Date: Tue, 27 Mar 2012 13:00:19 -0700 From: eric@boostpro.com To: daniel.j.walker@gmail.com CC: boost@lists.boost.org Subject: Re: [boost] [result_of] Make `cpp0x_result_of_impl` public
On 3/27/2012 12:48 PM, Daniel Walker wrote:
On Mar 27, 2012, at 2:53 PM, Eric Niebler wrote:
On 3/27/2012 11:49 AM, Daniel Walker wrote:
On Mar 27, 2012, at 1:35 PM, Eric Niebler wrote:
On 3/24/2012 1:52 AM, Michel Morin wrote:
There are two implementations of boost::result_of: a TR1-style implementation and a decltype-based implementation. While the TR1-style implementation has a public interface `boost::tr1_result_of`, decltype-based one doesn't have a public interface.
Yes, I'm the one responsible for this change.
By defining BOOST_RESULT_OF_USE_DECLTYPE, boost::result_of use decltype-based implementation. But this is not always a viable solution, since this breaks some Boost libraries.
So how about adding `boost::cxx11_result_of` as public interface of the decltype-based implementation? Attached a patch to add `boost::cxx11_result_of`. (This patch also changes the name of `cpp0x_result_of_impl` to `cxx11_result_of_impl` to reflect the recent discussion on the cpp/cxx naming.)
The patch looks fine, and I guess I'm as qualified to apply it as anybody. But it doesn't have docs and tests. Care to address that? The docs probably only need a line or two, and you can copy the tests for tr1_result_of.
I'm not sure that I agree that cxx11_result_of is a good idea. The plan was for boost::result_of to become a C++11 result_of as soon as we're comfortable flicking the switch so that it's on by default (on platforms that can support it). Michel, do you just want a C++11 result_of that works out-of-the-box or do you really need a separate interface in addition to boost::result_of?
There are places where a decltype-based result_of is safe, even if N3256 isn't implemented. In that case, cxx11_result_of would be the only option, since boost::result_of would still defer to tr1_result_of.
I would prefer, rather than fracturing the API, that we provide decltype-based boost::result_of by default on compilers that provide a reasonable decltype implementation, even if it's not fully N3256 compliant, with a well-documented caveat that boost::result_of depends on the compiler's decltype. For those who would rather have TR1 result_of than a result_of using non-N3256 decltype, they can use the existing tr1::result_of or boost::tr1_result_of.
This will badly and needlessly break valid code both within boost and in the wild for a large segment of Boost's users. Why would you prefer doing that than taking the safer course?
Why not implement boost::result_of using decltype only on compilers that have N3256 decltype, and give users with compilers that have non-N3256 decltype the option of turning on BOOST_RESULT_OF_USE_DECLTYPE? Regards, Nate