Le 26/04/13 14:22, Pierre T. a écrit :
Hello,
I wrote a proposal for the Boost.Expected project. You can find a pdf version here:
http://hyc.io/boost/boost-expected-proposal.pdf
It's about the Boost.Expected project, I'll add my own informations by the end of the week.
Finally, I propose a single class design, and facilities for custom error code. It's based on the Alexendrescu idea upgraded by Vicente J. Botet Escriba and myself. I also took into account the Boost.Optional class. If needed for this proposal, I can add documentation per method (such as in official proposal), but I'm not sure it's useful without a frozen interface.
Please do not hesitate to comment it and request further clarification,
Hi, first of all, thanks for writing this proposal. I have some concerns * Single class: I'm all for a single class; but having an ExceptionalType template parameter which defaults to exception_ptr. template <typename ValueType, typename ExceptionalType=exception_ptr> class expected; We could define the traits that make the difference between having an exception_ptr and another error template <typename ExceptionalType> struct exceptional_traits { typedef ExceptionalType exceptional_type; template <class E> static exceptional_type make_exceptional(E const& except) { return exceptional_type(except) } static exceptional_type current_exceptional() { return exceptional_type() } static void rethrow(exceptional_type except) { boost::throw_exception(bad_expected_access<exceptional_type>(except)); } }; template <> struct exceptional_traits<exception_ptr> { typedef ExceptionalType exceptional_type; template <class E> static exceptional_type make_exceptional(E const& except) { return boost::make_exception_ptr(except); } static exceptional_type current_exceptional() { return boost::current_exception() } static void rethrow(exceptional_type except); { boost::rethrow_exception(except); } template <class E> static bool has_exception(bool valid, exceptional_type except) const BOOST_NOEXCEPT { try { if (!valid) rethrow(except); } catch (const E& ex) { return true; } catch (...) { } return false; } }; } With these traits we are able to define the functions that have a specific behavior as e.g. typedef detail::exceptional_traits<ValueType, ExceptionalType> traits; expected() BOOST_NOEXCEPT : except_(traits::current_exceptional()) , has_value_(false) {} const T& get() const { if (!valid()) traits::rethrow(except_); return value_; } template <class E> bool has_exception() const BOOST_NOEXCEPT { return traits::has_exception(except_); } * Default Constructor or constructor from nullexpect What is the advantage of having a expected instance that doesn't have neither a value nor an exception? How would the user manages with this possibility? Are you looking to make expect movable? * then/otherwise issues Humm, I don't agree here with the proposed design (even if I made the then suggestion). What returns the then function if the instance has a value? I declared it as template <typename F> expected<typename boost::result_of<F(const expected&)>::type> then(F&& fuct) Let me name typedef typename boost::result_of<F(const expected&) RT; so the result would return the expected<RT>(fuct(value_)). Chaining this temporary with an otherwise call will work with the new temporary, and not with the original expected. I don't think this was what was expected ;-) E.g. string f(int); int f(string); expected<int> e= 1; e.then(f).otherwise(g); Here g will be called with expected<string>(fuct(value_)).except_. Hrrr expected<int> e= make_exceptional_expect(X); e.then(f).otherwise(g); Here g will be called with the exception_ptr stored in e. Hrrr That is g don't know if it is called with an exception stored on e or on the temporary resulting for the call to f. But the design error is not on the otherwise function but on the then function. Resuming, I'm not more for the 'then' function. Using a visitor would as you proposed initially would be better. template <class V> void accept_visitor(V& visitor) const BOOST_NOEXCEPT { if (valid()) visitor.visit(value_); else visitor.visit(expectional, except_); } HTH, Vicente