Serialization in Qt - part 3

In the previous post I already wrote about backward and forward compatibility when serializing and deserializing data into a binary stream. Let's summarize:

  • backward compatibility - the ability to deserialize data serialized by and older version of the application
  • forward compatibility - the ability to deserialize data serialized by a newer version of the application

I said that backward compatibility can be achieve by storing a version tag in the data stream and conditionally changing the deserialization routine based on the version of data. However forward compatibility cannot be achieved this way because we can't predict what changes will be made in the future. This is fine for configuration data, but in case of documents it's not always acceptable.

The best solution would be to allow the application to skip and ignore information it doesn't understand, and extract as much information as it can. Note that it's not always possible. In case of a text document, the content can be preserved even if some fancy formatting is lost. However, let's recall the example in which we added child bookmarks to the Bookmark class. Even if we could skip loading the child bookmarks, we would still lose a lot of information, as only the top level bookmarks would be available in the old version. So before we start thinking about a fancy solution, we should first ask ourserlves if it's really worth the effort.

There is also a relatively simple workaround available. The new version of the application can be forced to save data in format compatible with an older version. This simply means that we have to add similar conditional code in serialization routines. Many applications work in this way, including MS Office applications. For example, the application could save all bookmarks in a linear fashion, losing the parent-child relationship, but still preserving all bookmarks.

But for true compatibility we need to design the data format in such way, that when the application encounters data that it doesn't understand, it can at least skip it and continue processing. But without additional metadata the application doesn't even know how many bytes it should skip.

A simple solution is to wrap all data in a QVariant before serializing, because QVariant writes a tag which identifies the type of the data before the actual data. Let's start with the following code:

template<typename T>
void operator <<( QVariant& data, const T& target )
    data = QVariant::fromValue<T>( target );

template<typename T>
void operator >>( const QVariant& data, T& target )
    target = data.value<T>();

These are generic function templates that convert any data to and from a variant. Now let's specialize these functions for our Bookmark type from the previous post. We will use a map to convert a bookmark to a variant and vice versa:

void operator <<( QVariant& data, const Bookmark& target )
    QVariantMap map;
    map[ "Name" ] << target.m_name;
    map[ "URL" ] << target.m_url;
    map[ "Children" ] << target.m_children;
    data << map;

void operator >>( const QVariant& data, Bookmark& target )
    QVariantMap map;
    data >> map;
    map[ "Name" ] >> target.m_name;
    map[ "URL" ] >> target.m_url;
    map[ "Children" ] >> target.m_children;

Note that because the bookmark object is converted to a QVariantMap before serializing, it can be successfully deserialized even if the application that reads the data doesn't know anything about the Bookmark type. What's more, we can add more elements to the map in the future without affecting either backward or forward compatibility. When reading a newer version of the file, the elements which are not understood are simply ignored. When reading an older version, missing elements are automatically replaced with default values for the given type.

When we try to compile the above code, we will receive a cryptic error similar to 'qt_metatype_id' : is not a member of 'QMetaTypeId<T>'. That's because a QList<Bookmark> cannot be converted into a QVariant. Since we know how to convert a Bookmark into a QVariant, we can easily convert a QList<Bookmark> into a QVariantList. This can even be done in a generic way:

template<typename T>
void operator <<( QVariant& data, const QList<T>& target )
    QVariantList list;
    list.reserve( target.count() );
    for ( int i = 0; i < target.count(); i++ ) {
        QVariant item;
        item << target[ i ];
        list.append( item );
    data = list;

template<typename T>
void operator >>( const QVariant& data, QList<T>& target )
    QVariantList list = data.toList();
    target.reserve( list.count() );
    for ( int i = 0; i < list.count(); i++ ) {
        T item;
        list[ i ] >> item;
        target.append( item );

This way any QList<T> can be converted from/to a QVariant as long as T can be converted from/to a QVariant. Note that we may also want to create additional specializations for QStringList and QVariantList, so that they are not unnecessarily converted, and to add similar conversion functions for maps and other containers.

To summarize, the following conversions are used before data is serialized:

  • Primitive types (numbers, strings and many other built-in types in Qt) are stored as QVariant
  • Objects are stored as QVariantMap that maps properties to values
  • List of various types are stored as QVariantList

The actual serialization consists of two steps: converting the serialized object into a QVariant and serializing the converted data into the stream. Deserialization is analogous and works in the opposite way.

You can notice that the converted data is somewhat similar to the DOM tree of XML document. A variant of a primitive type is analogous to a leaf XML node, and a map of variants is similar to an XML element with child nodes. However, this approach is more compact, faster and easier to use than XML.

Note that simple custom types don't necessarily have to be stored as a QVariantMap. For example, in a financial application, there may be a Money class, which is really a wrapper over some simple numeric value. We can directly place this numeric value in a variant (e.g. as a qlonglong) without wrapping it in a map.

We can also combine the method of serialization based on QVariantMap with the traditional approach, as described in the previous article, for certain types that highly unlikely to change, and can be treated as primitive types. For example, in a graphic application we might define a Circle class which consists of a central QPoint and a radius. We can use the Q_DECLARE_METATYPE macro and the qRegisterMetaTypeStreamOperators function, so that the Circle object can be directly wrapped into a variant and serialized without any conversions.

Just remember that when the Circle type is introduced in a later version of the application, previous versions will not be able to load a file that contains it, so we must remeber about the version tag, as described in the previous post. Also the version of the data format used by built-in Qt types is important to maintain compatibility across different environments.

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