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Re: Defect Report: packaged_task is too strongly typed

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[resend, with cc this time -- my last try got lost, and the usual
channels for tracking down lost articles aren't working]

This may be considerably more clear once I fix the obvious errors.  I
misread the signature of packaged_task.


On 11/04/2012 07:16 PM, Daniel Kr=FCgler wrote:
>
>
> Am 04.11.2012 20:42, schrieb Andy Lutomirski:
>>
>> packaged_task is almost perfect for use in custom thread pools, like
>> this:
>>
>> packaged_task<int> task([] { /* something */ });
>
> This example cannot work, because packaged_task can only be
> instantiated with a function type, but 'int' isn't such a beast.

This should be:

packaged_task<int ()> task([] { /* something */ });
future<int> result = task.get_future();
threadpool.submit(task);

// later on
int x = result.get();


>>
>> 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.

>
>> Any number of fixes are possible.  For example, packaged_task<void,
>> ArgTypes...>
>
> Do you mean packaged_task<void(ArgTypes...)> ?

Yes.

>
>> 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 don't see how this would work, if either the return type or the
> argument types are missing.

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).

--Andy


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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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