db_column

Binds a specified column to a variable in the rowset.

[ db_column( 
      ordinal, 
   dbtype, 
   precision, 
   scale, 
   status, 
   length 
) ]

Parameters

• ordinal
  The ordinal column number (DBCOLUMNINFO ordinal) or column name (ANSI or Unicode string) corresponding to a field in the rowset to which to bind data. If you use numbers, you can skip consecutive ordinals (for example: 1, 2, 3, 5). The name may contain spaces if the OLE DB provider you use supports it. For example, you can use either of the following formats:

  [db_column("2")] TCHAR szCity[30];
  [db_column(L"city_name")] TCHAR szCity[30];
  

• dbtype (optional)
  An OLE DB Type Indicator for the column entry.

• precision (optional)
  The precision to be used for the column entry. For details, see the description of the bPrecision element of the DBBINDING structure

• scale (optional)
  The scale to be used for the column entry. For details, see the description of bScale element of the DBBINDING structure

• status (optional)
  A member variable used to hold the status of this column. The status indicates whether the column value is a data value or some other value, such as NULL. For possible values, see Status in the OLE DB Programmer's Reference.

• length (optional)
  A member variable used to hold the size of the column in bytes.

Remarks

db_column binds the specified table column to a variable in the rowset. It delimits member data that can participate in OLE DB IAccessor-based binding. This attribute sets up the column map normally defined using the OLE DB consumer macros BEGIN_COLUMN_MAP, END_COLUMN_MAP, and COLUMN_ENTRY. These manipulate the OLE DB DBBINDING structure to bind the specified column. Each member you mark with the db_column attribute will occupy one entry in the column map in the form of a column entry. Therefore, you call this attribute where you would put the column map, that is, in the command or table class.

Use db_column in conjunction with either the db_table or db_command attributes.

When the consumer attribute provider applies this attribute to a class, the compiler will rename the class to _YourClassNameAccessor, where YourClassName is the name you gave the class, and the compiler will also create a class called YourClassName, which derives from _YourClassNameAccessor. In Class View, you will see both classes.

For examples of this attribute used in an application, see the samples AtlAgent, and MultiRead.

Example

This sample binds a column in a table to a long data member and specifies status and length fields.

// db_column_1.cpp
// compile with: /LD
#include <atlbase.h>
#include <atlplus.h>
#include <atldbcli.h>

[ db_command(L"Select * from Products") ]
class CProducts {
   DBSTATUS m_dwProductIDStatus;
   DBLENGTH m_dwProductIDLength;

   [ db_column("1", status="m_dwProductIDStatus", length="m_dwProductIDLength") ] LONG m_ProductID;
};

This sample binds four columns to a long, a character string, a timestamp, and a DB_NUMERIC integer, in that order.

// db_column_2.cpp
// compile with: /LD
#include <atlbase.h>
#include <atlplus.h>
#include <atldbcli.h>

[ db_command(L"Select * from Products") ]
class CProducts {
   [db_column("1")] LONG m_OrderID;
   [db_column("2")] TCHAR m_CustomerID[6];
   [db_column("4")] DB_NUMERIC m_OrderDate;   
   [db_column("7", dbtype="DBTYPE_NUMERIC")] DB_NUMERIC m_ShipVia;
};

Requirements

Attribute Context

For more information about the attribute contexts, see Attribute Contexts.

See Also

Concepts

OLE DB Consumer Attributes

Class Attributes

ATL Samples