C 11 packaged_task doesn't work as expected: thread quits and no output-CodePudding

I've this code snippet:

#include<future>
#include<iostream>
using namespace std;
int main() {
  cout << "---------" << endl;
  packaged_task<int(int, int)> task([](int a, int b){
    cout << "task thread\n";
    return a   b;
  });
  thread tpt(move(task), 3, 4);
  cout << "after thread creation\n";
  future<int> sum = task.get_future();
  cout << "before join\n";
  tpt.join();
  cout << "after join\n";
  sum.wait();
  cout << "after wait\n";
  cout << sum.get() << endl;
  return 0;
}

It only printed

---------
after thread creation
task thread

then hang for about 2 seconds, and ends. I don't see my packaged_task function execute. It didn't print "after join\n" and "after wait\n"

Why my program ended unexpectedly, how to fix?

CodePudding user response：

You're moving the packaged task into the thread and then trying to get the future object from the moved-from task which no longer has any state. For me this throws an exception: https://godbolt.org/z/Gh534dKac

terminate called after throwing an instance of 'std::future_error'

what(): std::future_error: No associated state

You need to instead get the future from the task before moving it into the thread:

...
future<int> sum = task.get_future();
thread tpt(move(task), 3, 4);
cout << "after thread creation\n";
...

https://godbolt.org/z/xhzKxh96K