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Subject: Re: [boost] [spirit] semantic action for mismatches?
From: caustik (caustik_at_[hidden])
Date: 2011-01-08 11:41:58

I might give that a shot as I can't seem to get local variables to solve my
problem. I would expect these two commands to have the same result (albeit
unnecessarily assigning the _a variable, which just gets tossed away
anyway), but apparently they are not?

r = p[_a = _1]
r = p

On Sat, Jan 8, 2011 at 8:36 AM, Eric Niebler <eric_at_[hidden]> wrote:

> On 1/7/2011 6:54 PM, caustik wrote:
> > I've come to notice that there seems to be a missing bit of symmetry in
> > spirit with regards to semantic actions.
> >
> > If a rule matches, and thus executes it's semantic action(s), but a rule
> > which includes that rule mismatches, there seems to be no way to "unwind"
> > the code executed down the chain. For example, if one of your semantic
> > actions allocates memory or increments a reference count, how do you free
> /
> > release that reference in the mismatch scenario? I've thought about using
> > something like a shared_ptr, but it seems like that gets pretty sloppy
> and
> > unnatural. Is there something you can think of that would work?
> Just for reference, xpressive doesn't have this problem because actions
> are executed lazily. When a sub-pattern matches, its action is placed on
> a queue. If the pattern matching engine then needs to backtrack, the
> action is un-queued. Only when the *whole* pattern matches successfully
> is the entire action sequence executed ... in order, of course.
> --
> Eric Niebler
> BoostPro Computing
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