SAMPLE: Dynamic Column Binding with MFC ODBC Database Classes

ID: Q141802

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 5.0, 6.0

This is a 32-bit version of the DYNCOL sample.

SUMMARY

The DYNC32 sample demonstrates how to dynamically determine the number, types, and names of each column in a given table and then bind these columns to dynamically allocated objects in your CRecordset derived class.

This sample has been updated to work with the new implementation of CRecordset in Visual C++ 4.2 while remaining compatible with 4.0 and 4.1.

The following file is available for download from the Microsoft Software Library:

~ Dync32.exe

For more information about downloading files from the Microsoft Software Library, please see the following article in the Microsoft Knowledge Base:

Q119591 How to Obtain Microsoft Support Files from Online Services

DYNC32.EXE contains subdirectories, so you will want to preserve the directory structure. After you download it, place it in an empty directory and extract the files it contains by using the -d option:

DYNC32.EXE -d

MORE INFORMATION

DYNC32 follows the basic procedures outlined in the documentation titled, "Dynamically Binding Data Columns," found in Database Encyclopedia of the Books OnLine. However, DYNC32 has no permanent members, therefore only one list is generated that contains information for all the columns in the table. If you have a subset of columns that are always present and are therefore member variables of the CRecordset derived class, you will need to follow the instructions in the article on generating only a list of the columns not already bound. DYNC32 only constructs a single list and implements methods for performing DoFieldExchange() and DoDataExchange() for this list of dynamically allocated objects.

Generating the List

The first thing DYNC32 must do is determine how many columns are in the database, and of what type. To do this, the CRecordset-derived object, CDynCol32Set, contains a CColumns object to query this information. In CDynCol32Set, the Open member function has been overridden so that the column information can be retrieved before the base-class CRecordset::Open function is called to open the database. After this information has been accessed, a list of CColumnData objects is allocated to hold a description of each column in the table, plus a pointer to a storage object allocated for that column (to be used in the record field exchange routines).


   BOOL CDyncol32Set::Open(UINT nOpenType, LPCSTR lpszSql, DWORD
dwOptions)
   {
     // use one CDatabase Object so we only prompt once for database Open
     // ***** allocated in constructor...

     if (! m_pDatabase->Open(NULL, FALSE, FALSE, "ODBC;"))
       return  FALSE;

     CTables tables(m_pDatabase);

     // prompt for a table to open
     CTablesDialog tableName(NULL,m_pDatabase);
     if (IDCANCEL == tableName.DoModal())
       return FALSE;

     // Specify the table to look at
     m_pColumns = new CColumns(m_pDatabase);
     m_pColumns->m_strTableNameParam = tableName.m_strTableNameSelected;

     // Set the database to be the CDynaSet's database if one hasn't
     // already been set (this should be the case)
     if (m_pColumns->m_pDatabase == NULL)
       m_pColumns->m_pDatabase = m_pDatabase;

     // Open the recordset to get the column info
     if (!m_pColumns->Open(CRecordset::forwardOnly, NULL,
   CRecordset::readOnly))

       return FALSE;

     // Initialize the number of fields dynamically allocated to CDynaSet
     m_nFields = 0;

     // Loop until we've seen all the columns
     while (!m_pColumns->IsEOF())
     {
       // Allocate a new CColumnData object for the current column
       CColumnData *pData = new CColumnData;

       // Store the colmun information
       pData->m_nDataType = m_pColumns->m_nDataType;
       pData->m_strColumnName = "[";
       pData->m_strColumnName += m_pColumns->m_strColumnName;
       pData->m_strColumnName +="]";

       // Allocate an object of the appropriate type to store
       // the column data
       switch(pData->m_nDataType)
       {
         case SQL_BIT:
           pData->m_pData = (void *)new BOOL;
           break;

         case SQL_TINYINT:
           pData->m_pData = (void *)new BYTE;
           break;

         case SQL_SMALLINT:
           pData->m_pData = (void *)new int;
           break;

          ...

         default:
           ASSERT(FALSE);
       }

       // Add the column descriptor to the list and
       // increment the number of columns in the
    CDynaSet

       m_pList.AddTail(pData);
       m_nFields++;

