On Wed, Nov 19, 2014 at 6:12 PM, Andrzej Krzemienski <akrzemi1@gmail.com> wrote:
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.
struct none_t {}; const none_t none = none_t(); Technically, this is dynamic initialization, but for our purpose it doesn't matter (there are no data members in none_t). 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(); const none_t& none = singleton< none_t >::instance;