Re: Automatially move from $n (was: C++11 move semantics)

[Hans Åberg]

> On 15 Sep 2018, at 23:26, Frank Heckenbach <address@hidden> wrote:
> 
> Hans Åberg wrote:
> 
>> But you can't safely or in general have Bison writing $$ =
>> std::move(a) directly as one might do something else to a
>> afterwards.
> 
> It would be safe if Bison checked that it's used only once in the
> action.

But in view of that is complicated, I was playing along with idea to somehow 
using C++ to recognize it. The only way seems to use function calls and return, 
as assignments can be applied more than once. So the idea is that if Bison now 
writes
  switch (yyn) {
    … 
    case k:
    {
      …
    }   
  }
where the action may have $$ = …, one has
    case k:
      $$ = std::move(action_k(…));
where the action_k is some function where one returns the value instead. I'm 
not sure exactly how to pick it together, perhaps fitting a lambda capture
    case k:
      auto action_k = […](){ … return …; };
      $$ = std::move(action_k(…));

>> So a way to make it safe is to jump out of the action statement.
> 
> Not necessarily: You could use it twice within one expression
> (unsafe even with jump)

That would probably not be possible, if one gets an automated break after it.

> or just not use it twice (safe even without
> jump).

But then one has to find a way to guard against that.

>>> What I want (or actually have, since I imeplented it :) is a way to
>>> make Bison apply std::move automatically.
>> 
>> But that just applies it always, which might be safe for your move only type,
> 
> In fact it's safe for copy-only types, and possibly unsafe precisely
> for movable types (move-only or move-and-copyable).

It is always unsafe with moves, unless one can find a guard against 
unintentional reuse of a moved-from element.