Fixing the Kotlin Error: “Smart Cast to ‘Type’ is Impossible”

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Kotlin, a modern programming language that runs on the Java Virtual Machine (JVM), is praised for its safety features, conciseness, and interoperability with Java. One of its safety features is the smart cast system, which automatically casts types in certain contexts. However, developers might occasionally encounter the error: “Smart cast to ‘Type’ is impossible,” which can be a source of confusion and frustration. This article explores the root causes of this error and provides strategies for resolving it, ensuring your Kotlin code remains both safe and efficient.

Table of Contents

Understanding Smart Casts in Kotlin

Before diving into the error, it’s crucial to understand what smart casts are. In Kotlin, variables can be automatically cast to a more specific type safely. For example, if you check that a variable is of a certain type, Kotlin allows you to use it as that specific type without explicit casting:

if (obj is String) {
    // obj is automatically cast to String in this scope
    println(obj.length)
}

This feature simplifies code, making it more readable and less error-prone.

The Error: “Smart Cast to ‘Type’ is Impossible”

The error occurs when the Kotlin compiler cannot guarantee that a smart cast variable meets the conditions for the cast to be safe at all points where it’s used. This typically happens in two scenarios:

1. Mutable Properties That Can Change

If the property or variable you’re checking can be changed by another thread or within the current scope after the type check, Kotlin will not allow a smart cast:

var obj: Any = "Hello, Kotlin!"

if (obj is String) {
    println(obj.length) // Error: Smart cast to 'String' is impossible
}

In this case, because obj is mutable (var), its type could change after the check, making a smart cast unsafe.

2. Custom Getters for Properties

Properties with custom getters are considered potentially changing each time they’re accessed, thus smart casting is also deemed unsafe:

val obj: Any
    get() = "Hello, Kotlin!"

if (obj is String) {
    println(obj.length) // Error: Smart cast to 'String' is impossible
}

Even though obj is a val, the custom getter means its value could differ between calls, preventing a smart cast.

Solutions and Workarounds

Fixing the “Smart cast to ‘Type’ is impossible” error involves making the code in question safe for the compiler to automatically cast the variable. Here are some approaches:

1. Using Immutable Variables

Whenever possible, use immutable variables (val) instead of mutable ones (var). This change assures the compiler that the variable’s type cannot change after the initial check, allowing smart casts:

val obj: Any = "Hello, Kotlin!"

if (obj is String) {
    // Smart cast is possible here
    println(obj.length)
}

2. Explicit Casting

If you cannot avoid using a mutable variable or a variable with a custom getter, use explicit casting instead of relying on smart casting:

var obj: Any = "Hello, Kotlin!"

if (obj is String) {
    val strObj = obj as String
    println(strObj.length)
}

3. Local Variables

Copy the mutable property to a local val variable within the scope where you need the cast. This approach lets you leverage smart casting on the local immutable copy:

var obj: Any = "Hello, Kotlin!"

if (obj is String) {
    val localObj = obj
    // Smart cast works on localObj
    println(localObj.length)
}

4. When and Let Functions

The when expression or let function can often be used to perform a safe cast implicitly, especially useful in functional-style operations:

var obj: Any = "Hello, Kotlin!"

(obj as? String)?.let {
    // it is smart cast to String
    println(it.length)
}

Conclusion

While Kotlin’s smart casting feature significantly enhances code readability and safety, encountering the “Smart cast to ‘Type’ is impossible” error indicates situations where the compiler cannot guarantee the safety of such an operation. By understanding the root causes of this error and applying the solutions outlined above, developers can write more robust and maintainable Kotlin code. Whether through the use of immutable variables, explicit casting, leveraging local variables, or utilizing Kotlin’s functional programming features, several strategies exist to overcome this challenge and harness the full power of Kotlin’s type system.

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Anastasios Antoniadis

Anastasios Antoniadis is the founder and editor-in-chief of BORDERPOLAR... He is a software engineer, blogger, and avid gamer covering tech, gaming, and coding guides for over 4 years. He is a 2014 graduate of the Department of Informatics and Telecommunications of the University of Athens, an M.Sc. holder in Computer Science, and a Ph.D. student in Program Analysis.