[Cppunit-devel] faq question about ExceptionTest.testAssignment

[Greg Z. <gr...@pa...>]

Hi,

In response to the faq question

Why does the test ExceptionTest.testAssignment failed in CppUnit test
suite?

I don't know if anyone has responded to this bug yet but here is what I
discovered.

As best as I can tell when cppunit Excepiton::operator= calls
SuperClass::operator= (which really is std::exception::operator=), the
cppunit Exception object's vtable address is overwritten with that of
std::exception's vtable address. So when you call the what() method (a
call through the vtable since it is virtual), instead of calling cppunit
Exception's what(), it calls std::exception's what. This returns the
value stored in std::exception's _m_what member, which is "Unknown
exception" (the default value). So either there is a bug in
std::exception operator= or the compiler is not properly setting up the
vtable when calling std::exception operator=. The copy constructor works
fine.

You can fix the bug by either

- Don't have cppunit's class Exception inherit from std::exception and
remove all use of std::exception.
NOTE: I tried this and it worked fine. I.e. ExceptionTest.testAssignment
passed. Not really sure why you are inheriting from std::exception since
nothing in it is really used.

or

- Still have cppunit's class Exception inherit from std::exception and
use it's _m_what member instead of cppunit Exception's  m_message. You
need to make these changes

* add std::exception initialization to the following cppunit Exception
constructor

Exception::Exception( std::string message,
                      long lineNumber,
                      std::string fileName ) :
    std::exception ( message.c_str() ),  <=== add this to initialize
std::exception's _m_what member
    // m_message( message ), <== remove this not needed any more. using
std::exception to hold the message
    m_lineNumber( lineNumber ),
    m_fileName( fileName )
{
}

* remove m_message and all it's uses
* remove the what() method from exception.h and it's implementation from
exception.cpp. You can still call what() as it will invoke
std::exception::what() which will return the value in _m_what as a const
char *.

NOTE: I tried this and it worked fine. I.e. ExceptionTest.testAssignment
passed. However, the problem with this is that the vtable address for
cppunit Exception is wrong if operator= is ever called. Could lead to
problems down the road.

or

- Leave everything as is and get MS to fix their bug.


~greg