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Swift

mutating and value types

Which line in this code fails to compile, and why?

struct Counter {
    var value = 0
    func increment() { value += 1 }
}
let c = Counter()
c.increment()
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See the short answer

A struct method treats self as immutable by default, so value += 1 errors; changing a property needs mutating, and then a let instance can't call it.

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