有哪些 C++ 替代 static 对象可以使销毁更安全（或更具确定性）？

Question

I'm working on a large code base that, for performance reasons, limits access to one or more resources. A thread pool is a good analogy to my problem - we don't want everyone in the process spinning up their own threads, so a common pool with a producer/consumer job queue exists in an attempt to limit the number of threads running at any given time.

There isn't an elegant way to make ownership of the thread pool clear so, for all intents and purposes, it is a singleton. I speak better in code than in English, so here is an example:

class ThreadPool {
public:
  static void SubmitTask(Task&& t) { instance_.SubmitTask(std::move(t)); }

private:
  ~ThreadPool() {
    std::for_each(pool_.begin(), pool_.end(), [](auto &t) {
      if (t.joinable()) t.join();
    });
  }

private:
  std::array<std::thread, 5> pool_;
  static ThreadPool instance_; // here or anonymous namespace
};

The issue with this pattern is instance_ doesn't go out of scope until after main has returned which typically results in races or crashes. Also, keep in mind this is analogous to my problem so better ways to do something asynchronously isn't really what I'm after; just better ways to manage the lifecycle of static objects.

Alternatives I've thought of:

1. Provide an explicit Terminate function that must be called manually before leaving main.
2. Not using statics at all and leaving it up to the app to ensure only a single instance exists.
3. Not using statics at all and crashing the app if more than 1 instance is instantiated.

I also realize that a small, sharp, team could probably make the above code work just fine. However, this code lives within a large organization that has many developers of various skill levels contributing to it.

Answer 1

You could explicitly bind the lifetime to your main function. Either add a static shutdown() method to your ThreadPool that does any cleanup you need and call it at the end of main() .

Or fully bind the lifetime via RAII:

class ThreadPool {
public:
  static ThreadPool* get() { return instance_.get(); }

  void SubmitTask(Task&& t) { ... }
  ~ThreadPool() { ... }

private:
  ThreadPool() {}

  static inline std::unique_ptr<ThreadPool> instance_;

  friend class ThreadPoolScope;
};

class ThreadPoolScope  {
public:
  ThreadPoolScope(){
    assert(!ThreadPool::instance_);
    ThreadPool::instance_.reset(new ThreadPool());
  }

  ~ThreadPoolScope(){
    ThreadPool::instance_.reset();
  }
};

int main() {
  ThreadPoolScope thread_pool_scope{};
  ...
}

void some_func() {
  ThreadPool::get()->SubmitTask(...);
}

This makes destruction completely deterministic and if you do this with multiple objects, they are automatically destroyed in the correct order.