How to override (re-implement) a member function i

Question

I've been struggling with this for a while.

Qt's QFileSystemModel is really slow when fetching several hundred files because of a really bad icon fetching algorithm.

I want to completely disable icons. They are fetched in QFileSystemModel::data method which is not virtual (the source of QFileSystemModel is available here: http://qt.gitorious.org/qt/qt/blobs/4.7/src/gui/dialogs/qfilesystemmodel.cpp)

This is the code I'm trying to run:

class FileModel : public QFileSystemModel {
    using QFileSystemModel::data;
public:
    QVariant QFileSystemModel::data(const QModelIndex &index, int role) const
    {
     // my own implementation here
    }
}

but it fails with

cannot define member function QFileSystemModel::data witin FileModel

UPDATE

I've managed to override the function and this is what I have now:

class FileModel : public QFileSystemModel {
public:
    QVariant data(const QModelIndex &index, int role) const
    {
    // Here goes Qt's implementation
    Q_D(const QFileSystemModel);
    if (!index.isValid() || index.model() != this)
        return QVariant();
    switch (role) {
    case Qt::EditRole:
    case Qt::DisplayRole:
        switch (index.column()) {
        case 0: return d->name(index);
        case 1: return d->size(index);
        case 2: return d->type(index);
        case 3: return d->time(index);
        default:
            qWarning("data: invalid display value column %d", index.column());
            break;
        }
        break;
    case FilePathRole:
        return filePath(index);
    case FileNameRole:
        return d->name(index);
    case Qt::DecorationRole:
        if (index.column() == 0) {
            QIcon icon = d->icon(index); // This is the part I need to change
            if (icon.isNull()) {
                if (d->node(index)->isDir())
                    icon = d->fileInfoGatherer.iconProvider()->icon(QFileIconProvider::Folder);
                else
                    icon = d->fileInfoGatherer.iconProvider()->icon(QFileIconProvider::File);
            }
            return icon;
        }
        break;
    case Qt::TextAlignmentRole:
        if (index.column() == 1)
            return Qt::AlignRight;
        break;
    case FilePermissions:
        int p = permissions(index);
        return p;
    }
    return QVariant();
}
};

However this code doesn't compile. This is the error I get: 'const QFileSystemModelPrivate* QFileSystemModel::d_func() const' is private

Answer 1

Looks like this was anticipated, as there is a method for setting the "icon provider":

http://doc.qt.io/archives/qt-4.7/qfilesystemmodel.html#setIconProvider

The parameter, a QFileIconProvider, looks to be a fairly simple class that you can implement an instance of yourself, with a method that fetches an icon from a QFileInfo (basically, a file name):

http://doc.qt.io/archives/qt-4.7/qfileinfo.html

You could implement one of these that just returns the same icon for everything. If you find that doesn't address your problem, the following compiled fine for me...FWIW:

class FileModel : public QFileSystemModel {
public:
    QVariant data(const QModelIndex &index, int role) const
    {
        if (role == Qt::DecorationRole) {
            return QVariant (QIcon ());
        }    

        return QFileSystemModel::data(index, role);
    }
};

Answer 2

If a function in a base class is virtual then it is virtual in derived classes as well. The following will print "C":

#include <iostream>

class A {
public:
  virtual void data() = 0;
};

class B: public A {
public:
  void data() { std::cout << "B\n"; }
};

class C: public B {
public:
  void data() { std::cout << "C\n"; }
};

int
main() {
  C c;
  A *a = &c;
  a->data();

  return 0;
}

QFileSystemDialog is derived from QAbstractItemModel in which data() is pure virtual. You couldn't even instatiate the former if it didn't override data() with its own implementation.

See http://qt-project.org/doc/qt-4.8/qabstractitemmodel.html#data