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Subject: Re: [boost] [outcome] High level summary of review feedback accepted so far
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-05-31 10:47:52

On 30/05/2017 21:12, Andrzej Krzemienski via Boost wrote:
> 2017-05-30 15:18 GMT+02:00 Niall Douglas via Boost <boost_at_[hidden]>:
>>> - value() throws if holding error or exception (or empty)
>>> - error() returns E{} if holding value or E{errc::has_exception} if
>>> holding exception, or E{errc::no_value} if empty (names made up on the
>>> spot, doesn't matter)
>> The current behaviour is "always throw on observing empty". That's what
>> makes empty special (and it traps unintentional propagation of empty).
>>> - exception() returns nullptr if holding value or error or empty
>> The current behaviour is to return null exception_ptr if valued (so
>> if(eptr) would be false i.e. "no exception here"),
>> std::make_exception_ptr(std::system_error(error())) if errored, throw on
>> empty.
>> So an errored state is also an excepted state, but an excepted state is
>> never an errored state. has_exception() returns true for either errored
>> or excepted states.
> But is there some mental model behind this decision?

Of course.

exception <= error < value. So errors are exceptions, but exceptions are
not errors.

empty is the wildcard. It is modelled like NaN in floating-point, it
propagates in a dominant fashion unless you explicitly deal with it.

>> std::make_exception_ptr(std::system_error(error())) is fairly cheap, a
>> few thousand CPU cycles. std::exception_ptr's are heavy anyway.
> Or are these decisions, what to return in tricky cases are only driven by
> performance?

If you are using result<T> instead of outcome<T>, it is assumed that you
must feel an exception_ptr to be too expensive and/or you are not using
exceptions at all. outcome<T> implicitly converts from result<T> on
demand, so some code you can write without exception_ptr, some with, and
the two interoperate seamlessly by all funnelling into outcome<T>.


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