2017-06-02 17:42 GMT+02:00 Peter Dimov via Boost <boost@lists.boost.org>:
Robert Ramey wrote:
And wouldn't making these things immutable, eliminate a large part of the
whole controversy?
An immutable result/outcome would have no default constructor (because it's default-constructed only in cases you need to put the value into it later) which would eliminate the motivation for an empty state as something into which to default-construct... but that's only a small part of the whole controversy.
Niall's argument for empty state does not rest entirely on default construction. He argues that it makes Outcome a four-state variant instead of a tri-state one and having the fourth state is useful in certain scenarios when the function that returns it has a legitimate need to report four states.
That is, he wants to be able to express this:
enum class no_difference { _ }
expected<std::path, no_difference, std::error_code, std::exception_ptr> void return_first_difference( std::path const & p1, std::path const & p2 ) { // ... }
(or something along these lines, I may not have gotten the example entirely correct, but that's the spirit of it.)
There are different legitimate ways to draw the line; you could say three states are it, for more use expected<>,
This is a valid point. If you need this tricky fourth state once in 1000 use cases, you can afford to use std::variant instead and leave without the convenience of `outcome`. Regards, &rzej;