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### Teardown

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 `tcp::socket` or `net::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.

##### User-defined Teardown

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`.

##### Invoking 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));
        }
    };