Re: [boost] [outcome] High level summary of review feedback accepted so far

Le 31/05/2017 à 12:55, Niall Douglas via Boost a écrit :

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Returning a pointer actually only superficially looks like you are protecting yourself from the narrow contract. You just force the caller to use narrow contract function on the pointer. This reminds me that at some point people here in Boost discussed that get<T>() accessor on variant should return optional. What the C++ 17 standard eventually did was to add a std::get_if<I>(&variant) which returns a pointer to the value if the variant is in that state. std::get<I>(variant) throws an exception if you don't use the right state. Hrr, I believe that it had a narrow contract. We have holds_alternative and index. When you know this you don't need a wide contract. I'll ask in std-discussion why we don't have a narrow access. Is it because we want to favor visit? I guess, ultimately, the question to be answered here is how much like std::variant<...> should Outcome and/or Expected be? std::variant<...> doesn't implement completely unchecked observers like optional's operator*(), the best you can get is a nulled pointer observer, no silent reinterpret_cast.

When I started the Expected proposal Variant was not yet there. I believe that we could add the following from variant: * visit * index * get_if/value_if * get If we reach to have a SumType [1] we could as well define * sum_type::size and * sum_type::alternative, Vicente [1] https://github.com/viboes/std-make/tree/master/include/experimental/fundamen...