Re: Handle C++ exception and structured exception together

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sat, 26 Jan 2008 20:24:36 +0100
Message-ID:
<13pn23sfo6k3t8c@corp.supernews.com>
* Pavel:

Alf P. Steinbach wrote:

* Pavel:

anon wrote:

George2 wrote:

Hello everyone,

I am learning set_se_translator, and there are some good resources
about how to translate structured exception into C++ exception, like,

http://www.codeproject.com/KB/cpp/seexception.aspx


LOL
this code looks like coming from a clown:

catch(CSeException *e)
{
  e->ReportError(MB_OK | MB_ICONSTOP);
  e->Delete();
}

Wondering what they do in Delete() method. Hope not "delete this"


I thought "delete this" was not bad-bad, although certainly not
ideal. Sometimes there is no good alternative to at least indirect
"delete this" or its equivalent ... or I simply do not know one. Do you?


The problem is mostly that with an exception there's no designated
"owner" that's responsible for deleting. And with a catch(...) you
have a memory leak. The code above is based on MFC exceptions, which
were a pre-standard hack to use exceptions in a language
implementation that didn't support exceptions.

Cheers, & hth.,

- Alf


I agree, in general catch(...) does not work for throwing pointers; but
in the example in question the structured exceptions are all inherited
from CSeException so they can be safely caught and deleted if needed.
Structured exceptions targeted C language, with no automatic stack
unwinding, kind of glorified setjmp()/longjmp() so throwing pointers and
deleting memory in the handler seems to be a more or less precise
mapping of the concept into the standard C++ syntax.

I believe the "clown" pan was aimed at "delete this" idea though.


* C++: I believe the "clown" comment was aimed at throwing pointers to
dynamically allocated objects. It's a recipe for disaster, because of
the lack of designated owner and possibility of "catch(...)". Using
"delete this", on the other hand, is a normal way to do destruction in
many situations, nothing wrong with that, although it is a power tool
that can be dangerous in untrained hands.

* Windows: structured exception handling (SEH) is an operation system
API way to communicate failure that targets no specific language, but
does require language support. Most of it is undocumented. As I recall
Matt Pietrik (not sure of speling) wrote a series of articles going into
depth of the undocumented aspects, including typical language support.

* MFC exceptions (MFC is an old C++ GUI framework from Microsoft):
throwing pointers had, as far as I know, nothing to do with SEH, and
these exceptions were not targeted at C, since MFC was and is a C++
class framework. Rather, it probably had to do with this exception
handling being implemented via macros and nifty hacking, before C++
exception handling became available. Keep in mind that MFC predates the
C++ standard, and that C++ exception handling wasn't frozen until late
in the standardization process.

The pre-standard macro way (example lifted from [1]):

     TRY
     {
        // Do something to throw an exception.
        AfxThrowUserException();
     }
     CATCH(CException, e)
     {
        if (m_bPassExceptionsUp)
           THROW_LAST();

        if (m_bReturnFromThisFunction)
           return;

        // Not necessary to delete the exception e.
     }
     END_CATCH

Using standard C++ exception handling the cleanup that was hidden in the
macros must be done manually:

     try
     {
        // Do something to throw an exception.
        AfxThrowUserException();
     }
     catch(CException* e)
     {
        if (m_bPassExceptionsUp)
           throw;

        if (m_bThrowDifferentException)
        {
           e->Delete();
           throw new CMyOtherException;
        }

        if (m_bReturnFromThisFunction)
        {
           e->Delete();
           return;
        }

        e->Delete();
     }

Of course, this mess is on its own a good reason not to use MFC... ;-)

Cheers, & hth.,

- Alf

Notes:
[1] <url: http://msdn2.microsoft.com/en-us/library/19z28s5c.aspx>

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