Re: [boost] [core] Breaking change to boost::ref in 1.56

Subject: Re: [boost] [core] Breaking change to boost::ref in 1.56
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-07-14 03:56:24

On Sunday 13 July 2014 17:04:09 Eric Niebler wrote:
> On 7/13/2014 2:48 AM, Peter Dimov wrote:
> > Andrey Semashev wrote:
> >> Perhaps Eric could provide more context on why Boost.Proto needs
> >> recursive reference wrappers? Frankly, the requirement looks strange
> >> to me.
> > 
> > Generic code that does
> > 
> >    boost::reference_wrapper<T> r = boost::ref( t );
> > 
> > is broken when t is a reference_wrapper.
> 
> Right. That's exactly what I argued in committee. Doug Gregor was the
> only person I was able to convince. C'est la vie. It's also why Proto broke.
> 
> Proto doesn't *need* the old behavior, strictly speaking. But the part
> of proto that builds expressions, proto::make_expr, was designed around
> the old semantics. See <http://bit.ly/1oUbgZr>.
> proto::result_of::make_expr is a metafunction for computing the result
> of the proto::make_expr function. It interprets reference types as the
> types of things to be held by reference, and non-reference types to be
> held by value. When calling proto::make_expr at run-time, each argument
> to be held by reference must be wrapped in boost::ref. It uses the
> generic-friendly invariant of boost::ref that Peter references above.
> 
> I've already hacked Proto and it's tests to accommodate the change, and
> the tests are turning green again, so it's not a huge deal. But end user
> code will have to change. Xpressive didn't break, and it looks like
> Spirit didn't break, so the scope of the breakage might be small. Hard
> to say.

Thank you Eric.

Am I right that the user's code that uses proto::result_of::make_expr (or any 
other Proto metafunction for result type computation) won't be broken? I'm 
omitting for now the possibility that the user's code could use the reference 
wrapper like in the Peter's code sample. From your changes to Proto tests I 
could not see what kind of changes would be required on the user's side.