第十三號艦隊
0%

Coroutines in Visual C++ 2015 Update 1

Posted on Edited on In

Visual C++ 2015提供了一個Coroutine的實驗性實作，未來可能會被列入C++17當中，因此先來體驗一下

Generator

這寫法就跟Python差不多了，直接用yield就行了
不過跟Javascript的Generator還是有一定程度的不同

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
#include <iostream>
#include <experimental/generator>
using namespace std::experimental;
using namespace std;

generator<int> fib()
{
	int a = 0;
	int b = 1;
	for (;;) {
		yield a;
		auto next = a + b;
		a = b;
		b = next;
	}
}

int main(int argc, char* argv[])
{
	for (auto v : fib()) {
		if (v > 50)
			break;
		cout << v << endl;
	}
}

async function

以下是個示範程式

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
#include <future>
#include <iostream>
using namespace std;
using namespace std::chrono;

// this could be some long running computation or I/O
future<int> calculate_the_answer()
{
	return async([] {
		this_thread::sleep_for(1s); return 42;
	});
}

// Here is a resumable function
future<void> coro() {
	cout << this_thread::get_id() << " Started waiting... \n";
	auto result = await calculate_the_answer();
	cout << this_thread::get_id() << " : woke up, get " << result << endl;
}

int main(int argc, char* argv[])
{
	coro().get();
	cout << this_thread::get_id() << ": back in main\n";
}

future<void>和await的語法實在是有點突兀(尤其是其他語言都用async和await之後)
如果我們拿掉await會造成怎樣的結果

1
2
3
4
5
6
7
8
9
10
11
void noncoro() {
	cout << this_thread::get_id() << " Started waiting... \n";
	auto result = calculate_the_answer().get();
	cout << this_thread::get_id() << " : woke up, get " << result << endl;
}

int main(int argc, char* argv[])
{
	noncoro();
	cout << this_thread::get_id() << ": back in main\n";
}

可以看到兩個函數的不同

  • coro: Main Thread Invoke calculate_the_answer -> Child Thread Return
  • noncoro: Main Thread Invoke calculate_the_answer -> Main Thread Return

Complex Example

這個範例有點複雜，有時間在研究吧

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
#include <windows.h>
#include <future>
#include <iostream>
#include <experimental/resumable>
using namespace std;
using namespace std::chrono;
using namespace std::experimental;

auto operator await(system_clock::duration duration) {
	class awaiter {
		static void CALLBACK TimerCallback(PTP_CALLBACK_INSTANCE, void *Context, PTP_TIMER) {
			coroutine_handle<>::from_address(Context)();
		}
		PTP_TIMER timer = nullptr;
		system_clock::duration duration;
	public:
		explicit awaiter(system_clock::duration d) : duration(d) {}
		bool await_ready() const { return duration.count() <= 0; }
		bool await_suspend(coroutine_handle<> resume_cb) {
			int64_t relative_count = -duration.count();
			timer = CreateThreadpoolTimer(TimerCallback, resume_cb.to_address(), nullptr);
			SetThreadpoolTimer(timer, (PFILETIME)&relative_count, 0, 0);
			return timer != 0;
		}
		void await_resume() {}
		~awaiter() { if (timer) CloseThreadpoolTimer(timer); }
	};
	return awaiter{ duration };
}

future<void> test() {
	cout << this_thread::get_id() << ": sleeping...\n";
	await 1s;
	cout << this_thread::get_id() << ": woke up\n";
}

int main() {
	test().get();
	cout << this_thread::get_id() << ": back in main\n";
}

Reference

– Resumable functions in C++
– Coroutines in Visual Studio 2015 - Update 1
– Stackless coroutines with Visual Studio 2015