Re: [boost] [outcome] High level summary of review feedback accepted so far

Subject: Re: [boost] [outcome] High level summary of review feedback accepted so far
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-05-30 23:43:34

On 31/05/2017 01:18, Niall Douglas wrote:
>> FWIW, my preferred options would be (which I think were the originals):
>>
>>   - 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).

That makes sense when empty is always "you forgot to return a value". 
However you've said yourself that sometimes you've hijacked it as an 
intentional return of no value (as in optional<T>), and that's where 
things get murkier.

>>   - 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.
> 
> std::make_exception_ptr(std::system_error(error())) is fairly cheap, a
> few thousand CPU cycles. std::exception_ptr's are heavy anyway.

I can understand that, but I'm not sure I can entirely agree with it. 
As previously mentioned an error_code return is supposed to indicate a 
non-exceptional error, while an exception is exceptional.  So this feels 
like promoting a non-serious error to a serious error.

Having said that, it's not a deal-breaker; both behaviours make sense in 
their own way, and that behaviour could be convenient in some cases.

Although... should it actually throw on empty or just return an 
exception_ptr containing an exception that indicates it was unexpectedly 
empty?  The only place you should be using outcome<> rather than 
result<> is inside noexcept methods, so throwing is fairly serious.