1. Problem It Solves
A coroutine task represents an operation that can pause and later deliver one result or exception to a continuation. It makes an important assumption visible and checkable.
2. Prerequisites
Awaiters, task results, handles, and scheduling boundaries.
You should be able to compile a short program and read its output.
3. Core Idea
The task is a receipt for unfinished work. co_await registers where to continue, while a scheduler or event source eventually resumes the frame. Read co_await as a precise promise; runtime preconditions still belong to the programmer.
4. Minimal Syntax
int value = co_await operation;
co_return value;5. How It Works
The program introduces the smallest relevant form of
co_await.It applies the feature to fixed data while required owners remain in scope.
It prints one result that can be checked against the source.
6. Common Mistakes
A coroutine is not automatically parallel or asynchronous; without a real scheduler, an awaiter may only simulate a suspension point.
Also check the required header, C++20 library support, lifetime, and deduced types.
7. When to Use It
Use it when sequential-looking code must coordinate genuine asynchronous completion.
Avoid it when the work is immediate, CPU-bound without a scheduler, or a plain future is enough.
8. Simple Example
A small task suspends once, is resumed explicitly by main, then returns a fixed integer through its promise. The companion .cpp uses no input, so its result is easy to reproduce.
Complete sample code
Source file
cpp20/48_asynchronous_coroutine_task/main.cpp
// Day 48: Asynchronous Tasks with Coroutines
#include <coroutine>
#include <iostream>
#include <stdexcept>
#include <utility>
struct Task {
struct promise_type;
using Handle = std::coroutine_handle<promise_type>;
struct promise_type {
int result{};
Task get_return_object() { return Task{Handle::from_promise(*this)}; }
std::suspend_never initial_suspend() noexcept { return {}; }
std::suspend_always final_suspend() noexcept { return {}; }
void return_value(int value) noexcept { result = value; }
void unhandled_exception() { std::terminate(); }
};
Handle handle{};
explicit Task(Handle value) : handle{value} {}
Task(Task&& other) noexcept : handle{std::exchange(other.handle, {})} {}
Task(const Task&) = delete;
~Task() { if (handle) handle.destroy(); }
void resume() { handle.resume(); }
bool done() const { return handle.done(); }
int value() const { if (!done()) throw std::logic_error("not ready"); return handle.promise().result; }
};
Task async_value() {
co_await std::suspend_always{}; // Educational scheduling boundary.
co_return 42;
}
int main() {
Task task = async_value();
std::cout << "ready before resume = " << std::boolalpha << task.done() << '\n';
task.resume();
std::cout << "result = " << task.value() << '\n';
}
9. Key Takeaways
co_awaitexpresses the central C++20 idea of this day.The example isolates one behavior with fixed data.
Compiler checks do not replace lifetime and runtime reasoning.
Prefer the smallest interface that states the real requirement.
10. Self-Check Questions
Easy — What is the main job of
co_awaitin the minimal example?Medium — Why is the result unavailable before the caller resumes the suspended task?
Hard — Which missing component prevents this educational example from performing real background I/O?