Re: Compile errors trying to derive from CException
On Tue, 8 Apr 2008 06:54:40 -0700 (PDT), bamford.chris@googlemail.com
wrote:
Hello all, I'm having a problem creating a class derived from
CException in VS2005. Here's the class.:
class CIndexException : public CException
{
public:
CIndexException (const CIndexException & oSourceObject) :
m_pszMessage(oSourceObject.m_pszMessage) {};
CIndexException & operator = (const CIndexException & oSourceObject)
{
m_pszMessage = oSourceObject.m_pszMessage;
return *this;
};
CIndexException () : CException(), m_pszMessage(0) {};
CIndexException (char * pszError = 0, bool bAutoDelete = true) :
CException(bAutoDelete), m_pszMessage(pszError) {};
Those are both default constructors, and you can usefully have at most one
in a class. Also, you're not making a copy of the string, which means its
lifetime must exceed that of the exception object, which ain't gonna work
for a non-static local char array. So that's pretty dangerous. Finally, you
should be using const char*.
const char * GetMessage() { return m_pszMessage; }
protected:
char * m_pszMessage;
};
The build errors I'm getting are as follows:
Error 2 error C2248: 'CObject::CObject' : cannot access private member
declared in class 'CObject' c:\program files\microsoft visual studio
8\vc\atlmfc\include\afx.h 898
Error 4 error C2248: 'CObject::CObject' : cannot access private member
declared in class 'CObject' c:\program files\microsoft visual studio
8\vc\atlmfc\include\afx.h 898
I introduced the copy and assignment constructors thinking they would
remove the errors but they haven't. Would appreciate any help/
suggestions to get this working.
The CObject class is not copyable, and you shouldn't try to make classes
you derive from it copyable. You don't need to, since the CException way is
to throw pointers to CException objects created with new. So get rid of the
copy ctor and assignment operator. For a correct implementation of a class
derived from CException, see MfcGenericException here:
http://members.cox.net/doug_web/eh.htm#Q6
Note the CString member variable to hold the error message (fixes your
lifetime issue) and the function GetErrorMessage, which overrides the
CException virtual function. You can still write your GetMessage function
for your own convenience, but you /must/ override GetErrorMessage so that
when your CIndexException* is caught as a CException*, the error message
will be available. Of course, you should derive your own class from
MfcGenericException along the same lines as MfcStdException.
--
Doug Harrison
Visual C++ MVP