}

See how we pass the child index as the new parent when we recurse into list_files() ?

Note that the model is likely incomplete at this stage, as it implements lazy reading - so don't expect to see all your files with this simple program.