std::future
- C++ course
- Advanced
- Lesson 23/83
- 5 min
Learn value passing with std::future and promise.
std::future
std::future<T> holds a T result that is not computed yet. It is created by std::async or std::promise. f.get() blocks until the result is ready and returns the value.
get can be called only once; a second call throws std::future_error. valid() checks whether the future still holds a result. wait() blocks without fetching; wait_for and wait_until accept a timeout. A future is ideal for single-consumer synchronization.
promise and future relationship
std::promise<T> is the side that sets the result; std::future<T> is the side that reads it. A future is taken from a promise with p.get_future(); a value is written with p.set_value(v). This is the low-level way to pass a value to another thread.
std::async wires this relationship automatically; the user does not handle promise and future directly. But when custom synchronization is needed the pair is used manually. This is the foundation of the producer-consumer pattern. Each promise is paired with one future; the relation is one-to-one.
Practice Fetch a result with future
Write int compute() with body return 100;. In main write std::future<int> f = std::async(std::launch::async, compute);, then fetch the result with int result = f.get(); and print std::cout << result;.
Starter code
#include <iostream>
#include <future>
// Write the compute function
int main() {
// Launch a future, fetch the result, print
return 0;
}Sample solution
#include <iostream>
#include <future>
int compute() {
return 100;
}
int main() {
std::future<int> f = std::async(std::launch::async, compute);
int result = f.get();
std::cout << result;
return 0;
}Expected output
100
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the get line
std::future<int> f = std::async(std::launch::async, compute);
int r = ____;std::future<int> f = std::async(std::launch::async, compute);
int r = f.get();Explanation: f.get() waits and returns the result.
2 What does future::get() do?
- Deletes the result
- Only waits
- Waits for the result and returns it Correct answer
- Starts the thread
Explanation: get() blocks until the result is ready and returns the value.
3 Match each expression to its role
| Term | Meaning |
|---|---|
f.get() |
Fetch the result |
f.wait() |
Only wait |
f.valid() |
Is valid? |
p.set_value(v) |
Write the value |