Examples of std::future in C++
Example 1: Using std::future to Print the Value Returned by Asynchronous Task
C++
// C++ Program to illustrate the use of std::future #include <chrono> #include <future> #include <iostream> using namespace std; // A simple function that returns some integer value int returnTwo() { return 2; } // driver code int main() { // creating a future object and a thread that executes // the function return two asynchronously future<int> f = async(launch::async, returnTwo); // getting and printing the result cout << f.get(); return 0; } |
Output
2
Example 2: Trying to Get the Value Multiple Times from std::future
C++
// C++ Program to illustrate the use of std::future #include <chrono> #include <future> #include <iostream> using namespace std; // A simple function that returns some integer value int returnTwo() { return 2; } // driver code int main() { // creating a future object and a thread that executes // the function return two asynchronously future<int> f = async(launch::async, returnTwo); // getting and printing the result cout << f.get(); // trying for second time cout << f.get(); return 0; } |
Output
terminate called after throwing an instance of 'std::future_error' what(): std::future_error: No associated state
The solution to the above problem is to use the future::valid() function to check the state of the future object before using get() method.
Example 3: Avoid the No Associate State Error using valid()
C++
// C++ Program to illustrate the use of std::future #include <chrono> #include <future> #include <iostream> using namespace std; // A simple function that returns some integer value int returnTwo() { return 2; } // driver code int main() { // creating a future object and a thread that executes // the function return two asynchronously future<int> f = async(launch::async, returnTwo); // getting and printing the result if (f.valid()) { cout << f.get() << endl; } else { cout << "Invalid State, Please create another Task" << endl; } // trying for second time if (f.valid()) { cout << f.get() << endl; } else { cout << "Invalid State, Please create another Task" << endl; } return 0; } |
Output
2 Invalid State, Please create another Task
Apart from the async() method, we can also get future objects using std::packaged_task() and std::promise(). The below example demonstrates the use of std::future with std::promise.
Example 4: Using std::future with std::promise
C++
// C++ Program to illustrate the use of std::future #include <chrono> #include <future> #include <iostream> using namespace std; // A simple function that returns some integer value void foo(promise<int> p) { p.set_value(25); } // driver code int main() { // creating a future object and a thread that executes // the function return two asynchronously promise<int> p; future<int> f = p.get_future(); ; // moving the task thread t(foo, move(p)); t.join(); cout << f.get(); return 0; } |
Output
25
std::future in C++
The C++ Concurrency support library includes the std::future class template, which has been available since C++11. This template is defined in the <future> header and provides a means to access the outcomes of asynchronous operations.