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in with their underlying file and file access functions. CArchiveException has its
own m_cause member to hold archive-specific cause codes, as shown in Table D.2. You
can raise archive exceptions yourself through the AfxThrowArchiveException() function,
which requires a cause code parameter and a lpszArchiveName string pointer for the
archive object throwing the exception.</P>
<P>
<H4>TABLE D.2.&nbsp;THE CArchiveException m_cause CODE VALUES.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Cause Code		</TD>
		<TD ALIGN="LEFT">Meaning		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::none		</TD>
		<TD ALIGN="LEFT">No error occurred.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::generic		</TD>
		<TD ALIGN="LEFT">The specific cause wasn't specified.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::badSchema		</TD>
		<TD ALIGN="LEFT">The wrong version of an object was read.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::badClass		</TD>
		<TD ALIGN="LEFT">The class of the object being read was unexpected.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::badIndex		</TD>
		<TD ALIGN="LEFT">The file format is invalid.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::readOnly		</TD>
		<TD ALIGN="LEFT">Attempt to write on an archive opened for loading.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::writeOnly		</TD>
		<TD ALIGN="LEFT">Attempt to read on an archive opened for storing.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">CArchiveException::endOfFile		</TD>
		<TD ALIGN="LEFT">The end of the file was reached unexpectedly while reading.		</TD>
	</TR>
</TABLE>

<H3><A NAME="Heading10"></A>Using the Database Exceptions</H3>
<P>There are two database exception classes: CDBException is used for ODBC-based
database access, and CDAOException is used for DAO-based database access. You can
throw these exceptions yourself with the AfxThrowDBException() function, which needs
three parameters. The first, nRetCode, specifies one of a huge number of database
return codes to define the type of error (you should look in the ODBC documentation
for these). The second parameter, pDB, is a pointer to the database associated with
the exception, and the third parameter, hstmt, is an ODBC handle to the SQL statement
object that was executed, causing the exception.</P>
<P>The RETCODE type is available from the CDBException object via its m_nRetCode
member. You can also access a human-readable piece of error text from the m_strError
member string and the error text returned from the ODBC driver itself in the m_strStateNativeOrigin
member.</P>
<P>The CDAOException class has a corresponding AfxThrowDaoException() function that
can throw the DAO exception objects. This function needs just two optional parameters.
The first, nAfxDaoError, is a DAO-specific error code that indicates problems with
DAO itself (see Table D.3). The second parameter is an OLE SCODE value that is the
return code from a DAO-based OLE call (see the section &quot;Using OLE Exceptions&quot;
for a definition of SCODEs).</P>
<P>
<H4>TABLE D.3.&nbsp;DAO COMPONENT-SPECIFIC ERROR CODES FROM nAfxDaoError.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Error Code		</TD>
		<TD ALIGN="LEFT">Meaning		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">NO_AFX_DAO_ERROR		</TD>
		<TD ALIGN="LEFT">The exception was due to a DAO-specific problem; you should check the supplied CDaoErrorInfo
			object and SCODE value.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">AFX_DAO_ERROR_ENGINE_INITIALIZATION		</TD>
		<TD ALIGN="LEFT">The Microsoft Jet Engine database engine failed during initialization.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">AFX_DAO_ERROR_DFX_BIND		</TD>
		<TD ALIGN="LEFT">A DAO record set field exchange address is invalid.		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">
			<PRE>AFX_DAO_ERROR_OBJECT_NOT_OPEN</PRE>
		</TD>
		<TD ALIGN="LEFT">
			<PRE>The queried table hasn't been opened.</PRE>
		</TD>
	</TR>
</TABLE>
</P>
<P>The CDAOException class has three member attributes: m_scode, which holds an asso-ciated
OLE SCODE value with the attempted operation; or S_OK, if the OLE operation was successful.
The m_nAfxDaoError member holds one of the DAO-specific values from Table D.3. The
m_pErrorInfo is a pointer to a CDaoErrorInfo structure that holds an error code,
descriptive error strings, and a help context ID that is defined like this:</P>
<P>
<PRE>struct CDaoErrorInfo
{
   long m_lErrorCode;
   CString m_strSource;
   CString m_strDescription;
   CString m_strHelpFile;
   long m_lHelpContext;
};
</PRE>
<P>By interrogating this structure, you can find most of the specific database error
details pertaining to the DAO exception.</P>
<P>DAO exceptions can describe more than one error at a time, so you can use the
GetErrorCount() member function to find out how many are being referenced. These
other errors can then be obtained by passing the GetErrorInfo() function a zero-based
index to the specific error. After calling GetErrorInfo() with a specific index in
the range returned by the GetErrorCount() function, m_pErrorInfo will be updated
to point to the specified object, and thus you can retrieve those values.</P>
<P>
<H3><A NAME="Heading11"></A>Using OLE Exceptions</H3>
<P>There are two types of OLE exceptions, represented by two classes: the COleException
class, which is normally used for server-side or OLE-specific operations, and the
COleDispatchException class, which is used when dealing with client-side IDispatch-based
operations such as calling ActiveX object functions.</P>
<P>The simpler of the two is the COleException class, which can be generated by calling
the AfxThrowOleException() function passing an OLE SCODE value. An OLE SCODE is a
32-bit error code that is used to represent any kind of error arising from an OLE
function.</P>
<P>This value would probably arise from the return code of a function call to a function
on one of the interfaces of an OLE object. This SCODE value will then be stored in
the exception's m_sc member for analysis from within a catch block.</P>
<P>There is also a Process() static member function that is passed an exception object
and will turn that exception into an SCODE value to represent that exception.</P>
<P>The COleDispatchException class is used in conjunction with OLE IDispatch interfaces
and is thrown by the AfxThrowOleDispatchException() function. This function has two
forms, both with two mandatory parameters and an optional parameter. The first parameter
for both forms is a wCode WORD value that is an application-specific error code.
