Managing Substorages

As mentioned earlier, Patron manages a substorage for each page in the overall document. This, of course, requires somewhat more complex code when pages are created or destroyed. Whenever Patron creates a new page in the document, it calls IStorage::CreateStorage using the name Page<ID>, where <ID> is the page identifier in ASCII. When Patron opens an existing page, it calls IStorage::OpenStorage with the appropriate name as well. Both operations occur in CPage::Open. The member CPage::GetStorageName simply generates the text name to use in both calls:

BOOL CPage::Open(LPSTORAGE pIStorage)
    {
    BOOL        fNULL=FALSE;
    HRESULT     hr=NOERROR;
    DWORD       dwMode=STGM_TRANSACTED | STGM_READWRITE
                    | STGM_SHARE_EXCLUSIVE;
    OLECHAR     szTemp[32];

    if (NULL==m_pIStorage)
        {
        fNULL=TRUE;

        if (NULL==pIStorage)
            return FALSE;

        GetStorageName(szTemp);

        hr=pIStorage->OpenStorage(szTemp, NULL, dwMode, NULL, 0
            , &m_pIStorage);

        if (FAILED(hr))
            {
            hr=pIStorage->CreateStorage(szTemp, dwMode, 0, 0
                , &m_pIStorage);
            }
        }

   §
   }


UINT CPage::GetStorageName(LPOLESTR pszName)
    {
    return wsprintf(pszName, TEXT("Page %lu"), m_dwID);
    }

The CPages class assigns the page ID through the CPage constructor, which saves the value in m_dwID. The CPages class, once again, keeps a persistent DWORD counter for page IDs in the Page List stream. IDs are not recycled when a page is destroyed, but the DWORD counter would overflow only if you sat down and created (for example) one page every second until the year 2129. I desperately hope this software is obsolete by then!

Speaking of destroying a page, this operation requires a call to IStorage::DestroyElement to counter the IStorage::CreateStorage in the preceding code. The function CPage::Destroy takes care of this in Patron:

//pIStorage is document's root storage.
void CPage::Destroy(LPSTORAGE pIStorage)
    {
    if (NULL!=pIStorage)
        {
        OLECHAR   szTemp[32];

        Close(FALSE);
        GetStorageName(szTemp)
        pIStorage->DestroyElement(szTemp);
        }

    return;
    }

void CPage::Close(BOOL fCommit)
    {
    if (NULL==m_pIStorage)
        return;

    if (fCommit)
        Update();

    if (0L==m_pIStorage->Release())
        m_pIStorage=NULL;

    return;
    }

BOOL CPage::Update(void)
    {
    if (NULL!=m_pIStorage)
        m_pIStorage->Commit(STGC_DEFAULT);

    return TRUE;
    }