# Are smart casts and null checking on global var thread safe?

**URL:** <https://discuss.kotlinlang.org/t/are-smart-casts-and-null-checking-on-global-var-thread-safe/1652>\
**Category:** Uncategorized\
**Created:** [May 6, 2016, 10:15am UTC](https://discuss.kotlinlang.org/t/are-smart-casts-and-null-checking-on-global-var-thread-safe/1652 "2016-05-06T10:15:43Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![sash](https://avatars.discourse-cdn.com/v4/letter/s/c6cbf5/32.png) [@sash](https://discuss.kotlinlang.org/u/sash)\
**Post date:** [May 6, 2016, 10:15am UTC](https://discuss.kotlinlang.org/t/are-smart-casts-and-null-checking-on-global-var-thread-safe/1652/1 "2016-05-06T10:15:43Z")

</div>

Let `a` is global variable of type `A` or field of current object of type `A`.

Is the following code thread safe?

```
if a is B { 
    a.methodOfB() 
}

```

and

```
if a != null { 
    a.methodOfA() 
}

```

`a` could be changed in other thread and these checks might not help.  
Does Kotlin save checked value of `a` to local variable?

In Swift it’s done slightly in other way and I know that this is thread safe in Swift:

```
if let a = a { // null check and assignment to local variable `a`
   a.methodOfA()
}

```

and

```
if let a = a as? B { // safe cast, check for null and assignment to local variable a. also it can be done with `where` construct, but it doesn't matter here
   a.methodOfB()
}

```

---

<div class="post-metadata">

**Author:** ![Dittert](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/dittert/32/1736_2.png) [@Dittert](https://discuss.kotlinlang.org/u/Dittert)\
**Post date:** [May 6, 2016, 2:18pm UTC](https://discuss.kotlinlang.org/t/are-smart-casts-and-null-checking-on-global-var-thread-safe/1652/2 "2016-05-06T14:18:27Z")

</div>

I believe they are:

```auto
var a: String? = null

fun useA() {
    if (a != null) {
        println(a.length)
                ^ Error:(8, 17) Kotlin: Smart cast to 'kotlin.String' is impossible, 
                  because 'a' is a mutable property that could have been 
                  changed by this time
    }
}

fun isA() {
    if (a is String) {
        println(a.length)
                ^ Error:(8, 17) Kotlin: Smart cast to 'kotlin.String' is impossible, 
                  because 'a' is a mutable property that could have been 
                  changed by this time
    }
}

```

The closest to what you want is probably this:

```auto
val ta = a
if (ta != null) {
    println(ta.length)
    // more statements working on ta
}

```

You can’t do it as compact as in your Swift example because you can’t declare variables in `if` statements. You also can’t use braces like this `(val ta = a)?.let { ... }`

---

<div class="post-metadata">

**Author:** ![paulblessing](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/paulblessing/32/1955_2.png) [@paulblessing](https://discuss.kotlinlang.org/u/paulblessing)\
**Post date:** [May 6, 2016, 4:00pm UTC](https://discuss.kotlinlang.org/t/are-smart-casts-and-null-checking-on-global-var-thread-safe/1652/3 "2016-05-06T16:00:13Z")

</div>

You can accomplish something very close to the Swift version in Kotlin. You can either re-use the name of the outer variable as the name of the inner scoped one, or you can rename it, or you can use the implicit `it` reference:

```
a?.let { a ->
    a.methodOfA()
}

a?.let { localA ->
    localA.methodOfA()
}

a?.let { 
    it.methodOfA()
}

(a as? B)?.let { a ->
    a.methodOfB()
}

(a as? B)?.let { b ->
    b.methodOfB()
}

(a as? B)?.let {
    it.methodOfB()
}

```

---

<div class="post-metadata">

**Author:** ![Dittert](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/dittert/32/1736_2.png) [@Dittert](https://discuss.kotlinlang.org/u/Dittert)\
**Post date:** [May 6, 2016, 4:10pm UTC](https://discuss.kotlinlang.org/t/are-smart-casts-and-null-checking-on-global-var-thread-safe/1652/4 "2016-05-06T16:10:27Z")

</div>

You’re right. I didn’t think of that.