       // Get the next column's information
       m_pColumns->MoveNext();
     }

     // Free the HSTMT used to get the table info
     RETCODE nRetCode;
     AFX_SQL_SYNC(::SQLFreeStmt(m_pColumns->m_hstmt, SQL_CLOSE));
     // Return the base class if we got this far
     lpszSql = m_pColumns->m_strTableNameParam;

     // we don't need the columns recordset any more
     delete m_pColumns;
     return CRecordset::Open(nOpenType, lpszSql, dwOptions);
   }

Getting the Column Data into the Recordset

Once the CColumnData list has been generated, DoFieldExchange() must be overridden to traverse this list and call the appropriate RFX routine for each CColumnData's storage object.


   void CDyncol32Set::DoFieldExchange(CFieldExchange* pFX)
   {
     // Set the type of exhange; same as AppWizard generated
     pFX->SetFieldType(CFieldExchange::outputColumn);

     // Get a pointer to the first CColumnData object in the list
     POSITION rPos = m_pList.GetHeadPosition();
     CColumnData *pData = (CColumnData *)m_pList.GetNext(rPos);

     // Loop until we've traversed all the columns
     while (pData)
     {
       // Call the appropriate RFX routine for the column's type
       switch(pData->m_nDataType)
       {
         case SQL_BIT:
           RFX_Bool(pFX, pData->m_strColumnName, *((BOOL *)(pData-
                                                            >m_pData)));
           break;

         case SQL_TINYINT:
           RFX_Byte(pFX, pData->m_strColumnName, *((BYTE *)(pData-
                                                            >m_pData)));
           break;

         case SQL_SMALLINT:
           RFX_Int(pFX, pData->m_strColumnName, *((int *)(pData-
                                                          >m_pData)));
           break;

         ...

         default:
            ASSERT(FALSE);
       }

       // Set pData to NULL if that was the last column
       if (rPos)
         pData = (CColumnData *)m_pList.GetNext(rPos);
       else
         pData = NULL;
     }
   }

Getting the Recordset Data into the View

After getting the data into the recordset, the last thing to do is to get this information into the CRecordView-derived class to be displayed. In the case of DYNC32, it simply uses a list control to do a straight dump of the data onto the view. In order to do this, the CColumnData list must again be traversed. In the following functions, data is populated to the List Control:


   void CDyncol32View::RefreshData()
   {
      VARIANT var;
      CRect rect;
      LVITEM lvitem;
      int iActualItem = 0;
   
      POSITION rPos ;
      CString csTemp;
      CColumnData *pData = NULL;
   
      VariantInit(&var);
      var.vt = VT_ERROR;
      var.scode = DISP_E_PARAMNOTFOUND;
   
      if (!m_pSet->IsOpen())
         return;
   
   
      // Get pointer to CDynaSet's CColumnData list.
      CPtrList *pList = &m_pSet->m_pList;
   
      m_lstData.GetWindowRect(&rect);
   
      rPos = pList->GetHeadPosition();
      pData = (CColumnData *)pList->GetNext(rPos);
      // Navigate through the CCOlumnData to get the field names.
      // Set the columns header text as field names.
      for(int nCol= 0; nCol < (int)m_pSet->m_nFields;nCol++)
      {
         csTemp = pData->m_strColumnName.AllocSysString();
   
         m_lstData.InsertColumn(0, csTemp, LVCFMT_LEFT,rect.Width() * 1/3,
         0);
   
         if (rPos)
            pData = (CColumnData *)pList->GetNext(rPos);
         else
            pData = NULL;
      }
   
      // Navigate through the recordset.
      for (int iItem = 0; m_pSet->IsEOF()  != TRUE < 20; iItem++) // insert
                              // the items and subitems into the list view.
      {
         // Get the first element.
         rPos = pList->GetHeadPosition();
         pData = (CColumnData *)pList->GetNext(rPos);
         // Navigate through the column list
         // and put the data to the list control.
         for (int iSubItem = 0; iSubItem < (int)m_pSet->m_nFields;
   iSubItem++)
         {
   
            lvitem.mask = LVIF_TEXT | (iSubItem == 0? LVIF_IMAGE : 0);
            lvitem.iItem = (iSubItem == 0)? iItem : iActualItem;
            lvitem.iSubItem = iSubItem;
   
            csTemp = GetData(pData);
            // calculate the main and sub-item strings
            // for the current item.
            (lvitem.pszText) = csTemp.GetBuffer(csTemp.GetLength());
   
            if (iSubItem == 0)
               iActualItem = m_lstData.InsertItem(&lvitem); // insert new
                                                            // item.
            else
               m_lstData.SetItem(&lvitem); // modify existing item
                                           // (the sub-item text).
   
