Re: [boost] [variant2] Need rationale for never-empty guarantee

Subject: Re: [boost] [variant2] Need rationale for never-empty guarantee
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2019-03-01 18:02:24

AMDG

On 3/1/19 9:41 AM, Niall Douglas via Boost wrote:
> <snip>
> I can see the logic, but it is wrong in my opinion. The variant should
> be put back into the state it was in beforehand, in my opinion.
> 
> (Personally speaking, I find the double buffering a step too far. I
> remember debating this with Anthony Williams a few years ago at ACCU. I
> think that if double buffering is necessary, then you weaken your
> guarantees to basic, and you provide a constexpr bool for static
> asserting when the guarantees are basic or strong.

I almost agree with this.  double buffering is definitely
the right choice when it is needed to get the basic guarantee.
I don't think it's worthwhile if all it gives you is
upgrading the basic guarantee to the strong guarantee.
I support the strong guarantee as long as it can be
achieved with some combination of moves.

> In any case, I find
> the valueless by exception state to be an abomination,

+1.  I don't really care whether variant has an
empty state or not, but if it exists at all it
needs to be a first class citizen.  As such I
was always quite happy with boost::blank or monostate.

In Christ,
Steven Watanabe