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Subject: Re: [boost] [spirit] semantic action for mismatches?
From: caustik (caustik_at_[hidden])
Date: 2011-01-07 20:57:34

On Fri, Jan 7, 2011 at 5:30 PM, Joel de Guzman <joel_at_[hidden]>wrote:

> On 1/8/2011 7:54 AM, 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?
> Sure:
> r = p | eps[cleanup];
> If p fails with side-effects from its direct or indirect semantic
> actions, the cleanup semantic action can roll them back.
> <>


Brilliant. You sir, are a gentleman and a scholar.

I wonder if this is FAQ material. It's elegant yet a bit non-intuitive.

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