State machines are a powerful way to model and control the behavior of software systems. Traditional state machines, implemented using switch statements or if-else blocks, can quickly become complex and hard to manage. Thankfully, modern C++ offers a more elegant solution: coroutine-based state machines.

What are Coroutines?

Coroutines are a language feature that allows functions to be suspended and resumed at specific points, providing an efficient and readable way to write asynchronous code. In C++, coroutines were introduced in C++20 with the std::coroutine library.

Implementing State Machines using Coroutines

To implement a state machine using coroutines, we can define each state as a separate coroutine function. Each coroutine represents a state that can yield control to another state when certain conditions are met. Here’s an example of a simple state machine that toggles an LED:

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

namespace coroutines = std::experimental;

class LedStateMachine {
public:
    LedStateMachine() = default;
    
    void start() {
        current_state = toggle_led();
    }
    
    void process_event() {
        if (current_state) {
            current_state.resume();
            
            if (current_state.done()) {
                current_state = nullptr;
                std::cout << "State machine completed.\n";
            }
        } else {
            std::cout << "State machine not started.\n";
        }
    }

private:
    coroutines::coroutine_handle<> current_state;
    
    coroutines::suspend_always toggle_led() {
        while (true) {
            std::cout << "LED ON.\n";
            co_await std::experimental::suspend_always{};
            std::cout << "LED OFF.\n";
            co_await std::experimental::suspend_always{};
        }
    }
};

In this example, we define a LedStateMachine class that manages the state machine. The start() function initializes the state machine by setting the initial state (toggle_led() coroutine) as the current state. The process_event() function is responsible for advancing the state machine by resuming the current state coroutine.

The toggle_led() coroutine represents the state where the LED is switched ON and OFF continuously. The co_await std::experimental::suspend_always{} expression suspends the coroutine, allowing other coroutines to execute.

Advantages of Coroutine-based State Machines

• Readability: With coroutines, state machines can be written in a more sequential and linear manner, making it easier to understand the control flow.
• Simplicity: Coroutines provide a simpler approach compared to traditional switch-based state machines, reducing the complexity and boilerplate code.
• Asynchrony: Coroutines can be used to handle asynchronous events or I/O operations gracefully, making them a natural fit for building event-driven systems.

Conclusion

Coroutines offer an elegant way to implement state machines in C++. They provide a more readable, concise, and efficient solution for managing complex control flows. By leveraging the power of coroutines, developers can create more maintainable and scalable software systems.

Implementing state machines using coroutines can be a game-changer, especially in event-driven systems, game development, or any application where managing control flow is crucial.

#C++ #coroutines