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Re: Defect Report: packaged_task is too strongly typed

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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.

> future<int> result;
> 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.

> Any number of fixes are possible.  For example, packaged_task<void,
> ArgTypes...>

Do you mean packaged_task<void(ArgTypes...)> ?

> could have a member
> 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.

HTH & Greetings from Bremen,

Daniel Krügler



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Thread

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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