Re: [boost] compact_optional -- prompting interest

Subject: Re: [boost] compact_optional -- prompting interest
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-09-27 18:07:52

On 26/09/2015 04:17, Andrey Semashev wrote:
> On 25.09.2015 18:55, Marcel Raad wrote:
>> Andrey Semashev-2 wrote
>>> 1. You chose not to provide relational operators for compact_optional
>>> because you don't know how to order 'empty' values. I think you don't
>>> have to make that decision and simply forward the call to the underlying
>>> type. I mean, you always have the stored object constructed in some
>>> state and as long as it implements operators you can always use them.
>>
>> This is exactly the problem I sometimes have with Boost.Optional: in some
>> use cases, I want the compiler to remind me that my special value is
>> special and always has to be checked explicitly instead of assuming it's
>> always the lowest possible value. That doesn't apply to operator== and
>> operator!=, of course.
>
> In my practice most of the time you just want some kind of ordering
> (e.g. for binary lookup). For these cases it's useful for ordering to
> just work out of the box. When the actual order matters you can always
> provide your own operator overloads - something you will have to do
> either way.

Isn't the solution there to specialize std::less and/or std::hash 
instead of providing operators?  (I vaguely recall there being a nicer 
way to do this proposed and/or implemented in one of the newer 
standards, but I don't recall the details.)