Convert QMap to JSON

Question

I have a QMap object and I would like to convert it to JSON. I am confused how I would accomplish this.

I read QT documentation saying that I can use QDataStream to convert QMap to JSON, but QDataStream seems to convert files: http://doc.qt.io/qt-4.8/datastreamformat.html

// c++
QMap<QString, int> myMap;

Answer 1

It would be easiest to convert the map to QVariantMap which can automatically be converted to a JSON document:

QMap<QString, int> myMap;
QVariantMap vmap;

QMapIterator<QString, int> i(myMap);
while (i.hasNext()) {
    i.next();
    vmap.insert(i.key(), i.value());
}

QJsonDocument json = QJsonDocument::fromVariant(vmap);

The same thing can be used to create a QJsonObject if you want, via the QJsonObject::fromVariant() static method. Although for QJsonObject you can skip the conversion to variant map step and simply populate the object manually as you iterate the map:

QMap<QString, int> myMap;
QJsonObject json;

QMapIterator<QString, int> i(myMap);
while (i.hasNext()) {
    i.next();
    json.insert(i.key(), i.value());
}    

Answer 2

If you are using Qt 5.5 or higher you could use QJsonDocument::fromVariant, your map could be converted easily to a QVariantMap. If not, try QJson

For your purpose, you are looking for QMAP serialization, see this link: Serialization Qt. Try to set up the constructor with a QByteArray, something like this:

   QByteArray serializeMap(const QMap<QString, int>& map) {
      QByteArray buffer;
      QDataStream stream(&buffer, QIODevice::WriteOnly);
      out << map;
      return out;
   }

That's, will serialize your map in a QByteArray wich could be easily converted to a QString or std::string.