std::future - cppreference.com (original) (raw)

The class template std::future provides a mechanism to access the result of asynchronous operations:

• An asynchronous operation (created via std::async, std::packaged_task, or std::promise) can provide a std::future object to the creator of that asynchronous operation.
• The creator of the asynchronous operation can then use a variety of methods to query, wait for, or extract a value from the std::future. These methods may block if the asynchronous operation has not yet provided a value.
• When the asynchronous operation is ready to send a result to the creator, it can do so by modifying shared state (e.g. std::promise::set_value) that is linked to the creator's std::future.

Note that std::future references shared state that is not shared with any other asynchronous return objects (as opposed to std::shared_future).

Contents

• 1 Member functions
  • 1.1 Getting the result
  • 1.2 State
• 2 Examples
  • 2.1 Example with exceptions
• 3 See also

[edit] Member functions

[edit] Examples

#include #include #include   int main() { // future from a packaged_task std::packaged_task<int()> task([]{ return 7; }); // wrap the function std::future f1 = task.get_future(); // get a future std::thread t(std::move(task)); // launch on a thread   // future from an async() std::future f2 = std::async(std::launch::async, []{ return 8; });   // future from a promise std::promise p; std::future f3 = p.get_future(); std::thread([&p]{ p.set_value_at_thread_exit(9); }).detach();   std::cout << "Waiting..." << std::flush; f1.wait(); f2.wait(); f3.wait(); std::cout << "Done!\nResults are: " << f1.get() << ' ' << f2.get() << ' ' << f3.get() << '\n'; t.join(); }

Output:

Waiting...Done! Results are: 7 8 9

[edit] Example with exceptions

#include #include #include   int main() { std::promise p; std::future f = p.get_future();   std::thread t([&p] { try { // code that may throw throw std::runtime_error("Example"); } catch (...) { try { // store anything thrown in the promise p.set_exception(std::current_exception()); } catch (...) {} // set_exception() may throw too } });   try { std::cout << f.get(); } catch (const std::exception& e) { std::cout << "Exception from the thread: " << e.what() << '\n'; } t.join(); }

Output:

Exception from the thread: Example

[edit] See also

| | runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | waits for a value (possibly referenced by other futures) that is set asynchronously (class template) [edit] |