cppreference 协程例子理解
我的理解是：C++ 的协程是无栈的，这意味着协程只是计算任务，仅在运行时需要栈，在 suspend 之后就会保存状态并脱离栈。要想要跨线程转移计算任务（比如实现工作窃取池），只需要将离栈协程在另外一个线程上 resume 即可。
#include <coroutine>
#include <iostream>
#include <stdexcept>
#include <thread>
auto switch_to_new_thread(std::jthread& out)
{
struct awaitable
{
std::jthread* p_out;
bool await_ready() { return false; }
void await_suspend(std::coroutine_handle<> h)
{
std::jthread& out = *p_out;
if (out.joinable())
throw std::runtime_error("Output jthread parameter not empty");
out = std::jthread([h] { h.resume(); });
// Potential undefined behavior: accessing potentially destroyed *this
// std::cout << "New thread ID: " << p_out->get_id() << '\n';
std::cout << "New thread ID: " << out.get_id() << '\n'; // this is OK
}
void await_resume() {}
};
return awaitable{&out};
}
struct task
{
struct promise_type
{
task get_return_object() { return {}; }
std::suspend_never initial_suspend() { return {}; }
std::suspend_never final_suspend() noexcept { return {}; }
void return_void() {}
void unhandled_exception() {}
};
};
task resuming_on_new_thread(std::jthread& out)
{
std::cout << "Coroutine started on thread: " << std::this_thread::get_id() << '\n';
co_await switch_to_new_thread(out);
// awaiter destroyed here
std::cout << "Coroutine resumed on thread: " << std::this_thread::get_id() << '\n';
}
int main()
{
std::jthread out;
resuming_on_new_thread(out);
}
可能结果：
Coroutine started on thread: 139972277602112
New thread ID: 139972267284224
Coroutine resumed on thread: 139972267284224
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