### SSL handshake token requirements

A handshake token is a completion token for completion signature `void(error_code)`.

##### Examples

A free function as a handshake token:

    void handshake_handler(
        const asio::error_code& ec)
    {
      ...
    }

A handshake token function object:

    struct handshake_handler
    {
      ...
      void operator()(
          const asio::error_code& ec)
      {
        ...
      }
      ...
    };

A lambda as a handshake token:

    ssl_stream.async_handshake(...,
        [](const asio::error_code& ec)
        {
          ...
        });

A non-static class member function adapted to a handshake token using `std::bind()`:

    void my_class::handshake_handler(
        const asio::error_code& ec)
    {
      ...
    }
    ...
    ssl_stream.async_handshake(...,
        std::bind(&my_class::handshake_handler,
          this, std::placeholders::_1));

A non-static class member function adapted to a handshake token using `boost::bind()`:

    void my_class::handshake_handler(
        const asio::error_code& ec)
    {
      ...
    }
    ...
    ssl_stream.async_handshake(...,
        boost::bind(&my_class::handshake_handler,
          this, asio::placeholders::error));

Using use_future as a handshake token:

    std::future<void> f = ssl_stream.async_handshake(..., asio::use_future);
    ...
    try
    {
      f.get();
    }
    catch (const system_error& e)
    {
      ...
    }

Using use_awaitable as a handshake token:

    asio::awaitable<void> my_coroutine()
    {
      try
      {
        ...
        co_await ssl_stream.async_handshake(..., asio::use_awaitable);
        ...
      }
      catch (const system_error& e)
      {
        ...
      }
    }