c++ - can rethrow_exception really throw the same exception object, rather than a copy? -

c++ - can rethrow_exception really throw the same exception object, rather than a copy? -

while examining exception_ptr does, c++11 standard says (18.8.5/7) that:

use of rethrow_exception on exception_ptr objects refer same exception object shall not introduce info race. [ note: if rethrow_exception rethrows same exception object (rather copy), concurrent access rethrown exception object may introduce info race...

i don't find case weird "note" applies, since described effect of rethrow_exception "throws: exception object p refers" 15.1/3, describing general exception throwing process mandates "throwing exception copy-initializes temporary object, called exception object."

the weird note imply rethrow_exception skips copy-initialization. possible?

yes, looks deficiency in standard. rethrowing throw-expression i.e. throw; without operand, 15.1p8 says:

a throw-expression no operand rethrows handled exception. exception reactivated existing exception object; no new exception object created. [...]

that is:

#include <exception> #include <cassert> int main() { std::exception *p = nullptr; seek { seek { throw std::exception(); } catch(std::exception &ex) { p = &ex; throw; } } catch(std::exception &ex) { assert(p == &ex); } }

if implementation of current_exception copies handled exception object, there's no way tell whether rethrow_exception copies or not, if refers exception object can check:

#include <exception> #include <iostream> int main() { std::exception_ptr p; seek { seek { throw std::exception(); } catch(...) { p = std::current_exception(); std::cout << (p == std::current_exception()) << ' '; std::rethrow_exception(p); } } catch(...) { std::cout << (p == std::current_exception()) << '\n'; } }

every implementation i've tried on prints 1 1; 0 0 allowed if current_exception copies; 0 1 impossible, while standard in current state appears require 1 0. prepare 18.8.5p10 clarified language similar 15.1p8, either allowing or mandating rethrow_exception not re-create exception object pointed exception_ptr.

most throws: specifications in standard name type (throws: bad_alloc) or utilize indefinite article (throws: exception of type ...); other exception specifications utilize definite article of future::get , shared_future::get, resolution should address well.

c++ c++11 language-lawyer