On 6 September 2016 at 11:56, John McCall <[email protected]> wrote:
> > On Sep 6, 2016, at 11:50 AM, Thiago Macieira <[email protected]> wrote: > > Em terça-feira, 6 de setembro de 2016, às 11:38:43 PDT, John McCall > escreveu: > >> This would be inappropriate; the appropriate solution would be to ask > the > >> committee for a new function in namespace std that could be implemented > >> with abi::__forced_unwind. I believe that's the chief purpose of that > >> function, to allow that sort of functionality to be implemented in the > >> future. > >> > >> If std::current_exception works, of course, all the better. > > > > I will post to std-discussion and see if there's a consensus. I'm not > sure > > there can be because this is outside of the C++ language: there's no such > > thing as forced unwinds in the standard. > > Hmm. I think this used to not be true; there were library features (e.g. > longjmp) > that were allowed, but not required, to be implemented by unwinding the > stack. > But now it seems that these places have all been changed to specify > undefined > behavior if they would bypass any non-trivial destructors. > That's true, but it does not place restrictions on intervening 'catch (...)' blocks, and (since no exception has been thrown) they presumably must /not/ be entered. (You don't get UB from a longjmp where a corresponding throw would enter a catch(...) block -- although that may just be a wording defect.) The possibility of forced unwinding invoking a 'catch (...)' block with no current exception seems broken to me. For instance, this may or may not call terminate() if f() results in a forced unwind, depending on how 'throw;' is implemented: try { f(); } catch (...) { log_error(); // might reasonably assert(std::current_exception()) throw; } Pretending that a forced unwind throws a C++ exception of some known type seems likely to be the best approach -- and I'd expect that exception to be capturable via std::current_exception and counted by std::uncaught_exceptions, or exception-safe code using those facilities will not function correctly in the presence of a forced unwind. (And likewise for foreign exceptions; we presumably want to pretend that they are just like C++ exceptions but are of a type that doesn't match that of any handlers.)
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