            if (rPos)
               pData = (CColumnData *)pList->GetNext(rPos);
            else
               pData = NULL;
         }
         m_pSet->MoveNext();
      }
   
   }
   
   // Function to convert the data in pData to Cstring.
   CString CDyncol32View::GetData(CColumnData *pData)
   {
      CString csTemp;
      CString strBuffer;
      CString *pSet;
   
      switch(pData->m_nDataType)
      {
         case SQL_BIT:
            strBuffer = *((BOOL *) (pData->m_pData)) ? _T("TRUE") :
   _T("FALSE");
            csTemp = strBuffer;
            break;
   
         case SQL_TINYINT:
            {
               char *pBuffer = strBuffer.GetBuffer(32);
               _itoa(*((BYTE *)(pData->m_pData)), pBuffer, 10);
            }
            strBuffer.ReleaseBuffer();
            csTemp= strBuffer;
   
            break;
   
         case SQL_SMALLINT:
         case SQL_INTEGER:
            {
               char *pBuffer = strBuffer.GetBuffer(32);
               _itoa(*((int *)(pData->m_pData)), pBuffer, 10);
            }
            strBuffer.ReleaseBuffer();
            csTemp = strBuffer;
            break;
   
         case SQL_REAL:
            {
               char *pBuffer = strBuffer.GetBuffer(32);
               sprintf(pBuffer, "%#f", *((float *) (pData->m_pData)));
            }
            strBuffer.ReleaseBuffer();
            csTemp = strBuffer;
            break;
   
         case SQL_FLOAT:
         case SQL_DOUBLE:
            {
               char *pBuffer = strBuffer.GetBuffer(32);
               sprintf(pBuffer, "%#f", *((double *) (pData->m_pData)));
            }
            strBuffer.ReleaseBuffer();
            csTemp = strBuffer;
            break;
   
         case SQL_DATE:
         case SQL_TIME:
         case SQL_TIMESTAMP:
            strBuffer = ((CTime *)(pData->m_pData))->Format( "%c" );
            csTemp = strBuffer;
            break;
   
         case SQL_BINARY:
         case SQL_VARBINARY:
            csTemp = "Binary Field";
            break;
   
         case SQL_DECIMAL:   // ODBC default xfer type
         case SQL_NUMERIC:   // ODBC default xfer type
         case SQL_CHAR:
         case SQL_VARCHAR:
             pSet = (CString *)(pData->m_pData);
             csTemp = *pSet;
            break;
   
         case SQL_LONGVARCHAR:
         case SQL_LONGVARBINARY:
            csTemp = "Longvarchar or Longvarbinary";
            break;
   
         default:
            ASSERT(FALSE);
      }
      return csTemp;
   }

The most fundamental concepts of the sample are contained within the CColumns and CColumnData objects. The first allows DYNC32 to determine the name and data type for each column in the table. With this information it is able to allocate a linked list of CColumnData objects. Each CColumnData object is a descriptor that contains the name, type and a pointer to a data object of the correct type. Once this list has been constructed, traversal routines must be put in the DoFieldExchange() and DoDataExchange() routines to perform the appropriate RFX/DDX function with the allocated data object.

Note that the implementation of CRecordset has changed from between version 4.1 and 4.2. As a result, you will notice that CColumns::Open() and CTables::Open(), the two overrides of CRecordset::Open() that are used to directly call ODBC API functions, now include conditional code. This code represents the minimum that must be done to update the recordsets to 4.2.

The sample code has changed in the functions CTables::Open and CColumns::Open due to changes in implementation of CRecordset between VC++ 4.2 and VC++ 5.0. Now it has calls to member functions of CRecordset to allocate and cache the field information, before calling MoveFirst.

Additional query words: kbvc400 kbvc410 kbvc420 kbvc500 kbvc600


Keywords          : kbcode kbsample kbusage kbDatabase kbMFC kbODBC kbVC 
Version           : 4.0 4.1 4.2 5.0 6.0
Platform          : NT WINDOWS 
Issue type        :

Last Reviewed: July 28, 1999