shared - i need it to be useful

[Manu]

On Thu, Oct 18, 2018 at 7:20 AM Steven Schveighoffer via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
>
> On 10/18/18 10:11 AM, Simen Kjærås wrote:
> > On Thursday, 18 October 2018 at 13:35:22 UTC, Steven Schveighoffer wrote:
> >> struct ThreadSafe
> >> {
> >>    private int x;
> >>    void increment()
> >>    {
> >>       ++x; // I know this is not shared, so no reason to use atomics
> >>    }
> >>    void increment() shared
> >>    {
> >>       atomicIncrement(&x); // use atomics, to avoid races
> >>    }
> >> }
> >
> > But this isn't thread-safe, for the exact reasons described elsewhere in
> > this thread (and in fact, incorrectly leveled at Manu's proposal).
> > Someone could write this code:
> >
> > void foo() {
> >      ThreadSafe* a = new ThreadSafe();
> >      shareAllOver(a);
>
> Error: cannot call function shareAllOver(shared(ThreadSafe) *) with type
> ThreadSafe *

And here you expect a user to perform an unsafe-cast (which they may
not understand), and we have no language semantics to enforce the
transfer of ownership. How do you assure that the user yields the
thread-local instance?
I think requiring the cast is un-principled in every way that D values.

My proposal doesn't rely on convention (that we *hope* the user does
correctly yield the thread-local instance)... it assures a set of safe
rules by default.

This is the core value proposition of my proposal. It's literally the
entire point.