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Comment by mFixman | Hacker News Reader

Is this the first type of sum-type option choosing statement present for C++ unions? I've been waiting for this feature since the year 1978.

Still, it's a wasted opportunity not to have a language-level overload to the `switch` statement that allows nice pattern matching. Even with std::is_within_lifetime C++ unions are prone to errors and hard to work with.

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Since C++17 there is std::variant

https://en.cppreference.com/w/cpp/utility/variant.html

These are not proper sum types, since you're limited to one variant per type.

I am not sure what you mean, you can definitely have e.g. an

    std::variant<int, std::string, bool>

Which is a sum of those three types.

See the first IpAddr example here[1], where you have separate variants, both with string representations. You can't do this with std::variant. You have to use separate types.

[1] https://doc.rust-lang.org/book/ch06-01-defining-an-enum.html

I see what you mean now, thanks. To reproduce that example with std::variant I would need some kind of strong type alias, which as far as I know is missing from C++; so the only feasible way to do that would be wrapping the string in another class or struct.

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