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Re: Defect Report: packaged_task is too strongly typed

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On 2012-11-13 21:39, Andy Lutomirski wrote:
> On 11/04/2012 07:16 PM, Daniel Kr=FCgler wrote:
>>> This doesn't work well, though: the threadpool.submit() function would
>>> like to accept packaged_task<T> for any T -- the only packaged_task
>>> functionality that depends on the return type (as opposed to the
>>> parameter type) is get_future, and get_future can only be called once.
>>>
>>> This means that the submit function would need to be a template and use
>>> some unnecessarily complicated trick to deal with different
>>> packaged_task types.
>>
>> I'm unconvinced that what you are describing here really is a defect.
>>
>> It sounds as if you say that for similar reasons std::function
>> shouldn't be a template that depends on a function type. The current
>> design of packaged_task makes perfect sense, because a packaged_task
>> represents a type that wraps any function-like object (very similar to
>> std::function) and it also allows to call invoke this callable via a
>> corresponding set of functions:
>>
>> void operator()(ArgTypes... );
>> void make_ready_at_thread_exit(ArgTypes...);
>>
>> Obviously these depend on the remaining parts of the signature. So
>> this seems to invalidate your claims above.
>
> std::function, when called, returns the value that the wrapped object
> returns, so its operator()'s return type depends on the function's
> return type.
>
> packaged_task's call operator returns void.  It should be possible to
> invoke it without depending on the wrapped function's return type.

I certainly agree, but my arguments were related to the fact, that *all*
types mentioned in the packaged_task template signature are part of
public API and thus are obviously adequate. For some use-cases (like
those you describe), parts of this type information can be superfluous,
but that fact alone is not a reason for a defect, because you can easily
type-erase such occurrences.

>>> could have a member
>>> template<typename F>
>>> static pair<future<R>, packaged_task<void, ArgTypes...>>
>>> create_void(F&& f);
>>
>
> template<typename F>
> static pair<future<R>, packaged_task<void (ArgTypes...)>>
> create_void(F&& f);

I guess such an extension *could* be added, but I don't see how the lack
of these can be considered as a defect. If you think that such an
addition should be standardized, I suggest to create a proposal
following the descriptions of

http://isocpp.org/std/submit-a-proposal

to add such a feature.

> Here's a full example:
>
> #include <future>
> #include <utility>
>
> using namespace std;
>
> // SubmitToThreadpool does not (and cannot) care about
> // the return type of task.
> void SubmitToThreadpool(packaged_task<void ()> &&task)
> {
>    // In real code, this would involve threads.
>    task();
> }
>
> int main()
> {
>    packaged_task<int ()> task([]{ return 1; });
>    auto result = task.get_future();
>
>    SubmitToThreadpool(move(task));  // Does not compile
>
>    if (result.get() != 1)
>      abort();
>
>    return 0;
> }
>
> It will compile and work if you change void to int in
> SubmitToThreadpool.  (If using gcc on linux, you'll need to compile with
> -pthread or it blows up.  I'm not sure if this counts as a bug.)
> There should be a way to generate a paired future<int> and a
> packaged_task<void ()> (or some equivalent object).

I understand that this example does not compile, but I believe there are
simple adapter techniques to make this happen without changing the
library specification of packaged_task for the moment. If you think that
your interface extension (or other means that you have mentioned) should
be standardized, I suggest to submit a corresponding proposal as
described by

http://isocpp.org/std/submit-a-proposal

HTH & Greetings from Bremen,

Daniel Krügler



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Defect Report: packaged_task is too strongly typed Andy Lutomirski <luto@amacapital.net> - 2012-11-04 13:42 -0600
  Re: Defect Report: packaged_task is too strongly typed Daniel Krügler <daniel.kruegler@googlemail.com> - 2012-11-04 21:16 -0600
    Re: Defect Report: packaged_task is too strongly typed Andy Lutomirski <luto@amacapital.net> - 2012-11-13 12:39 -0800
      Re: Defect Report: packaged_task is too strongly typed Daniel Krügler <daniel.kruegler@googlemail.com> - 2012-11-21 11:23 -0800
      Re: Defect Report: packaged_task is too strongly typed Daniel Krügler <daniel.kruegler@googlemail.com> - 2012-11-21 11:27 -0800

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