C++ 20标准协同程序（协程）基于编译器展开的 stackless 协程。
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
#include
struct MyTask {
struct promise_type {
public:
MyTask get_return_object() {
return MyTask{ std::coroutine_handle::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 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 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
#include
struct MyTask {
struct promise_type {
public:
MyTask get_return_object() noexcept {
return MyTask{ std::coroutine_handle::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 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 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
#include
struct MyTask {
struct promise_type {
public:
MyTask get_return_object() noexcept {
return MyTask{ std::coroutine_handle::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 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 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
#include
struct MyTask {
struct promise_type {
public:
MyTask get_return_object() {
return MyTask{ std::coroutine_handle::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 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 coroutine;
};
struct MyTaskInt {
struct promise_type {
public:
MyTaskInt get_return_object() {
return MyTaskInt{ std::coroutine_handle::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 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 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;
}