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

Subject: Re: [boost] [optional] How to define boost::none?
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-11-19 11:04:34

On Wed, Nov 19, 2014 at 6:56 PM, Andrzej Krzemienski <akrzemi1_at_[hidden]> wrote:
> 2014-11-19 16:40 GMT+01:00 Sylvester-Bradley, Gareth <
> Gareth.Sylvester-Bradley_at_[hidden]>:
>
>> Hi Andrey,
>>
>> On Wed, Nov 19, 2014 at 3:27 PM, Andrey Semashev wrote:
>> > Since none has internal linkage, it is translation unit-specific.
>> > Technically, this can cause ODR violations in some cases. To avoid
>> > that you can declare it like this:
>> >
>> >  struct none_t {};
>> >
>> >   template< typename T >
>> >   struct singleton
>> >   {
>> >     static const T instance;
>> >   };
>> >   template< typename T >
>> >   const T singleton< T >::instance = T();
>> >
>>
>>   // Do we need...
>>   namespace
>>   {
>>
>> >   const none_t& none = singleton< none_t >::instance;
>>
>>   }
>>   // ?
>>
>> Thanks,
>> Gareth
>>
>
> With anonymous namespace the variable becomes TU-local (translation
> unit-local), which guarantees no ODR violation. But instead repeats the
> same declaration in every TU.

The reference becomes TU-local, the singleton instance has external
linkage, so it's shared between TUs. I'm not sure if the reference
declared this way actually adds anything to the binary size.