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?
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";
...