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DB2 Access Classes

nickadams
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Nov 28, 2003

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A set of classes to encapsulate DB2 CLI APIs in ADO style.

Contents

Overview

These classes encapsulate the complicated DB2 CLI APIs, so that developers can access DB2 database in a simple, ADO style way.

    CSqlDatabase Represents a connection to a database
    CSqlCommand Defines a specific command (SQL statement)
    CSqlRecordset Represents a set of records selected from table(s) or the results of an executed command
    CSqlParameter Represents a parameter or argument associated with a SQL statement or stored procedure
    CSqlField Represents a column of data in a recordset
    CSqlLocator Manipulates a locator to access large object data
    CSqlErrorInfo Provides information associated with a DB2 error

Environment

These classes can be used in either Windows or AIX environment. The compilers use predefined macro to identify themselves. for MSVC it's _MSC_VER; for xlC it's __xlC__.

In the current version, I use STL (Standard Template Library) to manipulate string and collections. Unfortunately the current version of AIX C++ compiler (xlC) doesn't support STL. So I used STLport, a free, multiplatform ANSI STL library.

STLport can be downloaded from: http://www.stlport.org/

Using the code

To use DB2 Access Classes in your program, you need to add the 2 .cpp files and 4 .h files in your project, and include sqldb2.h in your source file.

While you compile sqldb2.cpp or sqldata.cpp with VC++, you might meet the error: fatal error C1010: unexpected end of file while looking for precompiled header directive. To solve this problem, you should include the precompiled header file (usually it is stdafx.h) in sqldb2.cpp and sqldata.cpp.

Connect to database and execute SQL statements

The application must at first call the global function CSqlDatabase::Initialize to initialize the DB2 CLI environment before accessing DB2, and call CSqlDatabase::Uninitialize to release the environment afterwards.

Use CSqlDatabase class to connect to a database; use CSqlCommand class to execute a SQL statement. 

#include <stdio.h>
#include "sqldb2.h"

void main()
{
    // Step 1: initialize the DB2 CLI environment
    CSqlDatabase::Initialize();

    // Step 2: Establish a connection to database
    CSqlDatabase db;
    db.Connect("mydb", "user_name", "password");

    // Step 3: Execute SQL statement
    CSqlCommand cmd;
    cmd.Create(&db, CSqlCommand::execDirect);
    cmd.SetCommand("CREATE TABLE Employee (Name VARCHAR(20), "
       "Sex CHAR(1), EmployDate DATE, Age INT)");
    cmd.Execute();

    // Step 4: Execute another SQL statement
    cmd.Reset();
    cmd.SetCommand("INSERT INTO Employee VALUES ('John', 'M', "
          "'2000-1-1', 25)");
    cmd.Execute();

    // Step 5: Close database connection
    db.Close();

    // Step 6: Cleanup environment and release resources
    CSqlDatabase::Uninitialize();
}

Execute parameterized SQL statements

Parameterized commands are more efficient if you need to execute a command repeatedly while only a few parameters need be to changed. 

    // Assume the database connection has been established
    CSqlCommand cmd;
    cmd.Create(&db);
    cmd.SetCommand("INSERT INTO Employee VALUES (?, ?, ?, ?)");

    // Add parameters to the command
    CSqlParameter param;
    param.CreateParameter(SQL_CHAR, SQL_PARAM_INPUT, 20);
    cmd.Parameters().Add(param);
    param.CreateParameter(SQL_CHAR, SQL_PARAM_INPUT, 1);
    cmd.Parameters().Add(param);
    param.CreateParameter(SQL_TYPE_DATE, SQL_PARAM_INPUT);
    cmd.Parameters().Add(param);
    param.CreateParameter(SQL_INTEGER, SQL_PARAM_INPUT);
    cmd.Parameters().Add(param);

    // Set the value of the parameters and execute the command
    cmd.Parameters()[0] = "Mike";
    cmd.Parameters()[1] = 'M';
    cmd.Parameters()[2] = "2000-1-1";
    cmd.Parameters()[3] = 25;
    cmd.Execute();

    cmd.Parameters()[0] = "Jane";
    cmd.Parameters()[1] = 'F';
    cmd.Parameters()[2] = "1999-10-20";
    cmd.Parameters()[3] = 22;
    cmd.Execute();

    cmd.Close();

Perform a query

CSqlCommand class provides a way to execute a SQL statement, but doesn't provide a way to access the result set that might be generated by that statement. To perform a query we should use CSqlRecordset class. 

