Why GC when you have reference counted smart pointers?

Reference counted smart pointers are a simple form of garbage collection usable from the C++ programming language. A recent question on Stack Exchange asks why anyone would want anything more when reference counted smart pointers are already available.

Other forms of garbage collection (most notably tracing GCs) have several advantages over reference counting:

  • Accuracy: Reference counting alone leaks cycles so reference counted smart pointers will leak memory in general unless other techniques are added to catch cycles. Once those techniques are added, reference counting's benefit of simplicity has vanished.

  • Throughput: Smart pointers are one of the least efficient forms of garbage collection, particularly in the context of multi-threaded applications when reference counts are bumped atomically. There are advanced reference counting techniques designed to alleviate this but tracing GCs are still the algorithm of choice in production environments.

  • Latency: Typical smart pointer implementations allow destructors to avalanche, resulting in unbounded pause times. Other forms of garbage collection are much more incremental and can even be real time, e.g. Baker's treadmill.

Many of the answers given perpetuate myths about garbage collection. There is a myth that scope-based reference counting guarantees that values are collected as soon as possible. In fact, tracing collectors can and do collect values before the end of their lexical scope if the value becomes unreachable sooner and a GC occurs. Another myth is that garbage collected languages cannot release resources deterministically. In fact, this is done in exactly the same way as in unmanaged languages. Finally, there is a myth that manual memory management minimizes latency. In fact, manual memory management often has poorer worst-case latency characteristics than garbage collection (this problem originally drove us from C++ to OCaml!) and optimizing latency in an unmanaged language is seriously hard work.

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