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Tracking

The Serialization library tracks the memory address (a.k.a pointers) of serialized objects to make sure objects are serialized only once. Whenever saving of a tracked object is requested, the library checks the memory address of the object. If it matches that of a previously saved object, only a reference to the previous object will be stored. When the objects are read back the library makes sure all pointer and references remain valid independent of the saving order. The programmer may however affect the performance of deserialization by the saving order.

If an object is first saved through a pointer, the Serialization library has no way of knowing whether it will later be saved as an object. Thus, if the object is later saved again as an object, it will be saved again. When reading the archive, the object will be first a memory location through the pointer. Later, the library will notice that the object itself was stored. The contents of the previously allocated object must be released. Better performance would be achieved by first storing the object itself.

To be able to preserve memory references, the Serialization library must know the final location of deserialized objects. Therefore, if a tracked object is deserialized into a temporary memory location and subsequently moved to its final location, the move must be told to the deserializer. The following example illustrates this. Note that PiiSerializationUtil.h provides a function for saving QLists, so you don't actually need to do this manually. 

 QList<MyObj> lst; // filled somehow

 // Read elements in sequence in your load() function:
 for (int i=0; i<elementCount; i++)
   {
     MyObj obj;
     archive >> obj;
     lst << obj;
     // "archive" now thinks &obj is the address of the deserialized object.
     // If a pointer refers to this object, it must be updated
     archive.objectMoved(obj, lst.last());
   }

The PiiInputArchive::objectMoved() function updates all references currently pointing to a certain memory address to a new address. If you don't signal the move, any subsequently restored pointer to the object will point to the local variable.

Tracking can be selectively turned off with the PiiSerializationTraits::Tracking trait.

The tracking mechanism is implemented in PiiOutputArchive and PiiInputArchive. The former uses PiiTrackedPointerHolder objects to store pointers to tracked objects. Please refer to the PiiSerialization::createTrackedPointerHolder() and PiiSerialization::rereferencePointer() functions if you want to serialize reference-counted objects or customize the tracking behaviour in other ways.

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