The second parameter is an lpszDescription string pointer in one form, or nDescriptionID
for a UINT resource code; both types represent either a verbal string or a string
resource code for a verbal string describing the error. The last optional parameter
is a help context ID.</P>
<P>These values are then available as member variables of the COleDispatchException
object via m_wCode, m_strDescription, and m_dwHelpContext. If a help context is specified
and a help file available, the framework will fill in an m_strHelpFile string identifying
the help file. The name of the application producing the error can also be sought
from the m_strSource attribute.</P>
<P>If you raise this exception from an OLE object such as an ActiveX control, Visual
Basic or any other application using the control or object will display these exception
details.</P>
<P>
<H3><A NAME="Heading12"></A>Using the Not Supported Exception</H3>
<P>The CNotSupportedException class represents exception objects that are generated
when an unsupported MFC, operating system, or user-application-specific feature is
requested. If you want to raise this exception, use AfxThrowNotSupportedException(),
which doesn't required any parameters. There are also no extended members or functions
associated with this exception--it just means unsupported.</P>
<P>
<H3><A NAME="Heading13"></A>Using the User Exception</H3>
<P>You can use the CUserException class to generate application-specific exception
objects. You might want to do this when your program is interacting with the user
to halt the process should she choose a certain option. For example, when you are
using the AppWizard, you can press Esc at any time to cancel the whole process. Microsoft
might have used CUserException to do this by detecting the Esc key and then raising
a user exception object.</P>
<P>This exception can be raised by a call to the AfxThrowUserException() function
and then caught in the usual try and catch blocks. There are some places in the MFC
where this exception is raised, such as during dialog box validation or if the file
is too big for an edit view.</P>
<P>
<H3><A NAME="Heading14"></A>Generating Your Own Custom Exception Classes</H3>
<P>You can derive your own exception classes from CException and add your specific
extended functionality. Listing D.6 shows the class definition for such a custom
exception class that extends the normal functionality by adding a m_strMessage CString
variable to the exception, enabling you to specify your own message when constructing
the exception.</P>
<P>
<H4>LISTING D.6.&nbsp;LST29_6.CPP--CLASS DEFINITION FOR CCustomException IMPLEMENTED
IN CustomException.h.</H4>
<PRE>1:  // ** CustomException.h
2:  // ** Header file for CCustomException
3: 
4:  class CCustomException : public CException
5:  {
6:      DECLARE_DYNAMIC(CCustomException);
7: 
8:  public:
9:      CCustomException(CString strMessage);
10: 
11:      CString m_strMessage;
12:  };
</PRE>
<P>In Listing D.6 the class is implemented in its own CustomException.h header file
and derives from CException in line 4. The DECLARE_DYNAMIC macro in line 6 supplies
the MFC CObject-derived runtime class information required for you to decide the
exception type in a catch-all catch block. The constructor definition in line 9 takes
a CString strMessage parameter to let you create the custom exception with the message
that will be stored in the m_strMessage CString variable declared in line 11.</P>
<P>The corresponding CCustomException class implementation is shown in Listing D.7.</P>
<P>
<H4>LISTING D.7.&nbsp;LST29_7.CPP--IMPLEMENTATION OF THE CCustomException CLASS.</H4>
<PRE>1:  // ** CustomException.cpp
2:  // ** Implementation for CCustomException exception
3: 
4:  #include &quot;stdafx.h&quot;
5:  #include &quot;CustomException.h&quot;
6: 
7:  IMPLEMENT_DYNAMIC(CCustomException,CException);
8: 
9:  CCustomException::CCustomException(CString strMessage)
10:      : m_strMessage(strMessage)
11:  {
12:  }
</PRE>
<P>In Listing D.7 the usual header files are included, and the IMPLEMENT_DYNAMIC
macro is used in line 7 to implement the MFC runtime class information functions.
The constructor in line 9 takes the strMessage parameters and initializes the m_strMessage<I>
</I>member variable with this string value in line 10.</P>
<P>You can then use the custom exception class in your application, as shown in Listing
D.8.</P>
<P>
<H4>LISTING D.8.&nbsp;LST29_8.CPP--USING THE NEW CCustomException CLASS.</H4>
<PRE>1: try
2: {
3:     // ** Something goes wrong
4:     CCustomException* pCustomEx = 
5:         new CCustomException(&quot;My custom error occured&quot;);
6:     throw(pCustomEx);
7: }    
8: catch(CCustomException* e)
9: {
10:     // ** Access the extended m_strMessage string
11:     AfxMessageBox(e-&gt;m_strMessage);
12:     e-&gt;Delete();
13: }
</PRE>
<P>In Listing D.8 a new CCustomException object is created with the application-specific
error text in lines 4 and 5 and is thrown in line 6. This is then caught by the catch
keyword in line 8 and the custom information used by the message box in line 11.
The exception is then deleted in line 12.</P>
<P>If you try this, remember that the implementation code must also have an #include
for the CustomException.h header file to retrieve the class definition like this:</P>
<P>
<PRE>#include &quot;CustomException.h&quot;</PRE>
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