在查阅本文之前，请先查看本人的另外一篇关于协同程序切换的文献，这对于如何正确协同程序编程很有价值。

C/C++ 如何正确的切换协同程序？（基于协程的并行架构）-CSDN博客

我本人相当反对，在项目之中使用 C++ 20标准的 stackless 无栈协程，在上述博文之中明确提到了大体的原因是什么，并且 C/C++ 越新的标准越存在不可预计的编译风险性问题，目前基本可以确保稳定的 C++ 编译器标准为 C++ 17。

stackless 协程在 C/C++ 之中是 “switch 基于步骤的状态机切换” 协同程序展开结构，即每一次 MoveNext 切换到下一个工作流片。

这就像在 C# 之中，我们使用 C# 语言的迭代器来模拟实现协程一样，首先本人提供一个简单的协同程序切换的流程，就只使用 co_await 编译器关键字，我们就可以实现一个完整的协同程序切换流程。

以下述代码为例，将直接展示 C++ 20 标准提供的 stackless 程序编译器语法支援，即三个关键字的用法例子：co_await、co_yield、co_return

在只使用 co_await 关键字的情况下实现协同程序正确切换。

例子：

#include <iostream>
#include <coroutine>struct MyTask {struct promise_type {public:MyTask                                  get_return_object() {return MyTask{ std::coroutine_handle<promise_type>::from_promise(*this) };}public:std::suspend_never                      initial_suspend() noexcept { return {}; }std::suspend_always                     final_suspend() noexcept { return {}; }void                                    return_void() noexcept {}void                                    unhandled_exception() noexcept {}};public:explicit MyTask(std::coroutine_handle<promise_type> coro) noexcept : coroutine(coro) {}~MyTask() noexcept {if (coroutine) {coroutine.destroy();}}public:bool                                        await_ready() const noexcept { return false; }void                                        await_suspend(std::coroutine_handle<>) const noexcept {}void                                        await_resume() const noexcept {}public:void                                        resume() noexcept {coroutine.resume();}private:std::coroutine_handle<promise_type>         coroutine;
};MyTask fork_stackless_coroutine() noexcept {std::cout << "协程，步一...\n";co_await std::suspend_always{};std::cout << "协程，步二.\n";co_await std::suspend_always{};std::cout << "协程，结束.\n";
}int main() {MyTask task = fork_stackless_coroutine();std::cout << "主线程，步一...\n";task.resume();std::cout << "主线程，步二.\n";task.resume();std::cout << "主线程，结束.\n";return 0;
}

人们可以通过 co_return 关键字以促使 C++ 20 stackless 协程，可以返回数据。

例子：（协程返回值类型为：int）

#include <iostream>
#include <coroutine>struct MyTask {struct promise_type {public:MyTask                                                      get_return_object() noexcept {return MyTask{ std::coroutine_handle<promise_type>::from_promise(*this) };}public:std::suspend_always                                         initial_suspend() noexcept { return {}; }std::suspend_always                                         final_suspend() noexcept { return {}; }public:void                                                        return_value(int value) {result = value;}void                                                        unhandled_exception() noexcept {}public:int                                                         result;};public:explicit MyTask(std::coroutine_handle<promise_type> coro) noexcept : coroutine(coro) {}~MyTask() {if (coroutine) {coroutine.destroy();}}public:int                                                             result() const noexcept {return coroutine.promise().result;}void                                                            resume() noexcept {coroutine.resume();}private:std::coroutine_handle<promise_type>                             coroutine;
};MyTask fork_stackless_coroutine() noexcept {std::cout << "协程，结束返回" << std::endl;co_return 42;
}int main() {MyTask task = fork_stackless_coroutine();task.resume();std::cout << "主线程，协程返回: " << task.result() << std::endl;return 0;
}

co_yield 迭代关键字在C#、之中的用法与 C++ 的用户几乎是一摸一样的用法，相对来说 C/C++，co_yield 关键更好用一点，co_await 跟 co_yield 都可以实现协同程序流程分片切换。

但区别是 co_yield 切换的分片，可以设置当前的迭代值，就像在C#当前，我们移动切换迭代器成功时，可以通过 Current 属性来获取当前的迭代值，co_await 不可以获得值。

