Coroutine patterns and design techniques in C++

25 Sep 2023

Coroutines have become a popular feature in modern programming languages, allowing developers to write more efficient and readable asynchronous code. In this article, we will explore some coroutine patterns and design techniques in C++.

What are Coroutines?

Coroutines are a generalization of subroutines, allowing multiple entry and exit points within a function. They enable suspension of execution, where the program can be paused and resumed at a later point without blocking the entire execution thread. Coroutines are especially useful in scenarios like asynchronous I/O, event-driven programming, and state machines.

Coroutine Design Techniques

1. Asynchronous I/O using Coroutines

Coroutines are a great fit for handling asynchronous I/O operations. By using coroutines, you can write code that appears to be synchronous, but is actually asynchronous under the hood. Here’s an example of using coroutines for asynchronous file I/O in C++:

#include <fstream>
#include <experimental/coroutine>

using namespace std;
using namespace experimental;

struct async_file_reader {
    ifstream file;
    string buffer;
    
    struct promise_type {
        async_file_reader get_return_object() { return {}; }
        suspend_never initial_suspend() { return {}; }
        suspend_never final_suspend() { return {}; }
        void unhandled_exception() { throw; }
        suspend_always yield_value(string value) { return {}; }
    };
    
    bool await_ready() { return !buffer.empty(); }
    void await_suspend(coroutine_handle<> handle) {
        if (!file.is_open()) file.open("myfile.txt");
        // Read from file into buffer
        // Resume the coroutine
        handle.resume();
    }
    string await_resume() { return buffer; }
};

async_file_reader read_file() {
    co_await suspend_always{};

    // Coroutine resumes here when await_suspend() is called
    
    co_return "Hello, world!";
}

int main() {
    async_file_reader reader = read_file();
    string content = reader.await_resume();
    cout << content << endl;
    
    return 0;
}

In this example, async_file_reader is a coroutine type that reads a file asynchronously. The co_await keyword is used to suspend execution of the coroutine until the file is read, and then the execution is resumed.

2. State Machines with Coroutines

Coroutines can simplify the implementation of state machines by allowing you to write code that looks more like sequential logic. Here’s an example of a simple state machine implemented using coroutines:

#include <iostream>
#include <experimental/coroutine>

using namespace std;
using namespace experimental;

enum class State { INIT, RUNNING, COMPLETED };

struct state_machine {
    State state = State::INIT;
    
    struct promise_type {
        state_machine get_return_object() { return {}; }
        suspend_never initial_suspend() { return {}; }
        suspend_never final_suspend() { return {}; }
        void unhandled_exception() { throw; }
        suspend_always yield_value(State& value) {
            state = value;
            return {};
        }
    };
    
    bool await_ready() { return state == State::COMPLETED; }
    void await_suspend(coroutine_handle<> handle) {
        if (state == State::INIT) {
            state = State::RUNNING;
            handle.resume();
        }
    }
    State await_resume() { return state; }
};

state_machine do_async_work() {
    co_yield State::INIT;

    // Simulate some work
    for (int i = 0; i < 10; i++) {
        cout << "Working..." << endl;
        co_yield State::RUNNING;
    }
    
    co_return State::COMPLETED;
}

int main() {
    state_machine machine = do_async_work();

    while (machine.await_resume() != State::COMPLETED) {
        // Do something while work is in progress
    }
    
    cout << "Work completed!" << endl;
    
    return 0;
}

In this example, state_machine is a coroutine type that represents a simple state machine with three states: INIT, RUNNING, and COMPLETED. The co_yield keyword is used to switch between states, and the main loop waits for the state machine to reach the COMPLETED state.

Conclusion

Coroutines provide powerful abstractions for handling asynchronous code and implementing state machines. By using coroutine patterns and design techniques in C++, you can write code that is both efficient and maintainable. Start exploring the world of coroutines and unlock their full potential in your C++ projects!

#C++ #Coroutines