niedz., 24 wrz 2023 o 02:39 Klemens Morgenstern < klemensdavidmorgenstern@gmail.com> napisał(a):
On Sun, Sep 24, 2023, 3:51 AM Andrzej Krzemienski via Boost < boost@lists.boost.org> wrote:
Hi Everyone. I am very uneasy about the dummy co_retuned value that I see in the examples of async::generator. Like the one in the echo server example: https://klemens.dev/async/tutorial.html#echo_server
async::generator
listen() { tcp_acceptor acceptor({co_await async::this_coro::executor}, {tcp::v4(), 55555}); for (;;) { tcp_socket sock = co_await acceptor.async_accept(); co_yield std::move(sock); } co_return tcp_socket{acceptor.get_executor()}; // :-( } The control will never get to the co_return. The caller never even tries to observe this value. and yet we are forced to return it.
You can throw a dummy exception, too. Never gets executed anyhow.
Interestingly, the coroutine example from ASIO doesn't have this due to a different design:
https://www.boost.org/doc/libs/1_83_0/doc/html/boost_asio/example/cpp20/coro...
std::generator doesn't have this.
In other examples the degenerate value is used to indicate the end of generation.
I do not know the coroutines to be able to tell if this is a design problem with async::generator or with C++ coroutines in general. But it feels wrong
Why? How would you want to communicate to the awaiter that the coro is done?
First, there are a bunch of use cases where the consumer of the generator doesn't need to know, like the one with the listener: keep generating until you are canceled. Second, std::generator somehow does it. (I do not know how.) Back to your question, when I see a code structure like this: async::generator<T> fun() { while(cond) { co_yield something; } } I know that there is nothing to do after the last co_yield in the loop. So maybe the compiler/library should also. Modulo that this may not be doable in the library, or the hacks are too expensive. Regards, &rzej;
that the end of generation should be signaled in this way.
Regards, &rzej;
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