swift - immutable of type only has mutating members -

following code throwing error "immutable value of type [user] has mutating members named append" please advise.

library or framework code (precompiled)

class database {     var data: any? //can change code has generic placeholder      init(data: any?=nil) {         self.data = data     } } 

users of library or framework code

class user {     var first: string     var last: string      init(first: string, last:string) {         self.first = first         self.last = last     } }  class test: database {      init() {         super.init(data: [user]())     }      func additem(item: user) { //edit user type problem persists         (self.data as! [user]).append(item) //error     } } 

attempt 1, seems working still wonder why above code doesn't work.

func additem(item: user) {     var users = self.data as! [user]     users.append(item);     self.data = users;  }  

don't re-assignment, how can in place mutability possible

i've written (what believe be) elegant solution problem.

public struct accessor<t, u> {     public let (getter: (t -> u), setter: (u -> t))      public init()     {         (getter, setter) = ({ $0 as! u }, { $0 as! t })     }      public init(_ getter: (t -> u), _ setter: (u -> t))     {         self.getter = getter         self.setter = setter     } }  extension accessor: nilliteralconvertible {     public init(nilliteral: ())     {         self.init()     } }  public class mediator<t, u> {     public var rawvalue: unsafemutablepointer<t>     public var accessor: accessor<t, u>      public init(_ rawvalue: unsafemutablepointer<t>, _ accessor: accessor<t, u> = nil)     {         self.rawvalue = rawvalue         self.accessor = accessor<t, u>({ $0 as! u }, { $0 as! t })     }      public var value: u     {                 {             return accessor.getter(rawvalue.memory)         }          set         {             rawvalue.memory = accessor.setter(newvalue)         }     } } 

which allows for:

mediator<any?, [user]>(&self.data).value.append(item)