C++ storing std::future form std::async in vector and wating for all

Question

I want to do several tasks in parallel with std::async and then wait until all the futures have completed.

void update() {
  // some code here
}

int main() {

  std::vector<std::future<void>> handles(5);

  for (int i = 0; i < 5; ++i) {
    auto handle = std::async(std::launch::async, &update);
    handles.emplace_back(std::move(handle));
  }

  for (auto& handle : handles) {
    handle.wait();
  }

  return 0;
}

But when executing the program i get an std::future_error thrown:

terminate called after throwing an instance of 'std::future_error'
  what():  std::future_error: No associated state
Aborted (core dumped)

And i wonder why. Shouldnt I be able to store the future objects?

score 7 · Accepted Answer · answered Mar 31 '21 at 11:20

You initialized your handles array with 5 default-constructed elements, then you emplaced 5 more into it. It now has 10 elements, the first 5 of which are default-constructed and therefore not associated with anything to wait for.

Don't create the vector with 5 elements. I think you were trying to reserve space for 5 elements - this can be done by calling reserve after the vector is constructed.

C++ storing std::future form std::async in vector and wating for all

1 Answers1