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

Subject: Re: [boost] [optional] How to define boost::none?
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2014-11-19 10:28:00

2014-11-19 16:12 GMT+01:00 Andrzej Krzemienski <akrzemi1_at_[hidden]>:

> Hi Everyone.
>
> I am puzzled with a tough development task. I wonder if anyone is able to
> find a good solution.
>
> I want to re-implement tag boost::none. The current implementation is using
> a pointer to member which allows a harmful conversion from literal 0.  Try
> this:
>
> boost::optional<boost::rational<int>> orat = 0;
> // this creates a disengaged optional.
>
> I want to rewrite the definition with the following goals in mind:
> 1. boost::none_t is not implicitly convertible from literal 0;
> 2. The solution works in C++98
> 3. The solution is implementable in a header-only library. (defining some
> member variable in a CPP file is not an option.
>
> The only solution I was able to find so far is this:
>
> [code]
> struct none_t
> {
>   enum creator_t {creator};
>   none_t(creator_t){}
> };
>
> namespace
> {
>   none_t none (none_t::creator);
> }
> [/code]
>
> It works, but I feel uncomfortable defining boost::none in every
> translation unit. Any better ideas are welcome.
>
> Regards,
> &rzej
>

Hi Andrzej,

Why do you need the creator_t? What's wrong with a user creating other
instances of none_t?

Did you forget to make none const?

And then, why wouldn't a straightforward solution work:

struct none_t {};

none_t const none = {}