C / C++ / MFC

1. pUserData has not been initialized and points to a random location.
   
2. Your program has changed the value of pUserData (by incrementing it, for instance), and the pointer does not refer to the proper original location.
   
3. You ave freed the memory previously pointed to by pUserData (with free(pUserData), presumably) and yet you're trying to use the pointer.

virtual BOOL OnCmdMsg( UINT nID, int nCode, void* pExtra, AFX_CMDHANDLERINFO* pHandlerInfo );

void foo(char *& p)
{
  p="Second";
}
...
char *p="First";
foo(p);
// now p points to "Second"

void foo(char *p)
{
  p="Second";
}
...
char *p="First";
foo(p);
// p still points to "First"

class Student
{
public:
    Student(char *name, char *ID)
    {
        strcpy(m_name, name);
        strcpy(m_ID, ID);
    }
private:
  char m_name[32];
  char m_ID[10];
};

Student(char *name, char *ID)
{
    strcnpy(m_name, name, 31);
    m_name[31] = '\0';
    strncpy(m_ID, ID, 9);
    m_ID[9] = '\0';
}

class Student
{
public:
    Student(LPCTSTR szName, LPCTSTR szID)
    {
        strName = szName;
        m_strID = szID;
    }
private:
  CString m_strName;
  CString m_strID;
};

class Student
{
public:
    Student()
    {
        strName = "";
        m_strID = "";
    }
 
    BOOL SetInformation(LPCTSTR szName, LPCTSTR szID)
    {
        strName = szName;
        m_strID = szID;
        //Return TRUE if OK, return FALSE if not.
        return TRUE;
    }
private:
  CString m_strName;
  CString m_strID;
};

Student A;
Student B:
.
.
.
B = A;