What is the difference of boost asio serial_port_service and serial_port

问题

I am to implement a (hopefully) robust asynchronious serial rs232 data transmission (via USB) - for both windows and linux, including esp. that nice embedded system called beagle bone black.

In the end I just want to be able to (compatibly) talk rs232 with robust deadline-timeouts, cancel() reset() etc. to not crash/hang when e.g. the tx or rx line disconnects accidently. So sure, I could simply copy/paste/adopt existing examples. But I also would like to become more enlightened ;-)

I decided to use boost:asio::serial_port. Now while reading the docs, I am confused about this two classes (well three with the typedef serial_port):

serial_port_service - Default service implementation for a serial port.

class serial_port_service : public io_service::service

basic_serial_port - Provides serial port functionality.

template< typename SerialPortService = serial_port_service>
class basic_serial_port :
  public basic_io_object< SerialPortService >,
  public serial_port_base

So faar I see, that I need a boost::asio::io_service to construct either boost::asio::serial_port or serial_port_service. I think I have understand the basic approach how asio does the job, like bespoken in e.g. this examples .

OK serial_port_service derives from io_service, its ctor takes an io_service, and its interface also offers those memberfuncs of basic_serial_port.

For me it looks like it's a io_service that also implements a basic_serial_port - what is the reason for having both classes? When to use the one when the other? Not sure about possible usecases, and what about this serial_port typedef. Maybe (well obviously) I am missing something - someone can give me more light?

回答1:

Often, the application should use the I/O object. In this case, that would be boost::asio::serial_port.

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The various classes are used to separate responsibilities and abstractions. The similarity in names can be confusing, so the documentation is very careful in its naming. The documentation states:

Class io_service implements an extensible, type-safe, polymorphic set of I/O services, indexed by service type. An object of class io_service must be initialised before I/O objects such as sockets, resolvers and timers can be used. These I/O objects are distinguished by having constructors that accept an io_service& parameter.

I/O services exist to manage the logical interface to the operating system on behalf of the I/O objects. In particular, there are resources that are shared across a class of I/O objects. For example, timers may be implemented in terms of a single timer queue. The I/O services manage these shared resources.

To explain this in context of serial ports:

• The io_service provides an event processing loop and manages I/O services, such as serial_port_service.
• The serial_port is an I/O object that provides an interface to perform serial port related operations. The implementation is very basic:
  • Determines how information is to be returned to the caller: throw if an error occurs, populate a std::future, suspend a coroutine, etc.
  • Defers the actual work to the serial_port_service, its I/O service.
  • Provides RAII semantics. When the serial_port is destroyed, it will cancel outstanding asynchronous operations and close the serial_port.
• The serial_port_service is an I/O service:
  • It provides an abstraction and implements the platform specific API.
  • It is shared amongst serial_ports that use the same io_service. When the serial_port is created with an io_service, the serial_port will use an existing serial_port_service registered to the io_service or create and register a new serial_port_service with the io_service.
  • It functions as an factory for an I/O object's implementation. For a serial_port, this is likely a basic file descriptor or handle.

来源：https://stackoverflow.com/questions/26562965/what-is-the-difference-of-boost-asio-serial-port-service-and-serial-port