Re: [boost] Outcome/expected/etc/etc/etc

On Mon, Jun 5, 2017 at 7:57 AM, Peter Dimov via Boost wrote:

Gottlob Frege wrote:

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Some days I'm like "man just accept empty, it would be a simple API", but then I think "it is stupid for variant to be empty". Other days I think "just double buffer when necessary" (I assume that's your direction Peter?), but then I think "I don't want double-buffering variant to go on/off based on whether I use MS std vs libc++ etc" and also "I don't want double buffering for cases that only happen in theory, not in practice".

That is my direction, yes. My variant uses double storage when (1) not all types have noexcept move constructors and (2) there isn't a noexcept default constructible type in the list.

So converting a Circle to a Triangle might result in a Rectangle, because Rectangle is the fallback? I'd rather have double buffering or valueless_by_exception. ie correctness or acknowledgement of error. Not silent incorrectness.

I think that in practice few variants will hit the double case, as it's rare to have variant without a scalar alternative, although who knows.

For variant specifically, when going from libstdc++ to MS STL the difference is that sizeof(variant) changes, but it changes for std::variant, too. Obviously, double storage can never be as good as single storage, but I view this as an acceptable compromise. You can guarantee single storage by putting a nothrow default constructible type in the list of alternatives.

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- I think std::expected should always throw something deriving from std::exception. So if E derives from std::exception, we can throw it. If E is exception_ptr we can (re)throw it. If E is error_code/system_error/etc we can figure out what to throw. If E is user-type, then we wrap it in bad_expected or whatever.

My suggested expected<> just calls `throw_on_unexpected(e)` unqualified (it's a customization point.) There are overloads for std::error_code and std::exception_ptr and the fallback default is to throw bad_expected_access<E>.

Why not add "throw E if derived from std::exception" as part of that fallback? It is probably what users want.(?) Tony