II: Set operand.p_ to NULL, add BOOST_ASSERT in get operations + good performance + provides noexcept guarantee for move constructor + optimization will be used even if varinat has no type with trivial default constructor + easy to implement - triggers an assert when user tries to reuse moved object - adds an empty state to the recursive_wrapper
My vote goes to II. - Using moved-from objects should rarely happen, if ever. - It is not even a breaking change since move from variants was not supported in the past. Moreover, I am explicitly against proposition III: - The state of the moved-from variant is not explicitly set by the programmer. I tend to prefer explicit to implicit. - The type held in the moved-from variant depends on the types in the variant, which could make it harder to use variants in generic code. - It is simpler to have a single, well-documented behavior. It introduces less complexity. Question: Would it be possible to assign to moved-from variant? If so, proposition III is equivalent to proposition II with the following, unless I am mistaken: typedef variant<int, recursive_wrapper<foo> > V; V v1(std::move(v2)); // using v2 now triggers an assertion // (except when trying to set its content) v2 = int(); // this is explicit and equivalent to III Just my 2 cents. Louis Dionne