...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Boost.Asio may be used to perform both synchronous and asynchronous operations on I/O objects such as sockets. Before using Boost.Asio it may be useful to get a conceptual picture of the various parts of Boost.Asio, your program, and how they work together.
As an introductory example, let's consider what happens when you perform a connect operation on a socket. We shall start by examining synchronous operations.
Your program will have at least one io_context object. The io_context represents your program's link to the operating system's I/O services.
boost::asio::io_context io_context;
To perform I/O operations your program will need an I/O object such as a TCP socket:
boost::asio::ip::tcp::socket socket(io_context);
When a synchronous connect operation is performed, the following sequence of events occurs:
1. Your program initiates the connect operation by calling the I/O object:
socket.connect(server_endpoint);
2. The I/O object forwards the request to the io_context.
3. The io_context calls on the operating system to perform the connect operation.
4. The operating system returns the result of the operation to the io_context.
5. The io_context translates any error
resulting from the operation into an object of type boost::system::error_code
.
An error_code
may be compared
with specific values, or tested as a boolean (where a false
result means that no error occurred). The result is then forwarded back
up to the I/O object.
6. The I/O object throws an exception
of type boost::system::system_error
if the operation failed.
If the code to initiate the operation had instead been written as:
boost::system::error_code ec; socket.connect(server_endpoint, ec);
then the error_code
variable
ec
would be set to the
result of the operation, and no exception would be thrown.
When an asynchronous operation is used, a different sequence of events occurs.
1. Your program initiates the connect operation by calling the I/O object:
socket.async_connect(server_endpoint, your_completion_handler);
where your_completion_handler
is a function or function object with the signature:
void your_completion_handler(const boost::system::error_code& ec);
The exact signature required depends on the asynchronous operation being performed. The reference documentation indicates the appropriate form for each operation.
2. The I/O object forwards the request to the io_context.
3. The io_context signals to the operating system that it should start an asynchronous connect.
Time passes. (In the synchronous case this wait would have been contained entirely within the duration of the connect operation.)
4. The operating system indicates that the connect operation has completed by placing the result on a queue, ready to be picked up by the io_context.
5. Your program must make a call to io_context::run()
(or to one of the similar io_context member
functions) in order for the result to be retrieved. A call to io_context::run()
blocks while there are unfinished asynchronous operations, so you would
typically call it as soon as you have started your first asynchronous operation.
6. While inside the call to io_context::run()
, the io_context
dequeues the result of the operation, translates it into an error_code
, and then passes it to your completion handler.
This is a simplified picture of how Boost.Asio operates. You will want to delve further into the documentation if your needs are more advanced, such as extending Boost.Asio to perform other types of asynchronous operations.