Storing and Loading CObjects via an Archive

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Storing and loading CObjects via an archive requires extra consideration. In certain cases, you should call the Serialize function of the object, where the CArchive object is a parameter of the Serialize call, as opposed to using the << or >> operator of the CArchive. The important fact to keep in mind is that the CArchive >> operator constructs the CObject in memory based on CRuntimeClass information previously written to the file by the storing archive.

Therefore, whether you use the CArchive << and >> operators, versus calling Serialize, depends on whether you need the loading archive to dynamically reconstruct the object based on previously stored CRuntimeClass information. Use the Serialize function in the following cases:

• When deserializing the object, you know the exact class of the object beforehand.

• When deserializing the object, you already have memory allocated for it.

The following example illustrates the cases:

class CMyObject : public CObject
{
// ...Member functions
public:
   CMyObject() { }
   virtual void Serialize( CArchive& ar );

// Implementation
protected:
   DECLARE_SERIAL( CMyObject )
};


class COtherObject : public CObject
{
   // ...Member functions
public:
   COtherObject() { }
   virtual void Serialize( CArchive& ar );

// Implementation
protected:
   DECLARE_SERIAL( COtherObject )
};


class CCompoundObject : public CObject
{
   // ...Member functions
public:
   CCompoundObject();
   ~CCompoundObject();
   virtual void Serialize( CArchive& ar );

// Implementation
protected:
   CMyObject m_myob;    // Embedded object
   COtherObject* m_pOther;    // Object allocated in constructor
   CObject* m_pObDyn;    // Dynamically allocated object
   //..Other member data and implementation

   DECLARE_SERIAL( CCompoundObject )
};

IMPLEMENT_SERIAL(CMyObject,CObject,1)
IMPLEMENT_SERIAL(COtherObject,CObject,1)
IMPLEMENT_SERIAL(CCompoundObject,CObject,1)


CCompoundObject::CCompoundObject()
{
   m_pOther = new COtherObject; // Exact type known and object already 
            //allocated.
   m_pObDyn = NULL;    // Will be allocated in another member function
            // if needed, could be a derived class object.
}

CCompoundObject::~CCompoundObject()
{
   delete m_pOther;
}

void CCompoundObject::Serialize( CArchive& ar )
{
   CObject::Serialize( ar );    // Always call base class Serialize.
   m_myob.Serialize( ar );    // Call Serialize on embedded member.
   m_pOther->Serialize( ar );    // Call Serialize on objects of known exact type.

   // Serialize dynamic members and other raw data
   if ( ar.IsStoring() )
   {
      ar << m_pObDyn;
      // Store other members
   }
   else
   {
      ar >> m_pObDyn; // Polymorphic reconstruction of persistent object 
      //load other members
   }
}

In summary, if your serializable class defines an embedded CObject as a member, you should not use the CArchive << and >> operators for that object, but should call the Serialize function instead. Also, if your serializable class defines a pointer to a CObject (or an object derived from CObject) as a member, but constructs this other object in its own constructor, you should also call Serialize.

See Also

Serialization: Serializing an Object