std::packaged_task<R(Args...)>::make_ready_at_thread_exit (3) - Linux Manuals

std::packaged_task<R(Args...)>::make_ready_at_thread_exit: std::packaged_task<R(Args...)>::make_ready_at_thread_exit

NAME

std::packaged_task<R(Args...)>::make_ready_at_thread_exit - std::packaged_task<R(Args...)>::make_ready_at_thread_exit

Synopsis

void make_ready_at_thread_exit( ArgTypes... args ); (since C++11)

Calls the stored task with forwarded args as the arguments. The return value of the task or any exception thrown by it is stored in the shared state of *this.
The shared state is only made ready after the current thread exits and all objects of thread local storage duration are destroyed.

Parameters

args - the parameters to pass on invocation of the stored task

Return value

(none)

Exceptions

std::future_error on the following error conditions:

* The stored task has already been invoked. The error category is set to promise_already_satisfied.
* *this has no shared state. The error category is set to no_state.

Example

// Run this code

  #include <future>
  #include <iostream>
  #include <chrono>
  #include <thread>
  #include <functional>
  #include <utility>

  void worker(std::future<void>& output)
  {
      std::packaged_task<void(bool&)> my_task{ [](bool& done) { done=true; } };

      auto result = my_task.get_future();

      bool done = false;

      my_task.make_ready_at_thread_exit(done); // execute task right away

      std::cout << "worker: done = " << std::boolalpha << done << std::endl;

      auto status = result.wait_for(std::chrono::seconds(0));
      if (status == std::future_status::timeout)
          std::cout << "worker: result is not ready yet" << std::endl;

      output = std::move(result);
  }

  int main()
  {
      std::future<void> result;

      std::thread{worker, std::ref(result)}.join();

      auto status = result.wait_for(std::chrono::seconds(0));
      if (status == std::future_status::ready)
          std::cout << "main: result is ready" << std::endl;
  }

Output:

  worker: done = true
  worker: result is not ready yet
  main: result is ready