Re: MT Design Question

"Chris M. Thomasson" <cristom@charter.net> wrote in message
news:Jfndo.83475$lS1.55496@newsfe12.iad...


Something like this should be able to "work around the problem":
_______________________________________________________
struct complete
{
    std::condition_variable m_cond;
    std::mutex m_mutex;
};

// new data-structure...
template<typename T>
struct future_ex
{
    std::future<T> m_future;
    bool m_ready; // == false;

    bool is_ready() const
    {
        return m_ready;
    }
};

template<typename T>
struct promise_ex
{
    std::promise<T> m_promise;
    complete& m_complete;

/* new -> */ std::future_ex<T>& m_future_ex;

    void set_value(T v)
    {
        m_complete.m_mutex.lock();
        m_promise.set_value(v);

/* new -> */ m_future_ex.m_ready = true;

        m_complete.m_mutex.unlock();
        m_complete.m_cond.notify_one();
    }
};

// new ... Changed all `std::future<T>' to `future_ex<T>'
template<typename T>
struct multi_wait
{
    complete m_complete;
    linked_list<future_ex<T>*> m_list;

    void push_future(future_ex<T>* f)
    {
        m_list.push(f);
    }

    future_ex<T>* wait()
    {
        future_ex<T>* result = NULL;

        m_complete.m_mutex.lock();

        while (! m_list.is_empty())
        {
            for each future_ex<T>* in m_list as f
            {
                if (f->is_ready())
                {
                     result = f;
                     m_list.pop(f);
                     goto bailout;
                }
            }

            m_complete.m_cond.wait(m_complete.m_mutex);
        }

    bailout:
        m_complete.m_mutex.unlock();

        return result;
    }
};
_______________________________________________________

Humm... I now have to explicitly extend futures and promises in order for
this `multi_wait<>' scheme to work out. Scott, is there truly no way to poll
a `std::future<>' for completion such that a subsequent call to
`std::future<>::get' will be 100% guaranteed to contain either a result or
an exception? What is that `std::future<>::valid' call all about!?

Confused again!

;^(...