...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
The WebSocket protocol requirements described in rfc6455 section 7.1.1 outline
an operation described as Close
the WebSocket Connection. This operation cleanly discards
bytes remaining at receiving endpoints and also closes the underlying TCP/IP
connection. Orderly shutdowns are always preferred; for TLS or SSL streams,
a protocol-level shutdown is desired. This presents a small issue for the
stream
implementation: the stream's NextLayer
template type requires only SyncStream or AsyncStream, but those concepts do not support
the operations to shut down the connection.
To enable the implementation to perform the shutdown components of the close operation, the library exposes two customization points expressed as free functions associated with the next layer type:
teardown
: Overloads of this
function drain and shut down a stream synchronously.
async_teardown
: Overloads of
this function drain and shut down a stream asynchronously.
The implementation provides suitable overloads of the teardown customization
points when websocket streams are instantiated using the Asio types boost::asio::ip::tcp::socket
or boost::asio::ssl::stream
for the next layer. In this case no user action is required. However, when
the websocket stream is instantiated for a user-defined type, compile errors
will result if the customization points are not provided for the user defined
type. Furthermore, user-defined types that wrap one of the Asio objects mentioned
earlier may wish to invoke a teardown customization point for the wrapped
object. This is how those tasks are accomplished.
To provide overloads of teardown for a user-defined type, simply declare the two free functions with the correct signature, accepting a reference to the user-defined type as the stream parameter:
struct custom_stream; void teardown( role_type role, custom_stream& stream, error_code& ec); template<class TeardownHandler> void async_teardown( role_type role, custom_stream& stream, TeardownHandler&& handler);
When the implementation invokes the asynchronous teardown function, it always
uses an invokable completion handler. It is not necessary to specify the
return type customization when creating user-defined overloads of async_teardown
.
To invoke the customization point, first bring the default implementation
into scope with a using
statement.
Then call the customization point without namespace qualification, allowing
argument-dependent lookup to take effect:
template<class NextLayer> struct custom_wrapper { NextLayer next_layer; template<class... Args> explicit custom_wrapper(Args&&... args) : next_layer(std::forward<Args>(args)...) { } friend void teardown( role_type role, custom_wrapper& stream, error_code& ec) { using boost::beast::websocket::teardown; teardown(role, stream.next_layer, ec); } template<class TeardownHandler> friend void async_teardown( role_type role, custom_wrapper& stream, TeardownHandler&& handler) { using boost::beast::websocket::async_teardown; async_teardown(role, stream.next_layer, std::forward<TeardownHandler>(handler)); } };