The Right Way To Do Object Serialization

Introduction

Object serialization is one of the most powerful features of MFC. With it you can store your objects into a file and let MFC recreate your objects from that file. Unfortunately there are some pitfalls in using it correctly. So it happened that yesterday I spent a lot of time in understanding what I did wrong while programming a routine that wrapped up some objects for a drag and drop operation. After a long debugging session I understood what happened and so I think that it might be a good idea to share my newly acquired knowledge with you.

The Wrong Way

Let's start with my wrong implementation of object serialization. A very convenient way to implement drag and drop and clipboard operations is to wrap up your objects in a COleDataSource object. This object can hold any data with a defined format or with a format that you can define and then it can be passed either to the clipboard by calling COleDataSource::SetClipboard or to the drag and drop handler by calling COleDataSource::DoDragDrop.

If you want to transfer a set of known objects between different parts of your applications, it is convenient to serialize them in a memory file and to wrap them up in a COleDataSource.

Let's start: This is my object:

class CLeopoldoOb : public CObject
{
    DECLARE_SERIAL(CLeopoldoOb)

// construction/destruction
public:
    CLeopoldoOb();
    // ....

protected:
    // some data....
    // some methods...
};

And this is my first version of the routine that wrapped up a set of those objects in the COleDataSource:

...

COleDataSource*	pDataSource = new COleDataSource;
if ( !pDataSource ) {
    return NULL;
}

// this is the memory file...
CSharedFile sf (GMEM_MOVEABLE|GMEM_DDESHARE|GMEM_ZEROINIT);
// this archive works on the memory file
CArchive    ar (&sf, CArchive::store);

// serialize the quantity
ar << m_nObjectCount;

for ( int i = 0; i < m_nObjectCount; i++ ) {
    CLeopoldoOb  leo;
    // now prepare the contents of the leopoldo object
    ...
    // object contains all data we need
    // serialize the object
    ar << &leo
}

ar.Flush ();
ar.Close ();

pDataSource->CacheGlobalData (g_cfLeopoldo, sf.Detach ());
pDataSource->DoDragDrop ();
if ( pDataSource->m_dwRef <= 1 ) {
    delete pDataSource;
}
else {
    pDataSource->ExternalRelease (); // !!!
}

...

And let's see the routine that unwraps the objects when they are dropped:

...

BOOL CMainFrame::OnDrop (COleDataObject* pDataObject, 
    DROPEFFECT dropEffect, CPoint point)
{
    if ( pDataObject->IsDataAvailable (g_cfLeopoldo) ) {
        CLeopoldoOb     *pOb;
        HGLOBAL         hMem = pDataObject->GetGlobalData (g_cfLeopoldo);
        CMemFile        mf;
        UINT            nCount;

        mf.Attach ((BYTE *)::GlobalLock (hMem), ::GlobalSize (hMem));
        CArchive ar (&mf, CArchive::load);

        // get the object count
        ar >> nCount;

        for ( UINT n = 0; n < nCount; n++ ) {
            try {
                // create the object out of the archive
                ar >> pOb;

                // now do what you have to do with the object
                ....
                // we do not need it any more...
                delete pOb;
            }
            catch { CException *pEx ) {
                // do some error handling
                pEx->Delete ();
            }

        }

        ar.Close ();
        mf.Detach ();
        ::GlobalUnlock (hMem);
        ::GlobalFree (hMem);
        return nCount > 0;
    }

    return FALSE;
}

What happened? Apparently this stuff worked, if you had only one object wrapped up. OK. Sometimes the application crashed but basically it worked. If instead more than one object was wrapped up, two strange things happened:

• Only the first object was unwrapped
• A CArchiveException with badIndex sometimes occurred

When tested under the debugger, I saw that the creation of the object from the serialization failed starting from the second time. Since there was definitively no error in the load routine, I reached the conclusion that the archive data must be messed up.

What Went Wrong?

After debugging in the profundities of CArchive I discovered that the process of storing dynamic objects is not as simple as I imagined it should be.

Obviously my error was during the creation of the archive. There are two important rules:

1. The objects must be dynamically created (with CRuntimeClass::CreateObject).
2. The objects must remain valid until the serialization has been finished.

If you are asking why, take a look into the source of CArchive and you will see that the archive stores additional information about those objects during its life. Furthermore the entire MFC seems to have knowledge about all dynamically created CObjects.

The Right Way

First of all we need a little helper class that will make life simpler:

class CAllocatedObArray : public CObArray
{
public:
    CAllocatedObArray           () { }
    virtual ~CAllocatedObArray  () { RemoveAll (); }

public:
    void  RemoveAll ();
};

void CAllocatedObArray::RemoveAll ()
{
    for ( int i = 0; i < GetSize (); i++ ) {
        CObject *pObject = GetAt (i);
        if ( pObject ) {
            delete pObject;
        }
    }
}

The following is the modified wrapper routine. You will see that all objects are created dynamically and stored into this array. After the serialization has finished, the array goes out of scope and our allocated objects will be automatically be destroyed.

...

COleDataSource*	pDataSource	= new COleDataSource;
if ( !pDataSource ) {
    return NULL;
}

// this is the memory file...
CSharedFile         sf (GMEM_MOVEABLE|GMEM_DDESHARE|GMEM_ZEROINIT);
// this archive works on the memory file
CArchive            ar (&sf, CArchive::store);
// this array holds our objects
CAllocatedObArray   tmpArray;
// This is needed to access the runtime class
CLeopoldoOb         leo;

// serialize the quantity
ar << m_nObjectCount;

for ( int i = 0; i < m_nObjectCount; i++ ) {
    CLeopoldoOb *pLeo = (CLeopoldoOb *) leo.GetRuntimeClass ()->CreateObject ();
    // store it in the array
    tmpArr.Add (pLeo);
    // now prepare the contents of the leopoldo object
    ...
    // object contains all data we need
    // serialize the object
    ar << pLeo
}

ar.Flush ();
ar.Close ();

pDataSource->CacheGlobalData (g_cfLeopoldo, sf.Detach ());
pDataSource->DoDragDrop ();
if ( pDataSource->m_dwRef <= 1 ) {
    delete pDataSource;
}
else {
    pDataSource->ExternalRelease (); // !!!
}

...

This works. Probably this will be nothing new for all experts among you, but since I did make this error after years of programming MFC, I hope that this article will help some beginner.