swift2 - Array mutability in Swift -

according apple's swift guide:

if create array, set, or dictionary, , assign variable, collection created mutable. means can change (or mutate) collection after created adding, removing, or changing items in collection. if assign array, set, or dictionary constant, collection immutable, , size , contents cannot changed.

but in xcode 7.2.1, these results:

import foundation  //**added per comments**  var data: [int] = [10, 20] print(unsafeaddressof(data))  data.append(30)  print(data) print(unsafeaddressof(data))  --output:-- 0x00007ff9a3e176a0 [10, 20, 30] 0x00007ff9a3e1d310 

because assigned array var, expected see same address data after appending value data.

another example:

class item { }  var data: [item] = [item(), item()] print(unsafeaddressof(data))  data.append(item())  print(data) print(unsafeaddressof(data))  --output:-- 0x00007f86a941b090 [item, item, item] 0x00007f86a961f4c0 

and another:

var data: [string] = ["a", "b"] print(unsafeaddressof(data))  data[0] = "a"  print(data) print(unsafeaddressof(data))  --output:-- 0x00007faa6b624690 ["a", "b"] 0x00007faa6b704840 

first of all, example compiles if has import foundation @ top. because without array structure not conform anyobject protocol used in unsafeaddressof(object: anyobject) signature. read more here:

instances of swift string structure type cannot represented anyobject type, because anyobject represents instances of class type. however, when bridging foundation enabled, swift string values can assigned constants , variables of anyobject type bridged instances of nsstring class.

that's true arrays (array bridged nsarray), sets (set bridged nsset), dictionaries (dictionary bridged nsdictionary) , numbers (int, uint, float, double, bool bridged nsnumber).

so every time call unsafeaddressof(data) data structure implicitly bridged unique nsarray object , because of see different address each time. if not mutate structure between calls unsafeaddressof() bridged nsarray object can same. think that's because of optimization.

try code in playground:

import foundation  func printdata(data: anyobject) {     //print(data) // uncomment see data value     print(unsafeaddressof(data)) }  var data = [10, 20]  printdata(data)  data.append(30)  printdata(data)  var nsarray = data nsarray  printdata(nsarray) 

run several times , you'll see 2 last printed addresses same , it's not.

in general question not relate mutability of value types. see happens because swift value types bridged foundation reference types.