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Subject: Re: [boost] [outcome] outcome without empty state?
From: Jonathan Müller (jonathanmueller.dev_at_[hidden])
Date: 2017-05-24 20:44:15

What about providing no default constructor? There are two valid choices so
why surprise half the users?

Yes, it makes it a bit harder to use in arrays, but how often would it need
to be stored in arrays anyways.


On May 24, 2017 10:38 PM, "Niall Douglas via Boost" <boost_at_[hidden]>

> >> 4. Function r.empty() is implemented as `r.has_error() && !error()`.
> >
> > So you are requesting that error is explicitly convertible to bool. This
> > could work for outcome and result as the errors provide it but not for
> > expected<T,E>.
> > I don't believe the library should make use of this bool conversion.
> > Even the opposite. I will add a pre-condition that the Error value if
> > convertible to bool is equal to false (this can be also cheched at
> > run-time).
> std::error_code does not have a constexpr constructor, and does not have
> a constexpr .value() nor explicit operator bool(). Very unfortunate. It
> also precludes doing a generic check that a default constructed E tests
> to boolean false, so you will need that generic check plus a special
> override for std::error_code and subclasses of std::error_code.
> >> 5. Framework requires that type E in `expected<T, E>` is
> >> default-constructible, and its default-constructed value represents no
> >> error condition (already the case for exception_ptr, error_code).
> > This is something that the library cannot check without adding more
> > constraints. The interpretation that the user does of each one of the
> > values of E is up to him.
> I would agree.
> A null error_code has the convention of "no error here", but as I've
> explained in previous discussion, because of the C++ standard requiring
> system_category with a code of 0 for default construction, it rules out
> anyone *portably* treating a null error_code as "no error here".
> Until the C++ standard gets fixed, we cannot assume this of the null
> error code any more than the Filesystem TS does (which it does,
> incidentally, and it should not).
> > If the user uses values of Error that doesn't mean an error, you have a
> > tri-state.
> > IMO, the use error_code{} or exception_ptr{} with outcome or expected is
> > an ERROR.
> I disagree. That is on the end user to decide, not us.
> > At the end I'm not sure result<T> or expected<T> should have a default
> > constructor. Note that we want variables initialized at the declaration
> > and the default construction of these types is always artificial.
> I too felt this, which was one of my rationales for the formal empty
> state. Completely eliminating the default constructor altogether does
> seem attractive ... but it would be a usability pain for end users.
> > Concerning the default construction, I'm inclined to think that in this
> > cases the best is to don't initialize as a T or as E. Note that this
> > doesn't mean to add an additional state. The result of default
> > construction is just a partial initialized state that needs either to be
> > assigned or destructed, as if we had do a move. Trying to use has_value
> > on this state will be UB.
> Ah, more UB. Yay.
> > The advantage is that IMO, is that we don't spend more time than we need.
> > Not all types should have a well formed object as default value (See
> > e.g. chrono::duration, Date).
> If the formal empty state is to be removed, then either you default
> construct T, or E, or you go the UB route. I still find the formal empty
> state the most attractive, but I recognise no one agrees with me.
> Niall
> --
> ned Productions Limited Consulting
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