Re: [boost] [result_of] Make `cpp0x_result_of_impl` public

Subject: Re: [boost] [result_of] Make `cpp0x_result_of_impl` public
From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2012-03-27 15:48:51

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.

- Daniel