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

From: Frank Heckenbach
Subject: Re: Automatially move from $n (was: C++11 move semantics)
Date: Sun, 16 Sep 2018 01:01:30 +0200

Hans Åberg wrote:

> I realize now that you want to wrap $k with std::move and guard against reuse 
> of that! (See below)

Yes, see $subject.

> > Sure it's possible: "make_pair ($1, $1)". Don't do that with
> > automatic move!
> Ah, your idea is to wrap std::move around the $k values! My idea
> is to let the C++ language recognise that, and then automate the
> assignment to $$.

Again, the assignment to $$ is <bold>*NOT*</bold> the issue! (*)
It's the passing (moving) of $k.

(*) Nitpick: Except in a case like "$$ = $2;", but then I'd argue
    it's the passing of $2 to $$'s assignment operator that's the
    issue. :)

> Maybe there is some more general C++ code checking program that
> can be linked into Bison to check the  actions.

You mean some kind of code analyzer? This might be possible, but may
be overkill. Bison could keep track if any $k is used several times
and warn about that; though it may give false positives in cases
such as:

  $$ = $1 ? foo (move ($2)) : bar (move ($2));  // safe

But the user could work around such (hopefully) rare cases with a
temporary variable holding move ($2).

Or can code analyzers recognize the above as safe? (Do you have
experience with any?)

On the same token, Bison could then also (optionally) warn if some
$k which has a type is not used. I don't know if there is interest
in such a feature. It could also be useful for other languages.

It might not be too hard to do for someone who's familiar with
Bison's internals. (Unfortunately, I'm not very much, and don't have
much free time right now.)


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