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Immutability in Kotlin/Native

Kotlin/Native implements strict mutability checks, ensuring the important invariant that the object is either immutable or accessible from the single thread at that moment in time (mutable XOR global).

Immutability is a runtime property in Kotlin/Native, and can be applied to an arbitrary object subgraph using the kotlin.native.concurrent.freeze function. It makes all the objects reachable from the given one immutable, such a transition is a one-way operation (i.e., objects cannot be unfrozen later). Some naturally immutable objects such as kotlin.String, kotlin.Int, and other primitive types, along with AtomicInt and AtomicReference are frozen by default. If a mutating operation is applied to a frozen object, an InvalidMutabilityException is thrown.

To achieve mutable XOR global invariant, all globally visible state (currently, object singletons and enums) are automatically frozen. If object freezing is not desired, a kotlin.native.ThreadLocal annotation can be used, which will make the object state thread local, and so, mutable (but the changed state is not visible to other threads).

Top level/global variables of non-primitive types are by default accessible in the main thread (i.e., the thread which initialized Kotlin/Native runtime first) only. Access from another thread will lead to an IncorrectDereferenceException being thrown. To make such variables accessible in other threads, you can use either the @ThreadLocal annotation, and mark the value thread local or @SharedImmutable, which will make the value frozen and accessible from other threads.

Class AtomicReference can be used to publish the changed frozen state to other threads, and so build patterns like shared caches.