Default implementation of Hashable in Swift 4.0

Question

Created Sep ’18

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In Swift 4.0 structs whose stored properties are all

Hashable

compiler provides the synthesized implementation of `hashValue`. How does it implemented? What hash function it uses? Where can I find this information by myself (I know that Swift is open-source)?

Answered by kelin in 330044022

Ok, so the answer I got there (https://forums.swift.org/t/how-is-synthesized-hashable-implemented/15960?u=kelin) is: SipHash-1-3 for Swift 4.2. I asked this question becasue I wanted to make my own hash implementation and I think using the same function as the default one would be a good solution.

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Answer 1

DTS Engineer OP

Apple

Sep ’18

I think you're referring to the implementation of this proposal:

github.com/apple/swift-evolution/blob/master/proposals/0185-synthesize-equatable-hashable.md

which says it was implemented in 4.1.

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Answer 2

kelin OP

Sep ’18

Oh, yes, sure. Didn't notice how Xcode was updated to Swift 4.1.

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Answer 3

DTS Engineer OP

Apple

Sep ’18

Note that Hashable conformance synthesis changed dramatically in Swift 4.2:

github.com/apple/swift-evolution/blob/master/proposals/0206-hashable-enhancements.md

I see that you asked about the implementation over on forums.swift.org. That's good, but remember that you likely won't get an answer about the 4.1 implementation, if that's what you were looking for specifically. (The old "hashValue" requirement is deprecated in 4.2.)

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Answer 4

kelin OP

Sep ’18

Accepted Answer

Ok, so the answer I got there (https://forums.swift.org/t/how-is-synthesized-hashable-implemented/15960?u=kelin) is: SipHash-1-3 for Swift 4.2. I asked this question becasue I wanted to make my own hash implementation and I think using the same function as the default one would be a good solution.

0