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How do you implement an efficient and thread safe reference counting system on X86 CPUs in the C++ programming language?
I always run into the problem that the critical operations not atomic, and the available X86 Interlock operations are not sufficient for implementing the ref counting system.
The following article covers this topic, but requires special CPU instructions:
http://www.ddj.com/architect/184401888
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In computer science, reference counting is a programming technique of storing the number of references, pointers, or handles to a resource, such as an object, a block of memory, disk space, and others. In garbage collection algorithms, reference counts may be used to deallocate objects that are no longer needed.
Reference counting is one such technique. This method is simply keeping an extra counter along with each object that is created. The counter is the number of references that exist to the object, in the C/C++ case this would how many pointers refer to this object.
Thread safety A threadsafe function protects shared resources from concurrent access by locks. Thread safety concerns only the implementation of a function and does not affect its external interface. In C language, local variables are dynamically allocated on the stack.
It is a reference counting ownership model i.e. it maintains the reference count of its contained pointer in cooperation with all copies of the std::shared_ptr. So, the counter is incremented each time a new pointer points to the resource and decremented when destructor of the object is called.
Nowadays, you can use the Boost/TR1 shared_ptr<> smart pointer to keep your reference counted references.
Works great; no fuss, no muss. The shared_ptr<> class takes care of all the locking needed on the refcount.
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