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)
  {
    ...
  }
}