Write a QList of QPair to a QSettings object

I want to persistently store a list of integer pairs, using Qt Framework.

I thought of using QList< QPair < int,int>> as a type for the list, and to store them in an .ini file using QSettings.

Unfortunately, the function QSettings::setValue raises an error:

no matching function for call to ‘QSettings::setValue(const char [19], FavoriteList&)’
note:   no known conversion for argument 2 from QList< QPair< int, int> > to ‘const QVariant&’

It seems that it cannot cast that type to a QVariant. I tried declaring it with Q_DECLARE_METATYPE, but it didn't work, it raises the same error.

Example of code that fails:

QList<QPair<int,int>> list;

if(!settings.contains("Radio/Favorites/FM"))
{
    settings.setValue("Radio/Favorites/FM", list);
}

How can I write that type to a QSettings object?

Solution

You simply don't have any compatible implicit conversion. The compiler doesn't know how to turn your list to a variant.

In fact, Q_DECLARE_METATYPE shoudln't be necessary at all, at least for me the following code compiles with no errors:

  QSettings s;
  QList< QPair < int,int>> l;
  s.setValue("key", QVariant::fromValue(l));

EDIT: while it doesn't get compile errors, it will still get a runtime error because it doens't know how to serialize and deserialize a list of int pairs. So you will have to tell it how:

QDataStream &operator<<(QDataStream &out, const QList<QPair<int,int>> &l) {
  int s = l.size();
  out << s;
  if (s) for (int i = 0; i < s; ++i) out << l[i].first << l[i].second;
  return out;
}
QDataStream &operator>>(QDataStream &in, QList<QPair<int,int>> &l) {
  if (!l.empty()) l.clear();
  int s;
  in >> s;
  if (s) {
    l.reserve(s);
    for (int i = 0; i < s; ++i) {
      int f, sec;
      in >> f >> sec;
      l.append(QPair<int, int>(f, sec));
    }
  }
  return in;
}

And also register stream operators for the type in the meta system:

qRegisterMetaTypeStreamOperators<QList<QPair<int,int>>>("whatever");

Aaand.. it works!

  QSettings s;
  QList<QPair<int,int>> l;
  l.append(QPair<int, int>(555, 556));
  s.setValue("key", QVariant::fromValue(l));
  QList<QPair<int,int>> l1 = s.value("key").value<QList<QPair<int,int>>>();
  qDebug() << l1.at(0).first << l1.at(0).second; // 555 556