C++ Coroutines, introduced in C++20, offer a powerful way to handle asynchronous tasks. This tutorial aims to guide you through the essentials of using C++ Coroutines for asynchronous programming. We will cover the fundamental concepts, the necessary syntax, and practical examples to ensure you understand how to effectively implement coroutines in your projects. The target audience for this tutorial is developers who are familiar with C++ and have some experience with asynchronous programming but are new to C++ Coroutines.
Understanding Coroutines
A coroutine is a function that can suspend its execution to be resumed later. Unlike regular functions, which run from start to finish, coroutines can pause execution at certain points, returning control to the caller and then resume from where they left off. This makes coroutines particularly suitable for asynchronous programming, where tasks often need to wait for I/O operations or other asynchronous events.
Key Concepts
Before diving into the code, let’s understand some key concepts related to coroutines:
- Coroutine State: Coroutines maintain their state across suspensions, allowing them to resume execution seamlessly.
- Suspension Points: Points within a coroutine where execution can be paused and later resumed.
- Promises and Futures: Mechanisms to handle values produced by coroutines.
Coroutine Syntax
In C++, coroutines are declared using the co_await, co_yield, and co_return keywords. Here’s a brief overview of these keywords:
co_await: Suspends the coroutine until the awaited task completes.co_yield: Produces a value and suspends the coroutine.co_return: Completes the coroutine and optionally returns a value.
Setting Up Coroutines
To use coroutines, you need to include the <coroutine> header. Let’s start by writing a simple coroutine.
#include <coroutine>
#include <iostream>
struct return_object {
struct promise_type {
return_object get_return_object() { return {}; }
std::suspend_never initial_suspend() { return {}; }
std::suspend_never final_suspend() noexcept { return {}; }
void return_void() {}
void unhandled_exception() { std::terminate(); }
};
};
return_object simple_coroutine() {
std::cout << "Hello from coroutine" << std::endl;
co_return;
}
int main() {
simple_coroutine();
std::cout << "Coroutine finished" << std::endl;
return 0;
}Code language: C++ (cpp)In this example, we define a return_object struct with a nested promise_type struct. The promise_type struct defines how the coroutine handles its lifecycle, including suspension and completion. The simple_coroutine function demonstrates a basic coroutine that prints a message and then completes.
Asynchronous Tasks with Coroutines
Let’s move on to a more practical example: performing asynchronous tasks. We’ll create a coroutine that simulates an asynchronous operation using std::future.
Step 1: Defining the Awaiter
First, we need to define an awaiter. An awaiter is a type that knows how to suspend and resume a coroutine.
#include <future>
#include <iostream>
#include <coroutine>
template <typename T>
struct async_task {
struct promise_type {
std::promise<T> promise;
async_task get_return_object() {
return async_task{ promise.get_future() };
}
std::suspend_never initial_suspend() { return {}; }
std::suspend_always final_suspend() noexcept { return {}; }
void return_value(T value) {
promise.set_value(value);
}
void unhandled_exception() {
promise.set_exception(std::current_exception());
}
};
std::future<T> future;
async_task(std::future<T>&& fut) : future(std::move(fut)) {}
};
template <typename T>
struct awaiter {
std::future<T> future;
bool await_ready() const noexcept {
return future.wait_for(std::chrono::seconds(0)) == std::future_status::ready;
}
void await_suspend(std::coroutine_handle<> handle) {
std::thread([this, handle]() mutable {
future.wait();
handle.resume();
}).detach();
}
T await_resume() {
return future.get();
}
};Code language: C++ (cpp)In this example, async_task is a coroutine return type that wraps a std::future. The promise_type struct defines how the coroutine interacts with the promise and future. The awaiter struct provides the necessary methods to integrate std::future with coroutines.
Step 2: Creating an Asynchronous Task
Next, let’s create an asynchronous task using our awaiter.
async_task<int> compute_async() {
std::cout << "Computing asynchronously..." << std::endl;
co_return 42;
}
async_task<void> async_example() {
auto result = co_await awaiter{ compute_async().future };
std::cout << "Result: " << result << std::endl;
}Code language: C++ (cpp)Here, compute_async is a coroutine that returns an async_task<int>. The async_example coroutine awaits the result of compute_async and prints it.
Step 3: Running the Coroutine
Finally, we need to run the coroutine.
int main() {
auto example = async_example();
example.future.wait();
return 0;
}Code language: C++ (cpp)This code creates an async_example coroutine and waits for its completion.
Handling Errors
Handling errors in coroutines is crucial. The promise_type struct’s unhandled_exception method can capture exceptions thrown within the coroutine.
template <typename T>
struct async_task {
struct promise_type {
std::promise<T> promise;
async_task get_return_object() {
return async_task{ promise.get_future() };
}
std::suspend_never initial_suspend() { return {}; }
std::suspend_always final_suspend() noexcept { return {}; }
void return_value(T value) {
promise.set_value(value);
}
void unhandled_exception() {
promise.set_exception(std::current_exception());
}
};
std::future<T> future;
async_task(std::future<T>&& fut) : future(std::move(fut)) {}
};
async_task<int> faulty_async() {
throw std::runtime_error("Something went wrong");
co_return 0;
}
async_task<void> async_example() {
try {
auto result = co_await awaiter{ faulty_async().future };
std::cout << "Result: " << result << std::endl;
} catch (const std::exception& e) {
std::cout << "Error: " << e.what() << std::endl;
}
}Code language: C++ (cpp)In this example, faulty_async throws an exception, which is caught in async_example.
