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

Subject: Re: [boost] [variant2] Andrzej's review -- design
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2019-04-04 22:35:01

On Thu, Apr 4, 2019 at 3:14 PM Gavin Lambert via Boost <
boost_at_[hidden]> wrote:
>
> On 4/04/2019 13:26, Emil Dotchevski wrote:
> > Consistent with the basic guarantee, if you assign one std::vector to
> > another and the copy operation fails, the contents of the target vector
is
> > not specified, but it is guaranteed to be a valid vector, so that if you
> > happen to handle it later, you can safely do with it what you do with
any
> > other vector: you can insert or delete elements, iterate, whatever.
> >
> > By analogy with your position, a better design for std::vector would be
to
> > define a special "error" state that you must check for *every* time you
> > handle a vector, and the justification for that design is that
otherwise it
> > might be unexpected to see a vector in a half-copied (but valid, by
> > definition) state. Do you see the practical difference now?
>
> No, because vector already has that state -- it's called "a vector with
> size zero" (aka "empty"), which is the same state it typically
> transitions to when successfully moved-from.

template <class T>
void f( std::vector<T> & x, std::vector<T> const & y )
{
  x = y;
}

Above, if the assignment fails, the content of x is unspecified, but it is
guaranteed to be a valid vector, containing valid objects (if not empty).

By analogy: if a variant2 assignment fails, its value is unspecified, but
it is guaranteed to be valid.