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Showing content from https://cplusplus.com/reference/future/packaged_task/valid/ below:

public member function

<future>

bool valid() const noexcept;

Check for valid shared state

For default-constructed packaged_task objects, this function returns false (unless move-assigned or swapped with a valid packaged_task).

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// packaged_task::get_future
#include <iostream>     // std::cout
#include <utility>      // std::move
#include <future>       // std::packaged_task, std::future
#include <thread>       // std::thread

// function that launches an int(int) packaged_task in a new thread:
std::future<int> launcher (std::packaged_task<int(int)>& tsk, int arg) {
	if (tsk.valid()) {
      std::future<int> ret = tsk.get_future();
	  std::thread (std::move(tsk),arg).detach();
	  return ret;
	}
	else return std::future<int>();
}

int main ()
{
  std::packaged_task<int(int)> tsk ([](int x){return x*2;});

  std::future<int> fut = launcher (tsk,25);

  std::cout << "The double of 25 is " << fut.get() << ".\n";

  return 0;
}

packaged_task::get_future

Get future (public member function)

packaged_task::operator()

Call stored task (public member function)

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