Combining Coroutines with I/O Operations
Coroutines are particularly useful for I/O operations, which often involve waiting. Let’s create an example that performs asynchronous file I/O using coroutines.
Step 1: Setting Up the Asynchronous I/O
We’ll use std::ifstream to read a file asynchronously.
#include <fstream>
#include <string>
struct file_awaiter {
std::ifstream file;
std::string filename;
file_awaiter(const std::string& filename) : filename(filename) {}
bool await_ready() const noexcept {
return false;
}
void await_suspend(std::coroutine_handle<> handle) {
std::thread([this, handle]() mutable {
file.open(filename);
handle.resume();
}).detach();
}
std::ifstream& await_resume() {
return file;
}
};Code language: C++ (cpp)The file_awaiter struct handles asynchronous file opening.
Step 2: Creating a Coroutine for File I/O
Next, we’ll create a coroutine that uses file_awaiter to read from a file asynchronously.
async_task<void> read_file_async(const std::string& filename) {
auto file = co_await file_awaiter{ filename };
if (!file.is_open()) {
std::cout << "Failed to open file" << std::endl;
co_return;
}
std::string line;
while (std::getline(file, line)) {
std::cout << line << std::endl;
}
file.close();
}Code language: C++ (cpp)In this coroutine, read_file_async, we await the file_awaiter to open the file asynchronously and then read its contents.
Step 3: Running the File I/O Coroutine
Finally, let’s run our file I/O coroutine.
int main() {
auto example = read_file_async("example.txt");
example.future.wait();
return 0;
}Code language: C++ (cpp)This code initiates the read_file_async coroutine and waits for it to complete.
Combining Multiple Coroutines
Coroutines can be combined to perform complex asynchronous workflows. Let’s create an example where multiple coroutines work together to process data.
async_task<std::string> fetch_data_async() {
std::cout << "Fetching data..." << std::endl;
co_return "data";
}
async_task<void> process_data_async(const std::string& data) {
std::cout << "Processing data: " << data << std::endl;
co_return;
}
async_task<void> combined_example() {
auto data = co_await awaiter{ fetch_data_async().future };
co_await process_data_async(data);
}Code language: C++ (cpp)In this example, fetch_data_async fetches data asynchronously, and process_data_async
processes the fetched data. The combined_example coroutine combines both tasks.
Advanced Coroutine Features
C++ Coroutines offer several advanced features that can further enhance asynchronous programming.
Custom Awaitable Types
You can create custom awaitable types to extend the capabilities of coroutines.
template <typename T>
struct custom_awaitable {
T value;
bool await_ready() const noexcept { return true; }
void await_suspend(std::coroutine_handle<>) {}
T await_resume() { return value; }
};
async_task<void> custom_awaitable_example() {
auto result = co_await custom_awaitable<int>{ 10 };
std::cout << "Custom awaitable result: " << result << std::endl;
}Code language: C++ (cpp)In this example, custom_awaitable is a simple awaitable type that immediately returns a value.
Coroutine Traits
You can define custom coroutine traits to control how coroutines are created and managed.
template <typename PromiseType>
struct custom_allocator {
void* allocate(std::size_t size) {
return ::operator new(size);
}
void deallocate(void* ptr, std::size_t size) {
::operator delete(ptr, size);
}
};
template <typename T>
struct custom_task {
struct promise_type {
custom_task get_return_object() { return {}; }
std::suspend_never initial_suspend() { return {}; }
std::suspend_never final_suspend() noexcept { return {}; }
void return_void() {}
void unhandled_exception() { std::terminate(); }
void* operator new(std::size_t size) {
return custom_allocator<promise_type>{}.allocate(size);
}
void operator delete(void* ptr, std::size_t size) {
custom_allocator<promise_type>{}.deallocate(ptr, size);
}
};
};
custom_task<void> custom_task_example() {
std::cout << "Using custom task" << std::endl;
co_return;
}Code language: C++ (cpp)In this example, custom_task uses a custom allocator for coroutine memory management.
Best Practices for Using Coroutines
Here are some best practices to keep in mind when using coroutines in C++:
- Understand the Lifecycle: Be aware of how coroutines are created, suspended, and destroyed. Properly managing coroutine state is crucial for preventing resource leaks and undefined behavior.
- Handle Exceptions: Always handle exceptions within coroutines to ensure they don’t cause crashes or unexpected behavior.
- Use Appropriate Return Types: Choose the right return types for your coroutines based on the tasks they perform. For asynchronous tasks, consider using
std::futureor custom awaitable types. - Minimize Blocking Operations: Coroutines are designed to be non-blocking. Avoid using blocking operations within coroutines to maintain their asynchronous nature.
- Optimize for Performance: Coroutines can introduce overhead due to context switching. Optimize your coroutines and avoid unnecessary suspensions to improve performance.
- Leverage Libraries: Consider using libraries like Boost.Asio or cppcoro, which provide additional utilities and abstractions for working with coroutines.
Conclusion
C++ Coroutines offer a powerful mechanism for handling asynchronous tasks. By understanding the fundamental concepts, syntax, and best practices, you can effectively integrate coroutines into your C++ projects. This tutorial covered the basics of coroutines, including creating simple coroutines, handling asynchronous tasks, combining coroutines, and using advanced features.
Coroutines can greatly simplify asynchronous programming, making your code more readable and maintainable. As you gain experience with coroutines, you’ll discover even more ways to leverage their power to handle complex asynchronous workflows.