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

Subject: Re: [boost] [core] Breaking change to boost::ref in 1.56
From: Eric Niebler (eniebler_at_[hidden])
Date: 2014-07-13 20:04:09

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.

\e