Re: MT Design Question

Chris M. Thomasson wrote:
 > ________________________________________________________
 > struct complete
 > {
 > std::condition_variable m_cond;
 > std::mutex m_mutex;
 > };
 >
 >
 >
 >
 > template<typename T>
 > struct promise_ex
 > {
 > std::promise<T> m_promise;
 > complete& m_complete;
 >
 >
 > void set_value(T v)
 > {
 > m_complete.m_mutex.lock();
 > m_promise.set_value(v);
 > m_complete.m_mutex.unlock();
 > m_complete.m_cond.notify_one();
 > }
 > };
 >
 >
 >
 >
 > template<typename T>
 > struct multi_wait
 > {
 > complete m_complete;
 > linked_list<std::future<T>*> m_list;
 >
 >
 > void push_future(std::future<T>* f)
 > {
 > m_list.push(f);
 > }
 >
 >
 > std::future<T>* wait()
 > {
 > std::future<T>* result = NULL;
 >
 > m_complete.m_mutex.lock();
 >
 > while (! m_list.is_empty())
 > {
 > for each std::future<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;
 > }
 > };

As I noted at the outset, I'm no MT expert, so my judgment is suspect, but this
looks like it would work. I like that I can wait for the first result to come
back, and if I don't like it (e.g., because it's an exception), I can easily
continue waiting on the rest of the worker threads. Once I have a result I
like, there a handy list of futures that tells me which workers need to be told
to stop working. (There's no API for that in C++0x, but Chris's solution
augments the promise API, so it's not surprising that we'd have to augment
(i.e., build on) the standard future API, too.)

Scott

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