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Local and top-level lateinit vars

  • Type: Design proposal
  • Author: Dmitry Petrov
  • Contributors: Alexander Udalov
  • Status: Implemented in 1.2
  • Prototype: Implemented


Problem description

Classes (including objects and enum entries) can contain lateinit properties, which provide postponed initialization semantics. If a property is read before it is initialized, an exception is thrown.

Similar functionality is useful not only for class properties, but also for top-level properties and local variables.

In case of local variables, it also allows initializing local variables in local functions and lambdas (which works as a makeshift replacement for an effect system):

fun foo() {
    lateinit var x: Bar
    synchronized { 
        x = bar()
    // ...

Design details

All relevant restrictions related to lateinit class member properties apply. lateinit modifier is not applicable to:

  • read-only properties (vals)
  • properties of nullable types (including generic types with nullable upper bound)
  • properties of primitive types
  • delegated properties
  • properties with initializer

On JVM, lateinit top-level properties expose their backing field with the same visibility as a property setter.

In case of lateinit local vars, optimizing compiler can eliminate redundant initialization checks (as a part of redundant null check elimination pass).