Re: [boost] variant2 never empty guarantees

Subject: Re: [boost] variant2 never empty guarantees
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-06-13 08:34:15

On 13/06/2017 19:31, Andrzej Krzemienski wrote:
> First of all. I haven't seen anyone give an example where they need a
> strong guarantee for variant's assignment. There was one attempt with a
> state machine, but even there it seamed only theoretical. I think the
> questions if we can implement it are secondary to if anyone really needs it.

Actually the state machine example was a thought exercise that (I think) 
proved a strong guarantee was *not* useful for that case.

> If whoever has a real (as opposed to theoretical) need for a strong
> guarantee, please respond.

I think the first barrier is that someone needs to come up with a case 
where a strong guarantee on assignment for *any* type (not variant) is 
useful.

Once some type T needs it, it would become easier to argue that 
variant<T, ...> should provide it as well (due to the desire that a 
variant containing a T should act as much like a T as possible).  I'm 
still not entirely sure I'd agree with that argument, but at least it 
becomes possible to have that argument with something concrete in mind.