Destructors, finalizers, and synchronization

Abstract

We compare two different facilities for running cleanup actions for objects that are about to reach the end of their life.Destructors, such as we find in C++, are invoked synchronously when an object goes out of scope. They make it easier to implement cleanup actions for objects of well-known lifetime, especially in the presence of exceptions.Languages like Java[8], Modula-3[12], and C\#[6] provide a different kind of "finalization" facility: Cleanup methods may be run when the garbage collector discovers a heap object to be otherwise inaccessible. Unlike C++ destructors, such methods run in a separate thread at some much less well-defined time.Languages like Java[8], Modula-3[12], and C\#[6] provide a different kind of "finalization" facility: Cleanup methods may be run when the garbage collector discovers a heap object to be otherwise inaccessible. Unlike C++ destructors, such methods run in a separate thread at some much less well-defined time.We argue that these are fundamentally different, and potentially complementary, language facilities. We also try to resolve some common misunderstandings about finalization in the process. In particular: