Antony Polukhin wrote:
III: Make recursive_wrapper and variant cooperate, enable move for variant in the presence of recursive_wrappers only when there is at least one type that is nothrow-default-constructible, regardless of whether it's current.
That's not quite correct. Move for variant would be always enabled, it just won't result in a NULL reference_wrapper. For example, if a variant<int, string, reference_wrapper<string2>> contains int(5), it will continue to do so after move; and if it contains string("..."), it will hold string() after move. Only if it had a recursive_wrapper( string2("...") ) would it be int() after move, and if there was no suitable int in the list, it would either contain recursive_wrapper( string2("...") ), or recursive_wrapper( string2() ), depending on how the move constructor of recursive_wrapper is implemented. In addition, I'd argue that regardless of whether recursive_wrapper is made to be NULL after move, variant should still try to prevent exposing it to the user in the situations where there is an "int" in the list.
- user may be obscured by the fact, that v2 from the example now contains int
Users who are obscured by seeing an int() in v2 would be even more obscured by seeing a crash if v2 had a NULL recursive_wrapper instead.