#include <iostream>
#include <coroutine>struct MyTask {struct promise_type {public:MyTask                                                      get_return_object() noexcept {return MyTask{ std::coroutine_handle<promise_type>::from_promise(*this) };}public:std::suspend_always                                         initial_suspend() noexcept { return {}; }std::suspend_always                                         final_suspend() noexcept { return {}; }public:std::suspend_always                                         yield_value(int value) noexcept {current_value = value;return {};}void                                                        return_void() noexcept {}void                                                        unhandled_exception() noexcept {}public:int                                                         current_value;};public:explicit MyTask(std::coroutine_handle<promise_type> coro) noexcept : coroutine(coro) {}~MyTask() {if (coroutine) {coroutine.destroy();}}public:int                                                             current_value() const noexcept {return coroutine.promise().current_value;}bool                                                            move_next() noexcept {if (coroutine) {if (coroutine.done()) {return false;}coroutine.resume();return true;}return false;}private:std::coroutine_handle<promise_type>                             coroutine;
};MyTask fork_stackless_coroutine() noexcept {std::cout << "协程，开始：" << 100 << std::endl;co_yield 100;for (int i = 1; i <= 5; i++) {std::cout << "协程，步骤：" << i << std::endl;co_yield i;}
}int main() {MyTask task = fork_stackless_coroutine();while (task.move_next()) {std::cout << "主线程，读值：" << task.current_value() << std::endl;}std::cout << "主线程，结束" << std::endl;return 0;
}

如何在 C++ 20 有栈协程之中在调用其它 stackless 协程？c20协程库高级应用，即：co_await、co_return 共同搭配使用。

#include <iostream>
#include <coroutine>struct MyTask {struct promise_type {public:MyTask                                  get_return_object() {return MyTask{ std::coroutine_handle<promise_type>::from_promise(*this) };}public:std::suspend_never                      initial_suspend() noexcept { return {}; }std::suspend_always                     final_suspend() noexcept { return {}; }void                                    unhandled_exception() noexcept {}void                                    return_void() noexcept {}};public:explicit MyTask(std::coroutine_handle<promise_type> coro) noexcept : coroutine(coro) {}~MyTask() noexcept {if (coroutine) {coroutine.destroy();}}public:bool                                        await_ready() const noexcept { return false; }void                                        await_suspend(std::coroutine_handle<> co) const noexcept {bool done = co.done();if (!done) {co.resume();}}void                                        await_resume() const noexcept {}public:bool                                        move_next() noexcept {if (coroutine) {if (coroutine.done()) {return false;}coroutine.resume();return true;}return false;}private:std::coroutine_handle<promise_type>         coroutine;
};struct MyTaskInt {struct promise_type {public:MyTaskInt                               get_return_object() {return MyTaskInt{ std::coroutine_handle<promise_type>::from_promise(*this) };}public:std::suspend_never                      initial_suspend() noexcept {return {};}std::suspend_always                     final_suspend() noexcept {return {};}void                                    return_value(int value) {result = value;}void                                    unhandled_exception() noexcept {}public:int                                     result;};public:explicit MyTaskInt(std::coroutine_handle<promise_type> coro) noexcept : coroutine(coro) {}~MyTaskInt() noexcept {if (coroutine) {coroutine.destroy();}}public:bool                                        await_ready() const noexcept { return false; }void                                        await_suspend(std::coroutine_handle<> co) const noexcept {bool done = co.done();if (!done) {co.resume();}}int                                         await_resume() const noexcept {return coroutine.promise().result;}private:std::coroutine_handle<promise_type>         coroutine;
};MyTaskInt fork_stackless_coroutine_return_100() noexcept {co_return 100;
}MyTaskInt fork_stackless_coroutine_return_200() noexcept {int result = co_await fork_stackless_coroutine_return_100();result = result << 1;co_return result;
}MyTask fork_stackless_coroutine_start() noexcept {int result = co_await fork_stackless_coroutine_return_100();std::cout << result << std::endl;result = co_await fork_stackless_coroutine_return_200();std::cout << result << std::endl;
}int main() {MyTask task = fork_stackless_coroutine_start();while (task.move_next());return 0;
}