"vicente.botet" <vicente.botet@wanadoo.fr> writes:
Currently when we set_wait_callback on a packaged_task, the packaged_task is given by reference to the callback function. This means that the packaged_task can not be moved.
As packaged_task is not CopyConstructible this limits quite a lot its use together with set_wait_callback. We need to pass it always by reference as functions parameters and can't be returned by a function.
I have tried to make a lazy_call function similar to spawn_task in the defined in N2709
template<typename F> std::unique_future<typename std::result_of<F()>::type> spawn_task(F f) { typedef typename std::result_of<F()>::type result_type; std::packaged_task<result_type()> task(std::move(f)); std::unique_future<result_type> res(task.get_future()); std::thread(std::move(task)); return res; }
Here it is
template<typename F> std::unique_future<typename std::result_of<F()>::type> lazy_call(F f) { typedef typename std::result_of<F()>::type result_type; std::packaged_task<result_type> task(std::move(f)); task.set_wait_callback(invoke_lazy_task<result_type>()) std::unique_future<result_type> res(task.get_future()); return res; }
but I need to return the packaged_task instead of the future because its lifetime must be longuer than the future->get() call. So I've tried to move the packaged_task which don't work either because the promise is broken
template<typename F> std::packaged_task<typename std::result_of<F()>::type> lazy_call(F f) { typedef typename std::result_of<F()>::type result_type; std::packaged_task<result_type> task(std::move(f)); task.set_wait_callback(invoke_lazy_task<result_type>()) return task; }
Why does this not work? Is it that the callback is broken (it gets passed the wrong task?) If so, that's the part that needs fixing, rather than the copyability of packaged_task.
What is the rationale for making packaged_task movable only? Why should not be CopyConstructible?
If packaged_task was copyable, people would copy it and expect the copy to be associated with the futures they obtained from the original. Not necessarily deliberately, but by mistake (e.g. copying into a vector rather than moving). This is not the case. Anthony -- Author of C++ Concurrency in Action | http://www.manning.com/williams just::thread C++0x thread library | http://www.stdthread.co.uk Just Software Solutions Ltd | http://www.justsoftwaresolutions.co.uk 15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976