Summary - Coroutines
- C++ course
- Advanced
- Lesson 72/83
- 5 min
Combine co_await, co_yield and Task to write an async counter.
Summary - Coroutines
In this group you learned coroutine, co_await, co_return, co_yield and Task type.
A coroutine is a function that can be suspended and resumed. co_await creates an await point; co_return finishes by returning a value; co_yield produces a value and pauses. A Task type defines the coroutine behaviour through promise_type. Now you will write an async counter that combines these four: it will produce values with co_yield and run inside a generator frame.
Practice Write an async counter
Write co_yield 5; and co_yield 6; in the body of the counter coroutine. The resume and std::cout calls in main are ready; do not touch them.
Starter code
#include <iostream>
#include <coroutine>
struct Generator {
struct promise_type {
int value_;
Generator get_return_object() { return {std::coroutine_handle<promise_type>::from_promise(*this)}; }
std::suspend_always initial_suspend() { return {}; }
std::suspend_always final_suspend() noexcept { return {}; }
std::suspend_always yield_value(int v) { value_ = v; return {}; }
void return_void() { }
void unhandled_exception() { }
};
std::coroutine_handle<promise_type> handle;
};
Generator counter() {
// Produce 5 and 6 with co_yield
}
int main() {
auto g = counter();
g.handle.resume();
std::cout << g.handle.promise().value_;
g.handle.resume();
std::cout << g.handle.promise().value_;
return 0;
}Sample solution
#include <iostream>
#include <coroutine>
struct Generator {
struct promise_type {
int value_;
Generator get_return_object() { return {std::coroutine_handle<promise_type>::from_promise(*this)}; }
std::suspend_always initial_suspend() { return {}; }
std::suspend_always final_suspend() noexcept { return {}; }
std::suspend_always yield_value(int v) { value_ = v; return {}; }
void return_void() { }
void unhandled_exception() { }
};
std::coroutine_handle<promise_type> handle;
};
Generator counter() {
co_yield 5;
co_yield 6;
}
int main() {
auto g = counter();
g.handle.resume();
std::cout << g.handle.promise().value_;
g.handle.resume();
std::cout << g.handle.promise().value_;
return 0;
}Expected output
56
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each idea to its description
| Term | Meaning |
|---|---|
coroutine |
Suspendable function |
co_await |
Await point |
co_yield |
Produce a value |
co_return |
Valued finish |
2 Which keyword produces a value and pauses the coroutine?
- co_await
- co_return
- return
- co_yield Correct answer
Explanation: co_yield produces a value and suspends the coroutine.
3 the counter call
auto g = ____();auto g = counter();Explanation: counter starts the generator coroutine.