    // Assume the database connection has been established
    // Create and initialize a CSqlRecordset object
    CSqlRecordset rs;
    rs.Create(&db);

    // Specify the search condition of the query
    rs.m_strFilter = "Age > ? AND Sex = ?";
    CSqlParameter param;
    param.CreateParameter(SQL_INTEGER, SQL_PARAM_INPUT);
    rs.Parameters().Add(param);
    param.CreateParameter(SQL_CHAR, SQL_PARAM_INPUT, 1);
    rs.Parameters().Add(param);

    // Search condition: Age > 20 AND Sex = 'M'
    rs.Parameters()[0] = 20;
    rs.Parameters()[1] = "M";

    // Specify the return columns in the result set.
    // If we don't specify the return columns, 
    // CSqlRecordset returns all columns as default.
    CSqlField fd;
    fd.CreateField("Name", SQL_C_CHAR, 20);
    rs.Fields().Add(fd);
    fd.CreateField("Age", SQL_C_LONG);
    rs.Fields().Add(fd);

    // Set the size of local row cache, by default the cache size is 1.
    rs.SetCacheSize(10);
    // Perform query
    rs.Open("Employee", CSqlRecordset::sqlCmdTable);
    rs.MoveNext();
    while (!rs.IsEOF())
    {
        printf("Name:%s Age:%d\n", (char*)rs.Fields()[0], 
           (int)rs.Fields()[1]);
        rs.MoveNext();
    }

    // Specify another search condition: Age > 30 AND Sex = 'F'
    rs.Parameters()[0] = 30;
    rs.Parameters()[1] = "F";

    // Perform query again
    rs.Requery();
    rs.MoveNext();
    ...

Call a stored procedure

    // Assume the database connection has been established
    // Create and initialize a CSqlRecordset object
    CSqlRecordset rs;
    rs.Create(&db);

    // Call AutoBindProcParams() to retrieve parameter 
    // information of the procedure from database
    // and bind these parameters automatically
    rs.AutoBindProcParams("ProcName", "SchemaName");
    // Sepcify the value of input parameters
    rs.Parameters()[0] = 1;
    ...

    // Call the stored procedure
    rs.Open("ProcName", CSqlRecordset::sqlCmdStoreProc, 
       SQL_CURSOR_FORWARD_ONLY);
    printf("Store procedure return code = %d\n", rs.GetRetStatus());

    // Print the result sets if available
    rs.MoveNext();
    while (!rs.IsEOF())
    {
        for (int n=0; n<rs.GetFields(); n++)
        {
            string strVal;
            rs.Fields()[n].ToString(strVal);
            printf("%s ", strVal.data());
        }
        printf("\n");
        rs.MoveNext();
    }
    rs.Close();

Access large object data

There are two way to access large object (BLOB, CLOB or DBCLOB) data:

  • Bind the LOB field (or parameter) to a file, by calling CSqlField::CreateFileField (or CSqlParameter::CreateFileParam);
  • Bind the LOB field to a locator, then use a CSqlLocator object to manipulate this locator. 
    // Assume the database connection has been established
    CSqlRecordset rs;
    rs.Create(&db);

    // There are two LOB columns in the table. 
    // uses file binding for the column lob_col1
    CSqlField fd;
    fd.CreateFileField("lob_col1");
    fd = "lob_col1.txt";      // Specify the file name to be bound
    rs.Fields().Add(fd);

    // Bind the column lob_col2 to a locator
    fd.CreateField("lob_col2", SQL_C_CLOB_LOCATOR);
    rs.Fields().Add(fd);

    rs.Open("lob_table", CSqlRecordset::sqlCmdTable);
    rs.MoveNext();
    while (!rs.IsEOF())
    {
        // Open file lob_col1.txt to get value of lob_col1
        FILE *pFile = fopen("lob_col1.txt", "r");
        ...

