Re: [boost] [outcome] Possible extensions/changes to std::experimental::expected

2017-05-25 23:57 GMT+02:00 Peter Dimov via Boost :

Andrzej Krzemienski wrote:

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2017-05-25 23:34 GMT+02:00 Peter Dimov via Boost :

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Andrzej Krzemienski wrote:

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Instead, rewrite observer functions like this:

bool has_value() const { if (BOOST_UNLIKELY(_is_in_empty_state())) __builtin_unreachable();

Please no. This is horrible. Actually it's doubly horrible; merely > horrible would be putting the above in value(). In an observer > function... it's just evil.

Can you explain?

What is there to explain? This makes calling has_value undefined if empty ("narrow contract"), which makes the user responsible of checking empty() before doing ANYTHING with a result/outcome object. This applies to each and every result/outcome object you receive from somewhere.

auto r = function(); if( r.empty() { /* always have to do this*/ } else if( r.has_value() ) { /* otherwise UB here */ }

As I said, I don't like undefined behavior for its own sake, and I most certainly don't like putting a "Requires: not empty" contract on every function, including observers.

Putting a contract is not even a good fit for value(), because it already checks and throws, to enable

auto r = function().value();

where you go from noexcept land to exception land. If UB on empty, goodbye one liner, you still need to save the outcome and check for empty first, before retrieving value().

Yes, it would be wrong if the message was, "whoever gets an `expected` object needs to check for the empty state". The ideal solution would be to put this responsibility on someone who produces the `expected` object. Also, they way I look at this solution is not "when I get this value I have to check ...", but "when I produce this value I have to make sure...". Is there no way to acheive this in the language? Regards, &rzej;