Re: [boost] [optional] How to define boost::none?

Subject: Re: [boost] [optional] How to define boost::none?
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2014-11-19 10:58:40

2014-11-19 16:43 GMT+01:00 Peter Dimov <lists_at_[hidden]>:

> Andrzej Krzemienski wrote:
>
>> 2014-11-19 16:28 GMT+01:00 Krzysztof Czainski <1czajnik_at_[hidden]>:
>> > And then, why wouldn't a straightforward solution work:
>> >
>> > struct none_t {};
>> >
>> > none_t const none = {}
>>
>> I think it validates ODR, or doesn't it?
>>
>
> It does violate ODR at places (without any ill effects AFAIK). Bind's
> placeholders have the same problem.

Hmm, looks like an option (although technically this is an UB)

> One option is to use
>
>    struct none_rt {};
>
>    typedef none_rt (*none_t)();
>
>    inline none_rt none() { return none_rt(); }
>

Not an option. According to GCC, the following compiles:

void d(none_t) {}
int main() { d(0); }

And this is exactly what I try to avoid.

>
> Another is to declare none as constexpr:
>
>    struct none_t {};
>
>    constexpr none_t none = {};

Not an option: requires C++11