        // Use CSqlLocator to get the value of lob_col2
        CSqlLocator loc;
        loc.Open(&db, rs.Fields()[1]);  // Associate the 
                  // locator to CSqlLocator object
        int nSize = loc.GetLength();    // Get the length of the LOB data
        char buf[100];
        loc.GetSubString(buf, 40, 80);  // Retrieve a substring 
                  // from the LOB data
        loc.Free();                     // Free the locator

        rs.MoveNext();
    }
    rs.Close();

Handle errors or exceptions

For each CSqlDatabase/CSqlCommand/CSqlRecordset object which is inherited from CSqlObject, the application can call its member function GetLastError to acquire the error information of database operation. 

void HandleError(CSqlObject* pSqlObject)
{
    CSqlErrorInfo e;
    while (pSqlObject->GetLastError(e))
    {
        printf("SQL error message: %s\n", e.GetErrorText());
    }
}

An alternative way is to override CSqlObject::OnSqlError function so that the application can handle the error immediately after it hapens.

CSqlObject class

CSqlObject is the base class of CSqlDatabase, CSqlCommand, and CSqlRecordset. It wraps a DB2 CLI handle (it could be a connection handle, or a statement handle), and manages the error messages occured on this handle. 

CSqlObject();
Creates a CSqlObject object. 
SQLHANDLE GetHandle() const;
Returns the DB2 CLI handle. 
BOOL IsOpen() const;
Returns TRUE if the DB2 CLI handle has been created, otherwise FALSE. 
CSqlDatabase* GetDatabase() const;
Returns the pointer of the database connection this object runs upon. 
virtual void Close();
Closes handle and releases resources. 
BOOL SqlCheck(SQLRETURN nSqlRet);
Checks the return code of a DB2 CLI API call. Returns TRUE if the call succeeded, otherwise FALSE. 
BOOL SetAttribute(SQLINTEGER nAttr, SQLPOINTER pValue, 
    SQLINTEGER nValueSize=0);
Sets an attribute of the handle.
Parameters
nAttr
Specifies the identifier of the attribute.
pValue
Pointer to the value of the attribute.
nValueSize
If pValue points to a character string or a binary buffer, this argument should be the length of the value. 
BOOL GetAttribute(SQLINTEGER nAttr, SQLPOINTER pValue, 
    SQLINTEGER nBuffSize=0, SQLINTEGER* pnValueSize=NULL);
Gets the current setting of an attribute of the handle.
Parameters
nAttr
Specifies the identifier of the attribute.
pValue
Pointer to a buffer to retrieve the attribute value.
nBuffSize
Specifies the length of the buffer.
pnValueSize
Pointer to an integer to retrieve the length of the attribute value. 
BOOL GetLastError(CSqlErrorInfo& rErrorInfo);
Gets the last-error occured on this object.

CSqlDatabase class

CSqlDatabase is inherited from CSqlObject class. 

CSqlDatabase();
Creates a CSqlDatabase object. 
static BOOL Initialize(BOOL bMultiThread=TRUE);
Initializes a DB2 CLI environment.
Parameters
bMultiThread
Specifies whether the application uses DB2 CLI in a multithreading environment. 
static void Uninitialize();
Releases the DB2 CLI environment.
See sample 
static BOOL SetEnvAttr(SQLINTEGER nAttr, SQLINTEGER nValue);
Sets an attribute of the DB2 CLI environment.
Parameters
nAttr
Specifies the identifier of the attribute.
nValue
Specifies the value of the attribute. 
static BOOL GetEnvAttr(SQLINTEGER nAttr, SQLINTEGER& nValue);
Gets the value of an attribute of the DB2 CLI environment.
Parameters
nAttr
Specifies the identifier of the attribute.
nValue
The integer variable to retrieve the value. 
BOOL Connect(PCSTR pszDB, PCSTR pszUser, PCSTR pszPwd, 
  DWORD dwOption=defaultOption, DWORD dwTxnIsolation=SQL_TXN_READ_COMMITTED);
Establishes a connection to a database.
Parameters
pszDB
Specifies the database name.
pszUser
Specifies the user name.
pszPwd
Specifies the password of the user.
dwOption
Specifies the attribute of the database connection. It can be 0 or a combination of the following values:
readOnly
Specifies the connection is read-only. By default it is read-write.
manualCommit
The application must manually, explicitly commit or rollback transactions with CommitTrans() or RollbackTrans() calls. By default DB2 implicitly commits each statement automatically.
autoUnlock
Specifies the read locks are released when the cursor is closed. By default the read locks are not released automatically
dwTxnIsolation
Sets the transaction isolation level.
See sample 
void Close();
Closes the current database connection. 
BOOL BeginTrans();
Begins a new transaction 
BOOL CommitTrans();
Saves any changes and ends the current transaction. 
BOOL RollbackTrans();
Cancels any changes made during the current transaction and ends the transaction.

