c++ - asio::strand 与 io_context::strand

Question

由于最新版本的 boost，asio 推出了新的执行器并提供了 asio::strand<Executor> .所以现在完全可以使用 asio::strand<asio::io_context::executor_type>而不是 io_context::strand .但它们不能互换使用。

有人可以用例子解释一下用法上的区别吗？

与其他相比，他们的优势/不方便？

Answer 1

io_context::strand是“遗产”。我认为它的存在是为了与仍然使用 boost::asio::io_service 的代码的接口(interface)兼容。 (也已弃用)。

As the comments reflect I've since found out that io_context::strand is not actually deprecated, although I see no reason why this is the case, and close reading of the implementation leaves me with the conclusion that

• asio::strand<Executor> is strictly better

• mixing both strand services is not the best idea. In fact both services are documented with the same tag line:

   // Default service implementation for a strand.
  

  I can't help feeling there should be only one default :)

现代链不引用执行上下文，而是包装一个执行器。
虽然在技术上有所不同，但在概念上是相同的。
发布任务的用法相同:

post(s, task); // where s is either legacy or modern 
defer(s, task);
dispatch(s, task);

In fact you may have tasks with an associated executor, see:

• When must you pass io_context to boost::asio::spawn? (C++)
• Which io_context does std::boost::asio::post / dispatch use?

您不能再使用旧链来构建 IO 服务对象(如 tcp::socket 或 stable_timer)。那是因为遗留链不能被类型删除为 any_io_executor :

using tcp = boost::asio::ip::tcp;
boost::asio::io_context ioc;

auto modern = make_strand(ioc.get_executor());
tcp::socket sock(modern);

boost::asio::io_context::strand legacy(ioc);
tcp::socket sock(legacy); // COMPILE ERROR

如果你真的想要，你可以不使用 any_io_executor 来强制它:

boost::asio::basic_stream_socket<tcp, decltype(legacy)> sock(legacy);
sock.connect({{}, 8989});