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