"The garbage col­lec­tor is not guar­an­teed to run the de­struc­tor for all un­ref­er­enced ob­jects. Fur­ther­more, the order in which the garbage col­lec­tor calls de­struc­tors for un­ref­er­ence ob­jects is not spec­i­fied. This means that when the garbage col­lec­tor calls a de­struc­tor for an ob­ject of a class that has mem­bers that are ref­er­ences to garbage col­lected ob­jects, those ref­er­ences may no longer be valid. This means that de­struc­tors can­not ref­er­ence sub ob­jects."

Now I have a class A and it refers to subobject B. But B may or may not have other objects referring to it. As a result, an object of A is GCed, but subobject B may or may not be getting garbage collected.