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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 20:54:32

On Mar 27, 2012, at 8:28 PM, Eric Niebler wrote:

> On 3/27/2012 4:13 PM, Daniel Walker wrote:
>> 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.
> I should say that Nate's suggestion *is* reasonable, but (IIUC) he's not
> suggesting turning on decltype-based result_of for non-N3256 compilers,
> as you are (right?). His suggestion is pretty much The Plan and has been
> all along. He's merely questioning the need for an additional
> cxx11_result_of. I think it has value for people with who want to
> selectively use decltype-based result_of in their code where they know
> it is safe, regardless of whether a compiler implements N3256.

That's right. Nate pretty much restated the plan. In considering alternatives, I don't mean to suggest that we should actually veer from the plan. :-) It still seems the best course of action to enable it by default only when N3256 is available.

> But now that I rethink this, the use of cxx11_result_of anywhere
> necessarily makes such code non-portable to c++03 compilers, so it'll
> probably never be used within Boost. It could only be used (a) by
> Boost's users who (b) have compilers that support decltype and (c) don't
> support N3256 and (d) don't have a std::result_of that uses decltype.
> I'm pretty sure that's exactly zero people.
> So I'm flip-flopping. Nate's suggestion is correct.
> As for your suggestion that Boost libraries can use tr1_result_of where
> necessary, I agree. Proto already does this.

Good. OK, I think we're on the same page.

>>> 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.
> Again, I ask why? Why, when there is a less risky path forward? When we
> specifically added a feature test for compliance with N3256 for this
> very purpose?

It's really nice to have a feature test for this. Thanks!

While I think it would be acceptable to release a decltype-based result_of on non-N3256 compilers, assuming the impact on boost libraries was minimal and easily correctable, I don't think it's necessary. Hopefully, things are moving quickly enough on the compiler front that N3256 will be widely implement and this whole discussion will be moot.

- Daniel

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