Can't read QList<Class*> from a file

Question

I have a problem with stream operator>>. I'm trying to save and load on file a QList of custom objects. The save routine seems working fine but reading the file causes a crash. I prepared a very minimal example. First of all the custom class:

class CustomObject : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString name READ name WRITE setName)
public:
    explicit CustomObject(QObject *parent = 0);
    CustomObject(const CustomObject & copy, QObject *parent = 0);

    QString name() const;
    void setName( const QString & name);

private:
    QString m_name;
};

Q_DECLARE_METATYPE( CustomObject )

QDataStream& operator<<( QDataStream& dataStream, const CustomObject * item );
QDataStream& operator>>( QDataStream& dataStream, CustomObject * item );

I have implemented stream operators this way:

QDataStream &operator<<(QDataStream &dataStream, const CustomObject *item)
{
    for(int i = 0; i < item->metaObject()->propertyCount(); ++i) {
        if(item->metaObject()->property(i).isStored(item)) {
            dataStream << item->metaObject()->property(i).read(item);
        }
    }
    return dataStream;
}


QDataStream &operator>>(QDataStream &dataStream, CustomObject *item)
{
    QVariant var;
    for(int i = 0; i < item->metaObject()->propertyCount(); ++i) {
        if(item->metaObject()->property(i).isStored(item)) {
            dataStream >> var;
            item->metaObject()->property(i).write(item, var);
        }
    }
    return dataStream;
}

This is the save() function (m_objectsList is QList<CustomObject*>)

QFile saveFile("/tmp/SaveFile");
if(!saveFile.open(QIODevice::WriteOnly)) {
    qWarning() << saveFile.errorString();
    return;
}

QDataStream outStream(&saveFile);
outStream.setVersion(QDataStream::Qt_4_8);
outStream << m_objectsList;
saveFile.close();

and this is the read() routine:

QFile saveFile("/tmp/SaveFile");
if(!saveFile.open(QIODevice::ReadOnly)) {
    qWarning() << saveFile.errorString();
    return;
}

QDataStream inStream(&saveFile);
inStream >> m_objectsList;
saveFile.close();

The application segfaults at the condition statement of for loop in operator>>:

i < item->metaObject()->propertyCount()

item is not accessible.

Can you explain me where is the error?

Very thanks.

Answer 1

Your code segfaults because item is a dangling pointer. Nobody has initialized it. It's on you to instantiate an item before you read it.

You should probably also attempt to store the dynamic properties, and ensure that there's at least some potential for backwards compatibility.

The code below provides two template functions that serialize an object list (as defined by its properties). First, let's re-iterate the object type:

// https://github.com/KubaO/stackoverflown/tree/master/questions/prop-storage-24185694
#include <QtCore>

class CustomObject : public QObject {
   Q_OBJECT
   Q_PROPERTY(QString name READ name WRITE setName STORED true)
   QString m_name;
public:
#ifdef Q_MOC_RUN
   Q_INVOKABLE CustomObject(QObject *parent = {})
#endif
   using QObject::QObject;

   QString name() const { return m_name; }
   void setName(const QString &name) { m_name = name; }
};

Some helpers:

/// Returns a zero-copy byte array wrapping a C string constant
static QByteArray baFromCStr(const char *str) {
   return QByteArray::fromRawData(str, qstrlen(str));
}

/// Returns a list of stored properties for a given type
QList<QMetaProperty> storedProperties(const QMetaObject *mo) {
   QList<QMetaProperty> stored;
   for (int i = 0; i < mo->propertyCount(); ++i) {
      auto prop = mo->property(i);
      if (prop.isStored())
         stored << prop;
   }
   return stored;
}

/// Caches strings for saving to a data stream
struct SaveCache {
   QMap<QByteArray, qint32> strings;
   QDataStream &save(QDataStream &str, const QByteArray &string) {
      auto it = strings.find(string);
      if (it != strings.end())
         return str << (qint32)it.value();
      auto key = strings.count();
      strings.insert(string, key);
      return str << (qint32)key << string;
   }
   QDataStream &save(QDataStream &str, const char *string) {
      return save(str, baFromCStr(string));
   }
};

