DREFs are handled via reference counting. Each time a DREF to an object is created, its reference count is incremented and the count is decremented when the reference is deleted. This is handled via the Dref class. When an object is instanciated, it is placed in an instance of Dref on the stack that maintains a reference count on the instance on the DREF loses scope:
USE_NS_RTGC
class A : public Void
{
}
main(int argc, char **argv)
{
initialize(argc, argv)
Dref a(new A);
}
If at any time all DREFs to an object are removed (the reference
count drops to zero), the object is reviewed.IREFs are handled by keeping a record of all referencing objects on the object being referenced via the Iref class. This allows the cycle (directed graph) of objects which reference a particular object to be constructed when an object is up for review. If none of the objects in this cycle has a DREF, the cycle is no longer referenced by the application and is freed:
USE_NS_RTGC
class A : public Void
{
Iref x;
A() : x(this, 0) {}
}
main(int argc, char **argv)
{
initialize(argc, argv)
pDref<A> a(new A);
a->x = new A;
}
Finaly, WREFs are weak references to an instance. A weak
reference can be used to access an instance but has no bearing on
when the instance is reclaimed by the system. In other words, as
long as there is some other reference to the pointer, the WREF can
be used to access it; however, once all other references are
removed, the WREF becomes invalid and will throw a
DanglingReference exception when accessed.
USE_NS_RTGC
class A : public Void
{
Iref x;
Wref w;
A() : x(this, 0) {}
}
main(int argc, char **argv)
{
initialize(argc, argv)
pDref<A> a(new A);
a->x = new A;
a->w = a->x;
}