C++
Task type
Learn Task and promise types.
Task and promise type
A Task type is the return type of a coroutine and it contains a promise_type inside. The promise_type defines how the coroutine starts, finishes and returns a value.
A promise_type must have these members: get_return_object (produces the Task object), initial_suspend (suspend at start), final_suspend (suspend at finish), return_void or return_value (finish) and unhandled_exception (error). These members form the skeleton of the coroutine. If any is missing, you get a compile error.
Suspend members
initial_suspend and final_suspend decide what happens at the start and end of the coroutine. If they return std::suspend_never the coroutine runs immediately; if they return std::suspend_always it suspends.
A Task commonly uses suspend_never for initial_suspend (start immediately) and suspend_always for final_suspend (keep the frame). This way the coroutine starts, produces a value, and when it finishes the frame stays; it can be read from outside. They are also marked noexcept because final_suspend must not throw.
Exercise Complete the Task promise_type
Add std::suspend_never initial_suspend() { return {}; } and std::suspend_never final_suspend() noexcept { return {}; } members into the promise_type. The code in main is ready; do not touch it.
Show example solution
#include <iostream>
#include <coroutine>
struct Task {
struct promise_type {
Task get_return_object() { return {std::coroutine_handle<promise_type>::from_promise(*this)}; }
std::suspend_never initial_suspend() { return {}; }
std::suspend_never final_suspend() noexcept { return {}; }
void return_void() { }
void unhandled_exception() { }
};
std::coroutine_handle<promise_type> handle;
};
Task run() {
co_return;
}
int main() {
run();
std::cout << "Task test passed";
return 0;
}
Test yourself
-
the promise_type member
struct promise_type { Task ____() { return {}; } };
Show answer
struct promise_type { Task get_return_object() { return {}; } };
-
What does promise_type do?
- It speeds up the coroutine
- It deletes the coroutine
- It defines the start, finish and return behaviour of the coroutine
- It counts the coroutine parameters
Show answer
It defines the start, finish and return behaviour of the coroutine — promise_type defines the behaviour contract of the coroutine.
-
Match each member to its role
- get_return_object
- Produces the Task object
- initial_suspend
- Start suspend
- final_suspend
- Finish suspend
- unhandled_exception
- Error handling
Next lesson: Summary - Coroutines
