Re: [boost] [thread] A simply static initializeable mutex

From: Chris Thomasson (cristom_at_[hidden])
Date: 2006-10-30 23:18:21

"Roland Schwarz" <roland.schwarz_at_[hidden]> wrote in message 
news:45463A2F.8030209_at_chello.at...
> Chris Thomasson wrote:

[...]

> In this discussion we came to the conclusion that the problem
> with this scheme is not the construction, but the destruction.
>
> When will the destructor of this object being run?

Well, I guess you can do something like this: First of all, imagine I added 
a cas function to atomic::var...

 template<typename T>
  class once {
    // I forgot to make this a ptr in the pseudo-code! Argh...
    var<T*> m_state;

    // we can add a counter...
    var<intword_t> m_count;

  private:
    bool try_dec() throw() {
      intword_t local;
      do {
        local = m_count.load(mb::naked);
        if (! local || local < 1) { return false; }
        // use mb::fence to cover *both acquire and release
        // wrt the result of the decrement
      } while(! m_count.cas(local, local - 1, mb::fence));
      return (local == 1);
    }

    bool try_inc() throw() {
      intword_t local;
      do {
        local = m_count.load(mb::naked);
        if (! local || local < 1) { return false; }
      } while(! m_count.cas(local, local + 1, mb::acquire));
      return true;
    }

  public:
    once() throw() : m_state(0), m_count(1) {}

    ~once() throw() {
      if (try_dec()) {
        try { delete local; }
          catch(...) { assert(false); throw; }
      }
    }

  public:
    // atomic load / thread-saftey: strong
    T* load() const {
      T *local = m_state.load(mb::depends);
      if (! local) {
        externlib_api::hashed_mutex::guard_t const &lock(this);
        if (! try_inc()) { return 0; }
        local = m_state.load(mb::naked);
        if (! local) {
          // call try_dec on exceptions...
          local = new T;
          m_state.store(local, mb::release);
        }
      } else if(! try_inc()) {
        return 0;
      }

      return local;
    }

    // dec the refcount
    void dec() { if (try_dec()) { delete local; } }
  };

* : 
http://groups.google.com/group/comp.programming.threads/browse_thread/thread/a8792b05f35174d3

What do you think?