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Subject: Re: [boost] [result_of] Make `cpp0x_result_of_impl` public
From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2012-03-27 19:13:21

On Mar 27, 2012, at 4:55 PM, Eric Niebler wrote:

> On 3/27/2012 1:10 PM, Nathan Ridge wrote:
>>> Date: Tue, 27 Mar 2012 13:00:19 -0700
>>> From: eric_at_[hidden]
>>> To: daniel.j.walker_at_[hidden]
>>> CC: boost_at_[hidden]
>>> 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

I think Nate's suggestion is reasonable. Boost libraries which require N3256 can use tr1::result_of, so that they are more portable to non-standard compilers and do not break as easily when users request decltype-based boost::result_of, which, of course, is a perfectly reasonable request.

> Because BOOST_RESULT_OF_USE_DECLTYPE is a big hammer, and if someone
> uses that hammer with a non-N3256 compiler, there will be much
> collateral damage. Innocent bystanders. Think of the children.

boost::result_of can guarantee backwards compatibility with TR1 in situations where the TR1 result_of protocol was used to generate the type of the given call-expression. If the TR1 protocol was used to generate an incomplete type, then boost::result_of can't make that guarantee on non-N3256 compliant compilers. I think most users would find this completely acceptable.

- Daniel

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