sob., 2 mar 2019 o 13:18 Andrzej Krzemienski <akrzemi1@gmail.com> napisał(a):
Hi Peter, Given that the never-empty guarantee appears to be at the heart of the motivation for boost::variant2, I would suggest to: 1. put in the documentation some motivation behind pursuing the never-empty guarantee. 2. Make it clear that never-empty guarantee is not a strong exception-safety guarantee. 3. Describe the algorithm for achieving the guarantee.
Somewhat related to that. When I try to study the documentation to see what the assignment guarantees, I get this: constexpr variant& operator=( const variant& r ) noexcept( mp_all<std::is_nothrow_copy_constructible<T>..., std::is_nothrow_copy_assignable<T>...>::value ); - Let j be r.index(). Effects: - If index() == j, assigns the value contained in r to the value contained in *this. - Otherwise, equivalent to emplace<j>(get<j>(r)). I am interested in the latter ("Otherwise") case, so, going to emplace: template<size_t I, class... A> constexpr variant_alternative_t<I, variant<T...>>& emplace( A&&... a ); - Requires: I < sizeof(T…). Effects: Destroys the currently contained value, then initializes a new contained value as if using the expression Ti(std::forward<A>(a)…). This doesn't mention any double buffering. No tricks, no "monostate". It reads as if the variant is left with no contained object whatsoever. Regards, Andrzej