Re: [boost] [variant2] Formal review

Subject: Re: [boost] [variant2] Formal review
From: Rainer Deyke (rainerd_at_[hidden])
Date: 2019-04-14 19:52:54

On 14.04.19 20:16, Emil Dotchevski via Boost wrote:
> On Sun, Apr 14, 2019 at 10:19 AM Rainer Deyke via Boost <
> boost_at_[hidden]> wrote:
>>
>> On 13.04.19 19:26, Bjorn Reese via Boost wrote:
>>>     * std::variant which enters an invalid (valuless_by_exception) state.
>>>       While that may be appropriate for some use cases, there are others
>>>       where it is not. For instance, the variant may be used for a state
>>>       machine where the alternative types are function object types to be
>>>       invoked when events arrive. Having a valueless state is the wrong
>>>       choice for this use case.
>>
>> Wait.  I don't understand how never-empty is an advantage in this example.
>>
>>    The invariants of the state machine can still be broken.
> 
> It is not the job of variant to maintain the invariants of the state
> machine, that is the job of the state machine.

Yes, but the question was about the benefits of the never-empty 
guarantee.  If the never-empty guarantee doesn't help with maintaining 
higher level invariants, then what benefit does it bring?

-- 
Rainer Deyke (rainerd_at_[hidden])