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Subject: Re: [boost] [spirit] semantic action for mismatches?
From: Eric Niebler (eric_at_[hidden])
Date: 2011-01-08 11:36:43

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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