Understanding the GC

[Jeremy DeHaan]

I've been reading TDPL book and was also reading some posts on 
these forums about the GC, but I wanted to clarify a few things 
to make sure I am understanding correctly.

 From what I understand, when an object is recovered by the GC, 
the destructor may or may not be called. Why is that? Is it for 
performace reasons? What about the destructors of objects that 
the original object contained? Are they called when the item 
finally get's taken care of by the GC, or when the object is 
originally recovered by the GC?

My other question is, at the end of the program will GC go 
through every object it knows still holds memory and call their 
destructors? My guess is that it does so that the GC ensures all 
memory is successfully released before the program ends. I just 
want to make sure since in http://dlang.org/class.html, it says, 
"The garbage collector is not guaranteed to run the destructor 
for all unreferenced objects." What exactly is an unreferenced 
object?

Some other questions:

At what point is the GC called to do it's thing while the program 
is running? Is it at certain intervals or is it when the 
program's memory reaches a certain point?



Thanks very much for all the help!