Re: Exceptions as smart pointers
on Thu Aug 07 2008, "Sergey P. Derevyago" <non-existent-AT-iobox.com> wrote:
Long ago I've posted some messages on this topic, in particular this one:
Message-ID: <410A48AB.95A97D57@iobox.com>
http://groups.google.com/group/comp.lang.c++.moderated/msg/41de6076288f7c89
I've never understood how your solution addresses the problem.
In the above posting, you write:
The solution is destructor with in_stack_unwinding parameter:
class A {
// ...
~A(bool in_stack_unwinding) {
if (in_stack_unwinding) { /* ... */ }
else { /* ... */ }
}
};
That appears to only change the interface, but not address the semantic
issues with uncaught_exception. More specifically:
under what conditions will in_stack_unwinding be true above?
If that bool has the same semantics as uncaught_exception(), then all
the problems described in http://www.gotw.ca/gotw/047.htm still apply
(let's ignore the "morality" argument given there; I object to the whole
approach).
If you're suggesting semantics equivalent to "is it safe to throw," I
don't _think_ that's actually implementable.
But there exist some points that deserve C++ specific attention. And one
of these points is "exceptions being thrown as smart pointers to actual
exception objects".
In particular, the object is being thrown is always sh_ptr<Exception>:
typedef sh_ptr<Exception> shException;
See http://ders.stml.net/cpp/mtprog/doc/exception_8hpp-source.html for
the details.
Can you give a summary of what you achieve by throwing smart pointers?
There also can be nested exceptions and used like this:
void f(mem_pool& mp, int i)
{
shException exc(mp, 0);
try {
g(mp, i);
return;
}
catch (shException she) { if (she->is<MsgException>()) throw; else
exc=she; }
catch (...) { exc=recatchException(mp, _FLINE_); } // recatch!
throw newException(_FLINE_, "Problems in f()", exc); // use as nested!
}
And you can browse the documentation to quickly get the remaining
details: http://ders.stml.net/cpp/mtprog/doc/exception_8cpp-source.html
I'm sorry, but that simply seems to link to an HTML-ized copy of the
code, and I don't have the time to try to analyze your code to figure
out what you're trying to accomplish. Summarizing the problem you're
trying to solve, and how your code solves the problem, would help lots.
--
Dave Abrahams
BoostPro Computing
http://www.boostpro.com
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