Re: [Boost-users] boost.xpressive: using placeholders with custom (check()) assertions

Subject: Re: [Boost-users] boost.xpressive: using placeholders with custom (check()) assertions
From: Eric Niebler (eric_at_[hidden])
Date: 2010-06-16 09:29:38

On 6/16/2010 3:04 AM, Alex Dubov wrote:
> Greetings.
> 
> Xpressive allows the use of object placeholders in semantic actions,
> which are being replaced by actual object references during the
> match, like in this simple example:
> 
> placeholder<string> p_string; sregex r =
> (expr)[do_something(p_string, _)];
> 
> When executed, do_something() functor will receive a reference to
> string, as expected.
> 
> However, if functor is wrapped with assertion, this won't work:
> 
> sregex r = (expr)[check(do_something(p_string, _))];
> 
> Compilation will fail, because do_something has no method which
> accepts xpressive::placeholder<> objects. 

It should if it is a lazy function. How is do_something defined? Can you
send a self-contained program that demonstrates the problem you're having?

> And even if such method was
> to be added, there's no obvious way to obtain a run-time reference to
> actual object out of placeholder.

xpressive will do that for you.

> This leads to some question: 1. Is there any trick allowing to use
> placeholders within check() assertions? 2. Alternatively, may there
> be a way to declare a semantic action functor, returning bool, in
> such a way, that it will act as custom assertion? It seems to me,
> that some cunning trick involving boost::proto construct may exist,
> even though I can't think of any particular way to do so.
> 
> In light of the above, I also thought about sort of feature request: 
> 3. Sometimes, custom assertion applies only to a part of the match,
> something like:
> 
> *((expr)[check(cond)] | _)
> 
> That is, a desire is to consume everything, until some specific
> "expr" token matches a precondition. 

Yes, that should work.

> However, raised assertion will
> not stop the matching, because it only applies to one branch of the
> regexp. It could be very handy if there was a way to signal from
> assertion that a larger containing expression matched it results.

I don't understand that last sentence. Can you clarify?

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com