Proto for Boost mailing list page: Re: [proto] Recent (rvalue support) changes in proto causes a whole bunch of regressions in Spirit

Subject: Re: [proto] Recent (rvalue support) changes in proto causes a whole bunch of regressions in Spirit
From: Eric Niebler (eric_at_[hidden])
Date: 2011-10-10 00:38:07

On 10/9/2011 8:32 PM, Joel de Guzman wrote:
> Hi,
> 
> This code:
> 
>   #include <boost/spirit/include/qi.hpp>
> 
>   int main()
>   {
>       namespace qi = boost::spirit::qi;
> 
>       qi::rule<char const*> r;
>       r %= (+qi::char_);
> 
>       return 0;
>   }
> 
> no longer works as expected. r %= (+qi::char_) becomes a no-op.
> Before it calls either:
> 
>   template <typename Expr>
>   friend rule& operator%=(rule& r, Expr const& expr);
> 
>   // non-const version needed to suppress proto's %= kicking in
>   template <typename Expr>
>   friend rule& operator%=(rule& r, Expr& expr);
> 
> Both defined in the rule class.
> 
> Correct me if I'm wrong, but it seems the changes in proto forces
> us to support rvalue refs, but we are not ready for that yet and
> it's too late in the release cycle.
> 
> Thoughts?

Bummer. I suggest adding an rvalue ref overload of operator%= that
shares an implementation with the const lvalue one. Should just be a few
lines of code. Is that a problem?

As an alternative, you can use a domain/grammar to disable Proto's
operator%= for Spirit rules.

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