INFO: Changes in Consumer Templates Since VC++ Tech Preview

ID: Q190472

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, version 6.0

SUMMARY

In November 1997 the Visual C++ Technology Preview was released to illustrate features being developed for the next version of Visual C++. Included were the OLE DB Consumer Templates. These templates have been further developed and are included in Visual C++ 6.0.

To help with projects that might have been started with the Visual C++ Technology Preview this article lists the changes made to the OLE DB Consumer classes since the Technology Preview.

MORE INFORMATION

Static Accessor Hierarchy

The user's record no longer derives from CAccessor, in fact it no longer needs to derive from any class. Now, you can make copies of the data in the record class and you don't need to worry about the extra 12 bytes of instance data that is in CAccessorBase. The class CAccessor is still templated on the user record class but now derives from the user record (instead of the other way around) and CAccessorBase.

As CAccessor now derives from the user's record, you no longer pass the user record as the first template parameter to CTable or CCommand when you use a static accessor. You now pass CAccessor (which is templated on the user record).

For example:


    CTable<CMyProduct> myTable;

becomes


    CTable<CAccessor<CMyProduct> > myTable;

   // note the extra space between > and >

Macros

The column entry macros that you use to specify the bindings have been simplified. Previously you had to specify the OLE DB type and the size of the column in the COLUMN_ENTRY macro. The OLE DB type and size are now worked out automatically at compile time based upon the Visual C++ data type that the column binds to. If you wish to explicitly specify the type, you can use the new macro COLUMN_ENTRY_TYPE. One of the advantages of the new COLUMN_ENTRY macro is that you can now have data members that are arrays of TCHAR's, and they are automatically bound as DBTYPE_STR or DBTYPE_WSTR depending upon whether you are building an ANSI or UNICODE version.

The DEFINE_COMMAND() macro now requires the class name to be passed in as the first parameter.

The following code snippets show the old way and the new way of accessing data with a static accessor.

Sample Code - the Old Way


   class CProductAccessor :
      public CAccessor<CProductAccessor>,
   {
   public:
      long   nProductID;
      char   szName[40];

      DEFINE_COMMAND("SELECT * FROM Products WHERE UnitsInStock < ?");

      BEGIN_COLUMN_MAP(CProductAccessor)
         COLUMN_ENTRY(1, DBTYPE_I4, 4, nProductID)
         COLUMN_ENTRY(2, DBTYPE_STR, 40, szName)
      END_COLUMN_MAP()

      BEGIN_PARAM_MAP(CProductAccessor)
         COLUMN_ENTRY(1, DBTYPE_I4, 4, nProductID)
      END_PARAM_MAP()
   };

   void OpenTable(CSession& session)
   {
     CTable<CProductAccessor> product;
     product.Open(session, ...
     ...
   }

Sample Code - the New Way


   class CProductAccessor
   {
     public:
     long   nProductID;
     char   szName[40];

     DEFINE_COMMAND(CProductAccessor,"SELECT * FROM Products WHERE
                    UnitsInStock < ?");

     BEGIN_COLUMN_MAP(CProductAccessor)
        COLUMN_ENTRY(1, nProductID)
        COLUMN_ENTRY(2, szName)
     END_COLUMN_MAP()

     BEGIN_PARAM_MAP(CProductAccessor)
        COLUMN_ENTRY(1, nProductID)
     END_PARAM_MAP()
   };

   void OpenTable(CSession& session)

   {
     CTable<CAccessor<CProductAccessor> > product;
     product.Open(session, ...
     ...
   }

GUIDs

The guids from Oledb.lib are now been in Uuid.lib along with other guids that used to require that you have a Guids.cpp in your project. You should therefore remove any Guids.cpp program you have in your project and remove Oledb.lib from your linker line.

Other Changes and Additions

There is another rowset and accessor class type. CNoAccessor is optimized for when there are no output columns or parameters and CNoRowset, which is optimized for when there is no output rowset.

It is no longer necessary to have an output column map in the static accessor. This means you can now create an accessor class with only a parameter map. This is useful if you have a command that has input parameters but no output columns. For example:


      class CShippers
      {
        public:
        long nShipID;
        BEGIN_PARAM_MAP(CShippers)
           COLUMN_ENTRY(1, nShipID)
        END_PARAM_MAP()
      };

      void DeleteShippers(CSession& session)
      {
        CCommand<CAccessor<CShippers>, CNoRowset> cmd;
        LONG nRowsAffected;
        cmd.nShipID = 3;
        cmd.Open(session,
        _T("DELETE * FROM Shippers2 WHERE ShipperID < ? "), NULL,
           &nRowsAffected);
      }

CTable and CCommand now default to CNoAccessor as the first template parameter instead of CDynamicAccessor.

The new standard Data Links dialog is supported by new Open function overloads in the CDataSource class.

There is a new function on the accessor classes called FreeRecordMemory. You can call this to automatically free any columns that need to be freed before you make another call to MoveNext. For example, the function releases any columns that are interface pointers, calls SysFreeString on any columns that are BSTRs, and calls VariantClear() on any variants.

There is a new function called GetDataHere on Crowset, which can be used to get data into a specific buffer. You can use this if you wish to read through a rowset on multiple threads. See the MultiRead sample for an example of its use.

The class CMultipleResultsCommand no longer exists because its functionality has been added to CCommand. Passing in CMultipleResults as the third template parameter to CCommand, instead of the default (which is CNoMultipleResults), activates this functionality.

The ATL Object Wizard now automatically generates an accessor class and a class with code to open the datasource and rowset.

Additional query words:


Keywords          : kbtemplate kbATL kbDatabase kbOLEDB kbVC600 
Version           : WINNT:6.0
Platform          : winnt 
Issue type        : kbinfo

Last Reviewed: August 8, 1999