You cannot do what you ask unless you track instance creation.  One way
to do it is to use a factory object as the only means of creating
instances of the class you're talking about, and keeping a record in the
factory object of the instances created.  If you do this then you may
want to employ weak references so that your records of instances do not
prevent the instances from being garbage collected.  It starts to get
complicated if you want to do it right.

On the other hand, it is almost always a flawed design that requires the
kind of accounting you describe.  Why do you think you want to do what
you ask?