Re: [boost] [variant2] Andrzej's review -- design

Subject: Re: [boost] [variant2] Andrzej's review -- design
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2019-04-06 18:53:26

On Fri, Apr 5, 2019 at 7:55 PM Larry Evans via Boost <boost_at_[hidden]>
wrote:
>
> On 4/5/19 7:46 PM, Emil Dotchevski via Boost wrote:
> > On Thu, Apr 4, 2019 at 4:30 PM Gavin Lambert via Boost <
> > boost_at_[hidden]> wrote:
> >> The problem with transitioning to an otherwise valid but unexpected
> >> state is that it may end up evaluating unintended code
> >
> > This is why I made the analogy with std::vector. Let's say you always
put
> > exactly 10 elements in all your vectors, but then you assign one vector
to
> > another, and that fails, and now (unexpectedly?) you have an (otherwise
> > valid) vector with fewer than 10 elements.
> >
> > Such is life. This is how the basic guarantee works, you get an
unspecified
> > but valid state.
> >
> >> (Granted, continuing evaluation on a faulted variable is already a bug
> >> in itself, but you are making detection of that bug harder by not
> >> putting the value into an obviously-faulted state.)
> >
> > I do not think that it is a bug to access an object that is in a valid
> > state. In fact, the reason why the state is defined as valid is so that
you
> > can safely work with that object, even after a failure was reported.
>
> I'm guessing "unexpected" could mean the std::vector
> could have some arbitrary number of elements all filled with random
> values.  I don't see how you can do anything useful with that.

The values aren't random, they are guaranteed to be valid. From the point
of view of the type system and what a vector is, it is illogical to
complain that it has fewer than 10 elements. It is fine for vectors to have
fewer than 10 elements.

The way I formulated the example makes it feel like there is something
wrong with that state, because we always put 10 elements in all vectors and
now we find one with fewer than 10. But this has nothing to do with vector.
To do this correctly, we need to write a wrapper type for vector that has a
size-10 invariant, then define the appropriate safety guarantees for that
type.