The algorithms considered so far have all been indirect. Each has traced the graph of live objects from a set of known roots to identify all live objects. In this chapter, we consider the last class of fundamental algorithms, reference counting [Collins, 1960]. Rather than tracing reachable objects and then inferring that all unvisited objects must be garbage, reference counting operates directly on objects as references are created or destroyed.
Reference counting maintains a simple invariant: an object is presumed to be live if and only if the number of references to that object is greater than zero.1 Reference counting therefore associates a reference count with each object managed; typically this count is stored ...
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