2014-11-19 16:28 GMT+01:00 Krzysztof Czainski <1czajnik@gmail.com>:
2014-11-19 16:12 GMT+01:00 Andrzej Krzemienski <akrzemi1@gmail.com>:
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 = {}
I think it validates ODR, or doesn't it?