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Execute a task asynchronously

12345678910111213141516171819#include <future> int func() { int some_value = 0; // Do work... return some_value; } int main() { std::future<int> result_future = std::async(func); // Do something... int result = result_future.get(); }

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High-level asynchronous execution of tasks.


std::async provides a high-level way to accomplish asynchronous execution of tasks, abstracting over std::thread and std::promise.

On line 14, we call std::async, passing func as the function to execute asynchronously. Arguments to func can be passed as additional arguments to std::async. The return value is a std::future<int>, representing an int value that will be returned from the task at some point in the future.

By default, std::async will decide whether to execute the task concurrently or to wait until we request the result and then execute the task. If we want to specifically request one of these behaviours, we can use a flag as the first argument. To request concurrent execution, if possible, we use std::launch::async. To request that the task is executed only when the result is needed, we use std::launch::deferred.

On line 18, we use the std::future’s get member function to get the result of the asynchronous task. This blocks until the result is available.

Note: It is important that we capture the future returned by std::async. If we do not, then the call will always block.


  • Joseph Mansfield
  • Mark A. Gibbs

Last Updated

11 February 2016


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