CSqlCommand class

CSqlCommand is inherited from CSqlObject class. 

CSqlCommand();
Creates a CSqlCommand object. 
BOOL Create(CSqlDatabase* pDB, DWORD dwOption=defaultOption);
Initializes a CSqlCommand object.
Parameters
pDB
Pointer to the database connection which this command runs upon.
dwOption
Specifies the command option. It can be 0 or a combination of the following values:
execDirect
Executes command directly without preparation.
autoCloseCursor
Automatically closes an open cursor if a second cursor is opened.
nonScanEscape
Disables the scan of SQL string for escape clauses. Don't use this option to call stored procedure.
preFetch
Tells the server to prefetch the next block of data immediately after sending the current block.
See sample 
virtual void Close;
Closes the command object. 
virtual void Reset();
Unbinds all parameters and resets the command handle for reuse. 
BOOL SetCommand(PCSTR lpszSQL);
Specifies the SQL statement to be executed, and prepare the statetment if execDirect option not specified.
Parameters
lpszSQL
String pointer of a SQL statement. 
BOOL Execute();
Executes the current SQL statement. 
SQLINTEGER GetRowCount();
Returns the count of rows that were affected by the SQL statement. 
CSqlParameters& Parameters();
Returns the collection of parameters bound on this command.

CSqlRecordset class

CSqlRecordset is inherited from CSqlCommand class. 

CSqlRecordset();
Creates a CSqlRecordset object. 
string m_strFilter;
Specifies the search condition of the query before calling Open() with nCmdType set to sqlCmdTable. 
string m_strSort;
Specifies the sort order for the result set before calling Open() with nCmdType set to sqlCmdTable. 
BOOL Open(PCSTR pszCommand, int nCmdType, 
   DWORD nCursorType=SQL_CURSOR_STATIC, BOOL bUseBookmarks=FALSE);
Opens a recordset by executing a query, store procedure or generic SQL statement.
Parameters
pszCommand
String pointer of the command to be executed.
nCmdType
Specifies how the command argument nCmdType should be interpreted:
sqlCmdSQL
Evaluates the command as a SQL statement.
sqlCmdTable
Evaluates the command as a table name.
sqlCmdStoreProc
Evaluates the command as the name of a store procedure.
nCursorType
Specifies the type of cursor. It can be one of the following values:
SQL_CURSOR_FORWARD_ONLY
SQL_CURSOR_STATIC (default)
SQL_CURSOR_KEYSET_DRIVEN
bUseBookmarks
Specifies whether enable bookmark. This argument is ignored if nCursorType is set to SQL_CURSOR_FORWARD_ONLY.
See sample 
BOOL Requery();
Re-executes the current command to refresh the entire result set. 
BOOL NextRecordset();
Clears the current recordset, and returns the next recordset by advancing through a series of commands. 
virtual void Close();
Closes the object and releases any resources. 
virtual void Reset();
Unbinds all columns and parameters, and resets the command handle for reuse. 
BOOL AutoBindProcParams(PCSTR pszProcName, PCSTR pszSchemaName=NULL);
Retrieves all the parameters associated with a procedure and add them to parameter list. This function must be called before Open().
Parameters
pszProcName
Specifies the name of a stored procedure.
pszSchemaName
Specifies the name of the schema which the procedure belongs to. 
BOOL IsScrollable() const;
BOOL IsUpdatable() const;
Returns TRUE if the current cursor is scrollable, updatable. 
BOOL SetMaxRows(DWORD nMaxRows=0);
Specifies the maximum number of rows to return from a query. The default value is 0 means all rows are returned. Must be called before Open(). 
void SetCacheSize(int nCacheSize=0);
Specifies the number of rows of the local row cache. A reasonable cache size can reduce the network traffic and improve performence. The default value is 1. Must be called before Open(). 
int GetCacheSize() const;
Returns the size of the local row cache. 
int GetRetStatus() const;
Returns the return code (specified by a RETURN statement) of this stored procedure. 
SQLUSMALLINT GetRowStatus() const;
Returns the status of current row. It can be one of the following values:
SQL_ROW_SUCCESS
The row was successfully fetched.
SQL_ROW_ERROR
An error occurred while fetching the row.
SQL_ROW_NOROW
The rowset overlapped the end of the result set and no row was returned.
SQL_ROW_DELETED
The row has been deleted (by callingDelete()) since it was last fetched from this result set.
SQL_ROW_UPDATED
The row has been updated (by calling Update()) since it was last fetched from this result set. 
BOOL IsDeleted() const;
Returns TRUE if the current row is marked to be deleted. This function is only valid for updatable cursor. 
BOOL IsBOF() const;
BOOL IsEOF() const;
Returns TRUE if the current cursor is positioned before the first row, or after the last row. 
BOOL Move(SQLINTEGER nOffset, BOOL bRefresh=FALSE);
Moves the cursor position to the row nOffset from the current row. This function is only valid for scrollable cursor.
Parameters
nOffset
Specifies the offset from the current row.
bRefresh
Specifies whether refresh the local row cache by acquiring data from server. 
BOOL MoveFirst();
BOOL MoveLast();
BOOL MovePrevious();
BOOL MoveNext();
Moves to the first, last, previous, next row. Except MoveNext these functions are only valid for scrollable cursor.
See sample 
BOOL Refresh();
Refreshes the status of current row. This function is only valid for scrollable cursor. 
BOOL GetBookmark(SqlBookmark& rBookmark);
Gets the bookmark of the current cursor position. This function is only valid for scrollable cursor.
Parameters
rBookmark
a reference to retrieve the bookmark. 
BOOL SetBookmark(const SqlBookmark& rBookmark);
Moves to the position specified by a bookmark. This function is only valid for scrollable cursor.
Parameters
rBookmark
The bookmark got from GetBookmark() before. 
BOOL Update();
BOOL Delete();
Updates or deletes the current row. These functions are only valid for updatable cursor. 
int GetFields() const;
CSqlFields& Fields();
Returns the collection of fields (columns) bound on this recordset.

