is there a reason to create the io_service outside the class? Can I create an instance of it directly into the connection object?
Yes, you can. But usually you don't want to create io_service for every connection or for every "active object" (you can use a single io_service for all these objects or use io_service-per-cpu strategy - see asio examples for details), that's why that you might want to manage io_service(s) creation outside these objects.
In your example you use io_.post() into the send method, why you don't use it also in the data_received?
Well, I meant that it's the handler of one of the asio standard async. read methods, like this: class connection : ... { //... asio::serial_port serial_port_; void receive() { serial_port_.async_read_some(myBuffer, bind(&connection::data_received, shared_from_this())) } void data_received() { // always executed in io_service thread(s) } }; But if you use some other i/o object to communicate with socket/serial port, and utilize io_service just to process functors asyncronously, and you wish to notify connection user from io_service thread - then you can use the same technique there. It's just matter of your design. Anyway, I recommend you to look at the asio tutorial and examples, to find out some common usage scenarios and design proposals.