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The All-in-One Guide to C++20

The co_return Keyword

Explore how the co_return keyword functions within C++20 coroutines to enable asynchronous programming. This lesson helps you understand managing coroutine results using futures, promises, and shared pointers, providing a foundation for advanced coroutine implementations.

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A coroutine uses co_return as its return statement.

A future

Admittedly, the coroutine in the following program is the simplest coroutine. I can imagine, that still does something meaningful: it automatically stores the result of its invocation.

C++
#include <coroutine>
#include <iostream>
#include <memory>
template<typename T>
struct MyFuture {
    std::shared_ptr<T> value;
    MyFuture(std::shared_ptr<T> p): value(p) {}
    ~MyFuture() { }
    T get() {
        return *value;
    }
    struct promise_type {
        std::shared_ptr<T> ptr = std::make_shared<T>();
        ~promise_type() { }
        MyFuture<T> get_return_object() {
            return ptr;
        }
        void return_value(T v) {
            *ptr = v;
        }
        std::suspend_never initial_suspend() {
            return {};
        }
        std::suspend_never final_suspend() noexcept {
            return {};
        }
        void unhandled_exception() {
            std::exit(1);
        }
    };
};
MyFuture<int> createFuture() {
    co_return 2021;
}
int main() {
    std::cout << '\n';
    auto fut = createFuture();
    std::cout << "fut.get(): " << fut.get() << '\n';
    std::cout << '\n';
}

MyFuture behaves as a future, which runs immediately. ...