Re: [boost] [config] BOOST_STATIC_CONSTEXPR in C++11 vs C++14

Subject: Re: [boost] [config] BOOST_STATIC_CONSTEXPR in C++11 vs C++14
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2017-08-22 16:53:12

On 08/22/17 19:18, Gary Furnish via Boost wrote:
> I'd take a look at making something like BOOST_CONSTEXPR_AND_CONST
> similarly to BOOST_CONSTEXPR_OR_CONST using BOOST_CXX14_CONSTEXPR.
> See http://www.boost.org/doc/libs/1_65_0/libs/config/doc/html/boost_config/boost_macro_reference.html
> But I'd name it something like BOOST_COMPILETIME_CONSTANT because
> BOOST_CONSTEXPR_AND_CONST would seem to imply always constexpr and
> const, which is not what you want.  Really though you want to get
> other people who are more knowledgeable then I am involved if you want
> to add a cross-library macro, so maybe make one for your use case to
> test it.

Gary, please, don't top post.

http://www.boost.org/community/policy.html#quoting

> On Tue, Aug 22, 2017 at 9:44 AM, Lakshay Garg <lakshayg373_at_[hidden]> wrote:
>> On 22 August 2017 at 20:12, Gary Furnish via Boost
>> <boost_at_[hidden]> wrote:
>>> On Tue, Aug 22, 2017 at 5:28 AM, Lakshay Garg via Boost
>>> <boost_at_[hidden]> wrote:
>>>> This conversation is in reference to
>>>> https://github.com/boostorg/math/pull/82 but I'm posting it here
>>>> because it might be of interest to some.
>>>>
>>>> On 22 August 2017 at 13:53, jzmaddock <notifications_at_[hidden]> wrote:
>>>>> Note that those changes don't actually make the functions constexpr - or
>>>>> indeed change the way the constants are initialized - in fact in C++14
>>>>> BOOST_STATIC_CONSTEXPR doesn't even imply the constants are "const
>>>>> (C++11 is different, and early implementations prevent you from writing
>>>>> "constexpr const" which is what C++14 permits).
>>>>
>>>> Since BOOST_STATIC_CONSTEXPR behaves differently in C++11 and 14,
>>>> would it be a good idea for boost.config to provide a macro for
>>>> handling the `static const var = ...;` kind of variable
>>>> initializations in a portable manner? Maybe something like
>>>> BOOST_COMPILETIME_CONSTANT?
>>>>
>>>> Such a macro might will be useful for declarations where we want the
>>>> variable to be a constant in all possible senses (compile-time,
>>>> run-time) and allow generating the most optimal code possible with the
>>>> language version, compiler being used in a portable manner.
>>>>
>>>> An example of the kind of constants I am talking about can be seen
>>>> here: https://github.com/boostorg/math/blob/develop/include/boost/math/special_functions/prime.hpp#L40
>>>>
>>>> Lakshay
>>>>
>>>>
>>> This seems like a reasonable idea.
>>
>> Since I am just getting started with boost, I don't have much idea on
>> how to go about doing this. It would be very helpful is people can
>> post in their suggestions/reference material so I can look at it and
>> write such a macro.