Cross-Version Object Serialization

Michael C. Hamilton -- mch@tiac.net
Wednesday, October 09, 1996

Environment: (1) VC++ 1.52 (MFC 2.52), WfW 3.11
             (2) VC++ 4.2 (MFC 4.2), Win NT 4.0

Greetings all,

An application that was developed in environment one above is being
ported to environment two.  One criterion for this porting process is
that files serialized from MFC 2.52 should be readable by MFC 4.2...
For the most part this was all done behind-the-scenes by MFC, but I'm
running into a peculiar problem.

Most of the objects I serialize contain CObLists, which I serialize
by simply calling CObList::Serialize.  For example, one of my
overridden Serializes:

void CSeriesInfo::Serialize(CArchive& ar)
{
        CStampedInfo::Serialize(ar);    // base class
        
        // Internal simple variables (use inline helper func)
        SFX_CString(ar, m_ImgParam_ID);
        
        // Lists
        m_lstSlices.Serialize(ar);
        m_lstParams.Serialize(ar);
}

This works on every occasion except when the list I'm serializing is
is empty.  If the list I'm serializing out is in fact empty, MFC 4.2
throws an endOfFile exception at the following location:

// FILE: ARCCORE.CPP
DWORD CArchive::ReadCount()     // Line 739
{
        WORD wCount;
        *this >> wCount;        <--- Chokes here
        if (wCount != 0xFFFF)
                return wCount;

        DWORD dwCount;
        *this >> dwCount;
        return dwCount;
}

I don't understand why there is an end-of-file exception only when
the list is empty.  If there is at least one item in the list, it
serializes everything fine.

Incidentally, the schema numbers for both versions of the program are
identical, since no new information was added in the Win32 port...

Anyone run into this?  I'm at a loss to explain it, and so far the
only way around it has been to serialize another byte out to keep
track if the list is empty or not, and not try to serialize it if
it is empty...  A solution that seems a bit -- yucky.

Thanks,
-Michael C. Hamilton
 mch@tiac.net
 Millennium Information Technology Inc.

---

Gerry Sweeney -- gerry@hornbill.com
Friday, October 11, 1996


Environment: (1) VC++ 1.52 (MFC 2.52), WfW 3.11
             (2) VC++ 4.2 (MFC 4.2), Win NT 4.0

Michael,

Our application depended on the serialization stuff in MFC. When we moved to 
Win32 we had all sorts of problems with cross compatibility. I can't 
remember exact details but for example, one of the MFC list classes was 
using an 'int' variable to maintain a count of the number of items in the 
list. This was true for both the 16bit and the 32bit versions. When this 
list would serialize the first thing it would write to the archive apart 
from the class id was this count value. When loading the reverse was true. 
This caused problems because of the size of an int under 16&32bit code. As I 
can remember there where about half a dozen or so cases in our app. What we 
decided to do was handle the lists our self. We override the 'Serialize' 
member of the list and write each contained object to the archive. Because 
of the types of data objects we were storing we knew exactly what was going 
into/out of the archive but the collection classes where a bit of a mystery. 
We decided to take this approach so our application would not break when MS 
decided to change things.

If you want to see what's going on in your instance try stepping through the 
serialize code of your collection class. Its very enlightening


I hope this helps

Gerry
 ----------------------------------------------------
Greetings all,

An application that was developed in environment one above is being
ported to environment two.  One criterion for this porting process is
that files serialized from MFC 2.52 should be readable by MFC 4.2...
For the most part this was all done behind-the-scenes by MFC, but I'm
running into a peculiar problem.

Most of the objects I serialize contain CObLists, which I serialize
by simply calling CObList::Serialize.  For example, one of my
overridden Serializes:

void CSeriesInfo::Serialize(CArchive& ar)
{
        CStampedInfo::Serialize(ar);    // base class

        // Internal simple variables (use inline helper func)
        SFX_CString(ar, m_ImgParam_ID);

        // Lists
        m_lstSlices.Serialize(ar);
        m_lstParams.Serialize(ar);
}

This works on every occasion except when the list I'm serializing is
is empty.  If the list I'm serializing out is in fact empty, MFC 4.2
throws an endOfFile exception at the following location:

// FILE: ARCCORE.CPP
DWORD CArchive::ReadCount()     // Line 739
{
        WORD wCount;
        *this >> wCount;        <--- Chokes here
        if (wCount != 0xFFFF)
                return wCount;

        DWORD dwCount;
        *this >> dwCount;
        return dwCount;
}

I don't understand why there is an end-of-file exception only when
the list is empty.  If there is at least one item in the list, it
serializes everything fine.

Incidentally, the schema numbers for both versions of the program are
identical, since no new information was added in the Win32 port...

Anyone run into this?  I'm at a loss to explain it, and so far the
only way around it has been to serialize another byte out to keep
track if the list is empty or not, and not try to serialize it if
it is empty...  A solution that seems a bit -- yucky.

Thanks,
 -Michael C. Hamilton
 mch@tiac.net
 Millennium Information Technology Inc.

---

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