CSqlVariant class

CSqlVariant is the base class of CSqlParameter and CSqlField. It represents a variant that has standard SQL data type. 

CSqlVariant();
CSqlVariant(const CSqlVariant& rVar);
Creates a CSqlVariant object. 
SQLSMALLINT GetSqlType() const;
SQLSMALLINT GetDataType() const;
SQLUINTEGER GetPrecision() const;
SQLSMALLINT GetScale() const;
Returns the SQL data type, the C data type, precision, scale of the variant. 
SQLINTEGER GetMaxSize() const;
Returns the maximum length (in bytes) of the buffer of the variant. 
SQLINTEGER GetDisplaySize() const;
Returns the length (in bytes) needed to display the variant data in character form. 
SQLINTEGER GetLength() const;
Returns the length (in bytes) of the variant value. (If the variant is a locator, the length of locator is returned, not the LOB data). The returned value might be SQL_NULL_DATA, indicates the value is null. 
BOOL IsNull() const;
Returns TRUE if the value is null. 
void SetNull();
Sets the value to null. 
BOOL SetValue(const void* pValue, SQLINTEGER nValueSize=0);
Sets the value of the variant without any conversion.
Parameters
pValue
Pointer to the value buffer.
nValueSize
Specifies the length of the value if it is variable (string or LOB). 
BOOL FromInteger(int nValue);
BOOL FromDouble(double dValue);
BOOL FromString(const char* pszValue);
Sets the value by converting from an int, double, char* parameter. 
int ToString(std::string& strValue);
Converts the value to a string. 
operator short() const;
operator int() const;
operator double() const;
operator const char*() const;
Converts the value to short, int, double, char* data type. 
SQLCHAR* GetBuffer() const;
const DATE_STRUCT* GetDate() const;
const TIME_STRUCT* GetTime() const;
const TIMESTAMP_STRUCT* GetTimeStamp() const;
Returns the pointer of the value and casts to different data types.

CSqlParameter class

CSqlParameter is inherited from CSqlVariant class. 

