Q152294: HOWTO: Accessing Binary Data Using dbDao

Article: Q152294
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,4.1,4.2,4.2b,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbole kbProgramming kbDAOsearch kbMFC kbVC400 kbVC500 kbVC600
Last Modified: 09-MAY-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 4.0, 4.1 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 4.2b, 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 4.2b, 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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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: kbgrpMFCOLE

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Keywords          : kbcode kbole kbProgramming kbDAOsearch kbMFC kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbVC410 kbVC420 kbVC500 kbVC600 kbVC32bitSearch kbVC420b kbVC500Search
Version           : winnt:4.0,4.1,4.2,4.2b,5.0,6.0
Issue type        : kbhowto

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