/// Caches strings while loading from a data stream
struct LoadCache {
   QList<QByteArray> strings;
   QDataStream &load(QDataStream &str, QByteArray &string) {
      qint32 index;
      str >> index;
      if (index >= strings.count()) {
         str >> string;
         while (strings.size() < index)
            strings << QByteArray{};
         strings << string;
      } else
         string = strings.at(index);
      return str;
   }
};

The format stored in the stream is may be described as follows:

template <typename T>
QDataStream &writeObjectList(QDataStream &str, const QList<T*> &items) {
   str << (quint32)items.count();
   if (! items.count()) return str;
   str << (quint8)1; // version

   SaveCache strings;
   for (QObject *item : items) {
      auto *mo = item->metaObject();
      // Type
      strings.save(str, mo->className());
      // Properties
      auto const stored = storedProperties(mo);
      auto const dynamic = item->dynamicPropertyNames();
      str << (quint32)(stored.count() + dynamic.count());
      for (auto &prop : qAsConst(stored))
         strings.save(str, prop.name()) << prop.read(item);
      for (auto &name : dynamic)
         strings.save(str, name) << item->property(name);
   }
   return str;
}

The reading method has to try two ways of instantiating the object:

template <typename T> QDataStream &readObjectList(QDataStream &str,
                                                  QList<T*> &items)
{
   quint32 itemCount;
   str >> itemCount;
   if (!itemCount) return str;
   quint8 version;
   str >> version;
   if (version != 1) {
      str.setStatus(QDataStream::ReadCorruptData);
      return str;
   }

   LoadCache strings;
   for (; itemCount; itemCount--) {
      QByteArray string;
      // Type
      T *obj = {};
      strings.load(str, string);
      if (T::staticMetaObject.className() == string)
         obj = new T();
      else {
         string.append('*');
         auto id = QMetaType::type(string);
         const auto *mo = QMetaType::metaObjectForType(id);
         if (mo)
            obj = qobject_cast<T*>(mo->newInstance());
      }
      if (obj)
         items << obj;
      // Properties
      quint32 propertyCount;
      str >> propertyCount;
      for (uint i = 0; i < propertyCount; ++i) {
         QVariant value;
         strings.load(str, string) >> value;
         if (obj) obj->setProperty(string, value);
      }
   }
   return str;
}

And a very simple test harness:

QDataStream &operator<<(QDataStream &str, const QList<CustomObject*> &items) {
   return writeObjectList(str, items);
}

QDataStream& operator>>(QDataStream &str, QList<CustomObject*> &items) {
   return readObjectList(str, items);
}

int main() {
   qRegisterMetaType<CustomObject*>(); // necessary for any classes derived from CustomObject*

   QBuffer buf;
   buf.open(QBuffer::ReadWrite);
   QDataStream ds(&buf);

   CustomObject obj;
   obj.setObjectName("customsky");
   obj.setProperty("prop", 20);
   QList<CustomObject*> list;
   list << &obj;
   ds << list;

   QList<CustomObject*> list2;
   buf.seek(0);
   ds >> list2;
   Q_ASSERT(list2.size() == list.size());
   for (int i = 0; i < list.size(); ++i) {
      auto *obj1 = list.at(i);
      auto *obj2 = list2.at(i);
      Q_ASSERT(obj1->objectName() == obj2->objectName());
      Q_ASSERT(obj1->dynamicPropertyNames() == obj2->dynamicPropertyNames());
      for (auto &name : obj1->dynamicPropertyNames())
         Q_ASSERT(obj1->property(name) == obj2->property(name));
   }
}

#include "main.moc"

Answer 2

Your code is crashing because in your input operator the item pointer does not actually point to an object, and is probably null. To fix this, the operator should take a reference to the pointer, and create a new instance of CustomObject(). Something like this:

QDataStream &operator>>(QDataStream &dataStream, CustomObject *& item)
{
    QVariant var;
    item = new CustomObject();
    for(int i = 0; i < item->metaObject()->propertyCount(); ++i) {
        if(item->metaObject()->property(i).isStored(item)) {
            dataStream >> var;
            item->metaObject()->property(i).write(item, var);
        }
    }
    return dataStream;
}