With the evolution of C++, many exciting features have been introduced to streamline parallelism and concurrency. One such feature is coroutines, which provide a powerful way to write asynchronous and parallel code. In this blog post, we will explore how to implement parallel algorithms using C++ coroutines.

What are C++ Coroutines?

C++ coroutines are a language feature introduced in C++20 that enable writing asynchronous code in a more sequential and readable manner. Coroutines allow you to write code as if it were executing synchronously, hiding the complexities of asynchronous operations and callbacks.

Parallel Algorithms and Coroutines

Parallel algorithms are an essential part of many applications, especially when dealing with large datasets or computationally intensive tasks. Traditionally, parallel algorithms were implemented using low-level constructs like threads and locks. However, with coroutines, we can simplify the implementation significantly.

Let’s take an example of a simple parallel algorithm: summing the elements of an array. Here’s how we can implement it using C++ coroutines:

#include <iostream>
#include <vector>
#include <coroutine>

template <typename T>
struct parallel_sum_coroutine {
    struct promise_type {
        T sum = 0;
        auto get_return_object() { return parallel_sum_coroutine{*this}; }
        std::suspend_never initial_suspend() { return {}; }
        std::suspend_never final_suspend() noexcept { return {}; }
        void unhandled_exception() { std::terminate(); }
        void return_value(T value) { sum = value; }
    };

    parallel_sum_coroutine(promise_type& promise) : sum_(promise.sum) {}

    bool await_ready() { return false; }
    void await_suspend(std::coroutine_handle<>) {}
    T await_resume() { return sum_; }

    T sum_;
};

template <typename InputIterator>
parallel_sum_coroutine<typename std::iterator_traits<InputIterator>::value_type>
parallel_sum(InputIterator begin, InputIterator end) {
    typename std::iterator_traits<InputIterator>::value_type sum = 0;
    for (auto it = begin; it != end; ++it) {
        sum += *it;
    }
    co_return sum;
}

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    auto result = parallel_sum(numbers.begin(), numbers.end());

    std::cout << "Sum: " << result.await_resume() << std::endl;

    return 0;
}

In the above code, we define a coroutine parallel_sum_coroutine that calculates the sum of a range of elements. The promise_type holds the state of the coroutine and the intermediate result. The get_return_object function creates an instance of the coroutine, and the return_value function sets the final result.

The parallel_sum function is used to initiate the coroutine and performs the actual calculation. We iterate over the range of elements and accumulate the partial sum. co_return is used to return the final result.

In the main() function, we create a vector of numbers and pass it to the parallel_sum function. Finally, we obtain the result using await_resume and print it.

Conclusion

C++ coroutines provide a powerful mechanism for writing parallel algorithms in a more declarative and readable way. By using coroutines, we can simplify the implementation of parallel code and hide the complexities of asynchronous operations. This allows us to write more efficient and maintainable code.

#programming #cpp #coroutines #parallel