Smart cast to Type is impossible because variable is a mutable property that could have been changed by this time

Question

And the Kotlin newbie asks   quot why won t the following code compile  quot   var left  Node    null      fun show         if  left    null            queue add left     ERROR HERE           Smart cast to  Node  is impossible  because  left  is a mutable property that could have been changed by this time  I get that left is mutable variable  but I m explicitly checking left    null and left is of type Node so why can t it be smart-casted to that type  How can I fix this elegantly

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Do this   var left  Node    null  fun show          val left   left      if  left    null             queue add left     safe cast succeeds            Which seems to be the first option provided by the accepted answer  but that s what you re looking for

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For there to be a Smart Cast of the properties  the data type of the property must be the class that contains the method or behavior that you want to access and NOT that the property is of the type of the super class     e g on Android  Be   class MyVM   ViewModel         fun onClick          Solution   From  private lateinit var viewModel  ViewModel To  private lateinit var viewModel  MyVM   Usage   viewModel   ViewModelProvider this  MyVM  class java  viewModel onClick      GL

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The practical reason why this doesn t work is not related to threads  The point is that node left is effectively translated into node getLeft     This property getter might be defined as   val left get     if  Math random    lt  0 5  null else leftPtr   Therefore two calls might not return the same result

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How I would write it   var left  Node    null  fun show          val left   left    return      queue add left     no error because we return if it is null

User · Answer

Try using the not-null assertion operator     queue add left

User · Answer

Change var left  Node    null to lateinit var left  Node  Problem solved

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There is a fourth option in addition to the ones in mfulton26 s answer   By using the    operator it is possible to call methods as well as fields without dealing with let or using local variables   Some code for context   var factory  ServerSocketFactory   SSLServerSocketFactory getDefault    socket   factory createServerSocket port  socket close    smartcast impossible socket  close    Smartcast possible  And works when called   It works with methods  fields and all the other things I tried to get it to work   So in order to solve the issue  instead of having to use manual casts or using local variables  you can use    to call the methods   For reference  this was tested in Kotlin 1 1 4-3  but also tested in 1 1 51 and 1 1 60  There s no guarantee it works on other versions  it could be a new feature    Using the    operator can t be used in your case since it s a passed variable that s the problem  The Elvis operator can be used as an alternative  and it s probably the one that requires the least amount of code  Instead of using continue though  return could also be used   Using manual casting could also be an option  but this isn t null safe   queue add left as Node     Meaning if left has changed on a different thread  the program will crash

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Your most elegant solution must be   var left  Node    null  fun show         left  also           queue add  it             Then you don t have to define a new and unnecessary local variable  and you don t have any new assertions or casts  which are not DRY   Other scope functions could also work so choose your favourite

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Between execution of left    null and queue add left  another thread could have changed the value of left to null  To work around this you have several options  Here are some   Use a local variable with smart cast   val node   left  if  node    null         queue add node       Use a safe call such as one of the following   left  let   node - gt  queue add node     left  let   queue add it     left  let queue  add    Use the Elvis operator with return to return early from the enclosing function   queue add left    return   Note that break and continue can be used similarly for checks within loops

User · Answer

1  Also you can use lateinit If you sure do your initialization later on onCreate   or elsewhere   Use this  lateinit var left  Node   Instead of this  var left  Node    null   2  And there is other way that use    end of variable when you use it like this  queue add left       add

[kotlin] Smart cast to 'Type' is impossible, because 'variable' is a mutable property that could have been changed by this time

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