CMemFile::GetBufferPtr

There has been a change in CMemFile::GetBufferPtr from Studio 6 to Studio 8.

Studio 6:

    if (nCommand == bufferCheck)
    {
        return 1;

Studio 8:

    if (nCommand == bufferCheck)
    {
        // only allow direct buffering if we're
        // growable
        if (m_nGrowBytes > 0)
            return bufferDirect;
        else
            return 0;

This appears to have broken some existing code of mine and I'm trying to
figure out why.

My code (actually somebody else's who is no longer with the company) pretty
much looks like this:

Saving:
CMemFile buffer;
CArchive archive(&buffer, CArchive::store);
object-->SaveToArchive(archive);
buffer.SeekToBegin();
buffer.Read( // into some static memory...

Loading:
BYTE *pBuffer;
pBuffer = // the static memory...
CMemFile buffer((LPBYTE) pBuffer, bufferSize);
CArchive archive(&buffer, CArchive::load);
object-->LoadFromArchive(archive);

The default value of nGrowSize ( the omitted value in the CMemFile::ctor) is
zero. The documentation states (and the code supports the statement) that if
you use a nGrowSize > 0 that CMemFile won't use the memory that you pass in.
So, therefore, nGrowSize must be zero.

Because I want CArchive to use the direct memory access mode. As near as I
can tell, CArchive calls CMemFile::GetBufferPtr to determine if direct access
is supported:

m_bDirectBuffer =
m_pFile->GetBufferPtr(CFile::bufferCheck)&CFile::bufferDirect;
if (!m_bDirectBuffer)
{
    // no support for direct buffering, allocate new buffer
    m_lpBufStart = new BYTE[m_nBufSize];
    m_bUserBuf = FALSE;
}
etc.

So, under the new MFC, GetBufferPtr returns 0, and my data is now wiped out.
 Under the old MFC, GetBuffferPtr returns 1 and I can use my data via
FillBuffer.

Am I missing something here?

Thanks in advance,
John