from: Herb Sutter's - Going Native 2013

get_widget.cpp

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

    std::lock_guard<std::mutex> hold(m);
    auto& wp = cache[id];
    auto sp = wp.lock();
    if (!sp)
    {
      wp = sp = load_widget(id);
    }

    return sp;
}