As we all know, Common-Lisp relies on garbage collection for deleting unreachable objects. However, it makes no provision for the equivalent of a C++ Destructor function that should be called when the object is eliminated by the garbage collector. The equivalent of such methods in a garbage collected environment is normally called a finalizer.
ECL includes a simple implementation of finalizers which makes the following promises.
The finalizer can be any lisp function, let it be compiled or interpreter.
Finalizers are not invoked during garbage collection. Instead, if an unreachable object is found to have an associated finalizer, it is pushed into a list and before the next garbage collection cycle, the finalizer will be invoked.
If the finalizer is invoked and it makes the object reachable, for instance, by assigning it to a variable, it will not be destroyed, but it will have no longer a finalizer associated to it.
ECL will strive to call finalizers before the environment is closed and the program is finished, but this mechanism may fail when exiting in a non ordinary way.