CreateParameter();
CreateParameter(const CSqlParameter& rValue);
Creates a CSqlParameter object. 
BOOL CreateParameter(SQLSMALLINT nSqlType, SQLSMALLINT nIoType, 
    SQLUINTEGER nPrecision=0, SQLSMALLINT nScale=0);
Initializes a parameter.
Parameters
nSqlType
Specifies the SQL data type of the parameter.
nIoType
Specifies the input/output type of the parameter. It can be one of the following values:
SQL_PARAM_INPUT
SQL_PARAM_OUTPUT
SQL_PARAM_INPUT_OUTPUT
nPrecision
Specifies the precision of the parameter.
For a binary or single byte string, it is the maximum length in bytes;
For a double byte string, it is the maximum length in double-bytes;
For NUMERIC or DECIMAL, it is the decimal precision;
For fixed-length data type, this argument is ignored.
nScale
Specifies the scale if nSqlType is SQL_NUMERIC, SQL_ECIMAL, or SQL_TIMESTAMP.
See sample 
BOOL CreateFileParam(SQLSMALLINT nSqlType);
Initializes a file-binding parameter. The value of this parameter is the file name, not the LOB data.
Parameters
nSqlType
Specifies the SQL data type of the parameter. It only can be SQL_BLOB, SQL_CLOB, or SQL_DBCLOB. 
CSqlParameter& operator=(const CSqlParameter& rValue);
CSqlParameter& operator=(int nValue);
CSqlParameter& operator=(double nValue);
CSqlParameter& operator=(const char* pszValue);
Sets the value of the parameter.

CSqlField class

CSqlField is inherited from CSqlVariant class. 

CSqlField();
CSqlField(const CSqlField& rData);
Creates a CSqlField object. 
BOOL CreateField(PCSTR pszFieldName, SQLSMALLINT nDataType, 
    SQLINTEGER nDataSize=0);
Initializes a field (column) object.
Parameters
pszFieldName
Specifies the name of the field. It can be a column name in a table, or a SQL expression.
nDataType
Specifies the C data type identifier of the field.
nDataSize
Specifies the maximum length in bytes of the field. This argument is ignored if nDataType specifies a fixed-length data type.
See sample 
BOOL CreateField(CSqlCommand* pStmt, int nOrdinal);
Initializes a field according to the attribute of a specified column in a recordset.
Parameters
pStmt
Pointer to the recordset object which contains the specified column.
nOrdinal
The ordinal number identifying the column. 
BOOL CreateFileField(PCSTR pszFieldName, 
   SQLUINTEGER nFileOption=SQL_FILE_OVERWRITE);
Initializes a file-binding field. The value of this field is the file name, not the LOB data.
Parameters
pszFieldName
Specifies the name of the field. It is usually the name of a LOB column in a table.
nFileOption
Specifies the file access option. It can be one of the following values:
SQL_FILE_CREATE
Create a new file. If a file by this name already exists, SQL_ERROR will be returned.
SQL_FILE_OVERWRITE
If the file already exists, overwrite it. Otherwise, create a new file.
SQL_FILE_APPEND
If the file already exists, append the data to it. Otherwise, create a new file.
See sample 
CSqlField& operator=(const CSqlField& rValue);
CSqlField& operator=(int nValue);
CSqlField& operator=(double nValue);
CSqlField& operator=(const char* pszValue);
Sets the value of the field.

CSqlLocator class

CSqlLocator has no base class. 

CSqlLocator();
Creates a CSqlLocator object. 
BOOL Open(CSqlDatabase* pDB, const CSqlVariant& rVariant);
Initializes the CSqlLocator object.
Parameters
pDB
Specifies a database connection to manipulate the locator.
rVariant
Specifies a variant (usually a field) object which represents the locator. 
SQLINTEGER GetLength();
Returns the total length of the LOB data represented by the locator. 
SQLINTEGER GetSubString(SQLPOINTER pBuff, SQLINTEGER nStartPos, 
    SQLINTEGER nLength);
Gets a portion from the LOB data.
Parameters
pBuff
Specifies a buffer to retrieve returned string.
nStartPos
Specifies the position of the first byte to be returned.
nLength
Specifies the length of the string to be returned. 
BOOL Free();
Frees the locator object.
See sample

CSqlErrorInfo class

CSqlErrorInfo has no base class. 

CSqlErrorInfo();
CSqlErrorInfo(const CSqlErrorInfo& rEI);
Creates a CSqlErrorInfo object. 
const char* GetErrorText();
Returns the text of an error object.
See sample