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Subject: Re: [SG10] P0304: constexpr addressof
From: Richard Smith (richard_at_[hidden])
Date: 2016-03-13 20:16:05


On Fri, Mar 11, 2016 at 11:30 AM, Daniel Krügler
<daniel.kruegler_at_[hidden]> wrote:
> 2016-03-11 20:27 GMT+01:00 Agustín Bergé <agustinberge_at_[hidden]>:
>> On 3/11/2016 4:14 PM, Nelson, Clark wrote:
>>> Agustin raised the question of whether there should be a macro to
>>> indicate the presence of the fix for LWG2296. I made a couple of
>>> replies suggesting that, to me, the answer is not obviously "yes".
>>>
>>> So far, no one (not even Agustin) has made any further comment.
>>
>> I said that if there's no macro I'd prefer to leave the code broken.
>> That should have implied my motivation is weak. I think a macro fits the
>> bill, but if lacks in popularity I'm not going to push for it.
>
> I tentatively agree, I see no need for a feature macro here.

I can imagine a macro being useful:

struct optional {
  bool b;
  T t;
  constexpr T *get() {
    return b ?
#if __cpp_constexpr_addressof
      std::addressof(t)
#else
      &t
#endif
      : nullptr;
  }
};

The idea here is that it's more important that get() is constexpr than
that it works if there's an overloaded operator&, so addressof is only
used if it would actually work in a constant expression. Achieving
this result with SFINAE seems difficult.


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