##### ssl::stream::async_handshake (2 of 2 overloads)

Start an asynchronous SSL handshake.

    template<
        typename ConstBufferSequence,
        typename BufferedHandshakeToken = default_completion_token_t<executor_type>>
    DEDUCED async_handshake(
        handshake_type type,
        const ConstBufferSequence & buffers,
        BufferedHandshakeToken && token = default_completion_token_t< executor_type >(),
        constraint_t< is_const_buffer_sequence< ConstBufferSequence >::value >  = 0);

This function is used to asynchronously perform an SSL handshake on the stream. It is an initiating function for an asynchronous operation, and always returns immediately.

###### Parameters

type

  The type of handshaking to be performed, i.e. as a client or as a server.

buffers

  The buffered data to be reused for the handshake. Although the buffers object may be copied as necessary, ownership of the underlying buffers is retained by the caller, which must guarantee that they remain valid until the completion handler is called.

token

  The completion token that will be used to produce a completion handler, which will be called when the handshake completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        const asio::error_code& error, // Result of operation.
        std::size_t bytes_transferred // Amount of buffers used in handshake.
      );

  Regardless of whether the asynchronous operation completes immediately or not, the completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `post`.

###### Completion Signature

    void(asio::error_code, std::size_t)

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`

if they are also supported by the `Stream` type's `async_read_some` and `async_write_some` operations.