HOWTO: Accessing Binary Data Using dbDao

• Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 4.2b, 5.0, 6.0

SUMMARY

When using the DAO SDK C++ classes to access binary data (such as a bitmap) you will find that the data is returned in a COleVariant. COleVariant is an MFC class that wraps the OLE VARIANT data type. Within the VARIANT, the data is stored as an OLE SAFEARRAY.

Extracting the binary data from the COleVariant requires some knowledge of VARIANTs and SAFEARRAYs. The sample code below illustrates how to work with these data types by providing a function for extracting binary data from a COleVariant and a function for storing binary data in a COleVariant.

MORE INFORMATION

Sample Code

   //Extensive error checking is left out to make the code more readable

   BOOL GetBinaryFromVariant(COleVariant & ovData, BYTE ** ppBuf,
                                unsigned long * pcBufLen)
   {
     BOOL fRetVal = FALSE;

   //Binary data is stored in the variant as an array of unsigned char
     if(ovData.vt == (VT_ARRAY|VT_UI1))  // (OLE SAFEARRAY)
     {
       //Retrieve size of array
       *pcBufLen = ovData.parray->rgsabound[0].cElements;

       *ppBuf = new BYTE[*pcBufLen]; //Allocate a buffer to store the data
       if(*ppBuf != NULL)
       {
         void * pArrayData;

         //Obtain safe pointer to the array
         SafeArrayAccessData(ovData.parray,&pArrayData);

         //Copy the bitmap into our buffer
         memcpy(*ppBuf, pArrayData, *pcBufLen);

         //Unlock the variant data
         SafeArrayUnaccessData(ovData.parray);
         fRetVal = TRUE;
       }
     }
     return fRetVal;
   }

   BOOL PutBinaryIntoVariant(COleVariant * ovData, BYTE * pBuf,
                                unsigned long cBufLen)
   {
     BOOL fRetVal = FALSE;

     VARIANT var;
     VariantInit(&var);  //Initialize our variant

     //Set the type to an array of unsigned chars (OLE SAFEARRAY)
     var.vt = VT_ARRAY | VT_UI1;

     //Set up the bounds structure
     SAFEARRAYBOUND  rgsabound[1];

     rgsabound[0].cElements = cBufLen;
     rgsabound[0].lLbound = 0;

     //Create an OLE SAFEARRAY
     var.parray = SafeArrayCreate(VT_UI1,1,rgsabound);

     if(var.parray != NULL)
     {
       void * pArrayData = NULL;

       //Get a safe pointer to the array
       SafeArrayAccessData(var.parray,&pArrayData);

       //Copy bitmap to it
       memcpy(pArrayData, pBuf, cBufLen);

       //Unlock the variant data
       SafeArrayUnaccessData(var.parray);

       *ovData = var;  // Create a COleVariant based on our variant
       VariantClear(&var);
       fRetVal = TRUE;
     }

     return fRetVal;
   }


   //How you might call these functions

   CdbRecordset rs;

   //Code for initializing DAO and opening the recordset left out...

   COleVariant ovData = rs.GetField(_T("MyBinaryField"));

   BYTE * pBuf = NULL;
   unsigned long cBufLen;

   if(GetBinaryFromVariant(ovData,&pBuf,&cBufLen))
   {
     //Do something with binary data in pBuf...

     //Write back a new record containing the binary data

     COleVariant ovData2;
     if(PutBinaryIntoVariant(&ovData2,pBuf,cBufLen))
     {
       rs.AddNew();
       rs.SetField(_T("MyBinaryField"), ovData2); //Write our COleVariant
   to the table
       rs.Update();
     }
     //Clean up memory allocated by GetBinaryFromVariant
     if(pBuf)
       delete pBuf;
   } 

Additional query words:

Keywords : kbcode kbole kbprg kbDAO kbMFC kbVC400 kbVC410 kbVC420 kbVC500 kbVC600
Version : WINNT: 4.0, 4.1, 4.2, 4.2b, 5.0, 6.0
Platform : NT WINDOWS
Issue type : kbhowto