Re: [boost] [optional] using name boost::in_place -- need your opinion/advice

Subject: Re: [boost] [optional] using name boost::in_place -- need your opinion/advice
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2016-05-30 05:29:30

On Monday, 30 May 2016 12:13:04 MSK Andrzej Krzemienski wrote:
> Hi Everyone,
> I have stumbled upon a problem that I do not know how to address. I would
> like to seek an advice from the people in this list.
> 
> I am trying to make boost::optional resemble std::optional (where it is
> reasonably easy to achieve). std::optional has this nice and useful
> "placement" constructor:
> 
>   std::optional<std::mutex> m {std::in_place};
> 
> std::in_place is a tag type, that instructs std::optional to create an
> object using placement-new syntax, from the provided arguments (zero in the
> example), without incurring any move construction.
> 
> I cannot easily copy 1-to-1 this solution, because in namespace boost name
> in_place is reserved for the in-place factories:
> http://www.boost.org/doc/libs/1_61_0/libs/utility/in_place_factories.html

If not the typed in-place factories, you could transform in_place to be a namespace 
scope function object and use it as a tag in the optional constructor. The typed in-place 
factories ruin the solution though, even if not used by optional.

> I could simply go with a different name for a tag, but something tells me
> it would introduce an unnecessary complications for users that want to
> migrate from one optional to another. I could hack in-place factories so
> that they can also be used as a tag, although, I am not sure it is doable
> in a type-safe manner, and if it would not confuse people to have this
> boost::in_place do a number of different things.
> 
> I could use a different namespace, but again, this might look surprising.I
> am a bit torn.

I think that would be the best solution. Additionally, I would suggest moving the whole 
library to its own namespace (e.g. optionals). Then the tag in the library namespace 
would look more natural. For backward compatibility you could import the optional class 
template into boost namespace.