boost::asio::spawn 将一统C++网络库
(金庆的专栏)
boost::asio::spawn()创建一个协程，使C++网络编程大大简化，
个人认为这使得 asio 成为C++首选网络库。
boost::asio::spawn(my_strand, do_echo);
一般输入2个参数，参数1是 io_service 或者是 strand，
参数2是协程函数，类型如下：
void coroutine(boost::asio::yield_context yield);
在协程函数中调用各个异步IO，异步操作将挂起协程，待异步操作完成后会自动继续协程。
示例：
// See: doc/html/boost_asio/example/cpp11/spawn/echo_server.cpp try { boost::asio::io_service io_service; using std::placeholders::_1; using std::ref; boost::asio::spawn(io_service, std::bind(LoopAccept, ref(io_service), unPort, _1)); io_service.run(); } catch (std::exception& e) { std::cerr << "Exception: " << e.what() << "\n"; }LoopAccept() 是实际的协程函数
// Accept coroutine.static void LoopAccept(boost::asio::io_service& io_service, unsigned short port, boost::asio::yield_context yield){ using boost::asio::ip::tcp; tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), port)); for (;;) { boost::system::error_code ec; std::shared_ptr<CAsioSvrSession> pSession( new CAsioSvrSession(io_service)); acceptor.async_accept(pSession->GetSocket(), yield[ec]); if (ec) continue; pSession->Go(); // Will spawn read and write coroutine. }}