Le 04/06/2017 à 17:55, Peter Dimov via Boost a écrit :
Vicente J. Botet Escriba wrote:
expected<T> is valid. Its purpose is to make expected{ expr } valid (the > equivalent of make_expected.)
But make_expected() returns expected<T, error_code> I understand that when designing expected<T, E...> we need to reconsider some functions.
BTW, to what it is useful to have a expected<T> without a possible error? is expected<T> convertible to expected<T,E...>?
expected<T, E1...> is convertible to expected<T, E2...>, where all error types in `E1...` also occur in `E2...`. So it follows that expected<T> is convertible to expected<T, E...> for any `E...`.
That is, you can `return expected{ x }` in a function returning expected<X, E...>. Great.
I don't see how expected<T, E...> could default to expected<T, error_code>?
It doesn't default to anything, no.
Does it mean that we need an additional template alias
template <class T, class R = error_code> using expected2<T, E>;
We could call this alias... `result<T>`! :-) Sure, and what about alias and factories.
expected<variant<int,string>, int, string> ? unexpected_type<E> was used for that to avoid ambiguity.
Yes, I suppose you could do that. I agree that unexpected_<E...> should be a distinct type. For now I've taken a shortcut, but this will be fixed. Glad to see we agree.
I don't see how get<I> can work here.
https://rawgit.com/pdimov/mp11/develop/doc/html/mp11.html#mp11.reference.alg...
I don't see it yet. What is the type of I. How can you use a function parameter as a template argument?
The type of I is mp_size_t<I'>, an alias for std::integral_constant<std::size_t, I'>. Since integral_constant has a constexpr conversion to its value, it can be used directly as if it were the compile-time value (modulo compiler bugs.) So get<I>, get<I.value> and get<decltype(I)::value> all amount to the same thing. C++14 is magic. Wow, I didn't was aware of this possibility.
Since we have argument deduction now, expected{ expr } and > unexpected_{ expr } can be used for the purpose, and I saw no need for > separate factory functions.
For the standard this *could* be good, but will your library be C++17 only?
For a library that targets C++20, it does seem reasonable for the proposed interface to be optimized for C++17. If this becomes a Boost library, I could include make_expected and make_unexpected to ease C++14 use, but I don't think we need them in the formal proposal. Agreed. the standard and Boost needs two different libraries.
What do you think of making expected constructor from T explicit? What we lost by making it explicit? Implicit use expected<double, unscoped_error> test() { return unscoped_other_error; // returns 7.0 } Explicit use expected<double, unscoped_error> test() { return expected{unscoped_other_error}; // returns value 7.0 // make_expected } expected<double, unscoped_error> test() { return unexpected_{unscoped_other_error}; // returns Error 7 // make_unexpected } Vicente