Herb Sutter's favorite C++ 10-liner has a memory management bug

In a recently-posted video, Herb Sutter (a prominent C++ expert) describes his favorite C++ 10-liner as “a thread-safe reference-counted object cache”:

shared_ptr<widget> get_widget(int id) {
  static map<int, weak_ptr<widget>> cache;
  static mutex m;

  lock_guard<mutex> hold(m);
  auto sp = cache[id].lock();
  if (!sp) cache[id] = sp = load_widget(id);
  return sp;
}

This example is very interesting. Firstly, it manages to pull in reference counting, weak references and a mutex which are all very rare in modern programming. Secondly, it contains a memory leak that is difficult to fix in C++ because APIs are burdened with memory management details and this API is incapable of expressing deterministic cleanup because there is no facility for a widget's destructor to remove its entry in the map. Finally, the correct name for this data structure is a concurrent weak dictionary, specifically one with weak values. You'll find correct implementations of this data structure are widely available for C#, F# and Java such as the one here.

The obvious fix is to sweep stale entries from the map when get_widget is called but this leaves floating garbage in the map between calls to get_widget, is asymptotically less efficient and incurs unbounded pauses for an unbounded number of threads.

Update: Matthew Avery (from the USA) suggests altering the API and semantics of the functions involved so load_widget returns a shared_ptr with a custom deleter that removes the stale map entry as soon as a widget is destructed. If this idea can be made to work then it would be the only deterministic solution to have been proposed to date.

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