5 ms·

So strings are mutable? I'm not sure that's a reason to celebrate.

• slavapestov 9y ago
  The example does not demonstrate mutability of strings; it is constructing a set of strings and inserting a string element. Only the set is being mutated. But yes, in Swift, arrays, dictionaries, sets and strings are all mutable. They are also value types, so mutation is a purely local effect.
  • xrisk 9y ago
    Would that not make passing around data expensive? Or are you expected to pass some kind of pointer instead?
    • slavapestov 9y ago
      Copy-on-write alleviates some of the overhead. Copies are only performed where required to preserve value semantics.
    • mantas 9y ago
      All of these can be both mutable or immutable. And converted as needed let array = [1, 2, 3] var mutableArray = [4, 5, 6] var anotherMutable = array Not sure how much optimisation is done under the hood, but Swift compiler loves to complain about mutable structures that are not ever edited.
    • masklinn 9y ago
      These are all copy-on-write boxed types with value semantics: the value you pass around is a pointer (with some additional data) and Swift has mechanisms/API which let the implementor copy the backing buffer on mutation if necessary: https://developer.apple.com/documentation/swift/2429905-isknownuniquelyreferenced https://developer.apple.com/documentation/swift/2429905-iskn...
  • fauigerzigerk 9y ago
    I think what the example is supposed to demonstrate is that champagne is a type of wine.
• mikeash 9y ago
  Strings (and arrays and sets and dictionaries) are neither mutable nor immutable in Swift, they're just values. Mutability is a property of where they're stored, not the types themselves. They work just like numbers do in most programming languages: you wouldn't say that Java's `int` is mutable or immutable, it just is. It may be stored in a mutable or immutable location, but the type itself doesn't have that property.