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])