Safe Initialization of Objects

Every newly created object goes through several initialization states: starting from a state where all fields are uninitialized until all of them are assigned. Any operation on the object during its initialization process, which usually happens in the constructor via \emph{this}, has to observe the initialization states of the object for correctness, i.e.~only initialized fields may be used. Checking safe usage of \emph{this} statically, without manual annotation of initialization states in the source code, is a challenge, due to aliasing and virtual method calls on \emph{this}. Mainstream languages either do not check initialization errors, like Java, C++, Scala, or they defend against them by not supporting useful initialization patterns, such as Swift. In parallel, past research has shown that safe initialization can be achieved for varying degrees of expressiveness but by sacrificing syntactic simplicity. We approach the problem by upholding \emph{local reasoning} about initialization which avoids whole-program analysis, and we achieve \emph{typestate polymorphism} via subtyping. On this basis, we put forward a novel type-and-effect system that can effectively ensure initialization safety while allowing flexible initialization patterns. We implement an initialization checker in the Scala 3 compiler and evaluate on several real-world projects.