async_connect (1 of 6 overloads)

Asynchronously establishes a socket connection by trying each endpoint in a sequence.

template<
    typename Protocol,
    typename Executor,
    typename EndpointSequence,
    typename RangeConnectToken = default_completion_token_t<Executor>>
DEDUCED async_connect(
    basic_socket< Protocol, Executor > & s,
    const EndpointSequence & endpoints,
    RangeConnectToken && token = default_completion_token_t< Executor >(),
    constraint_t< is_endpoint_sequence< EndpointSequence >::value >  = 0);

This function attempts to connect a socket to one of a sequence of endpoints. It does this by repeated calls to the socket's async_connect member function, once for each endpoint in the sequence, until a connection is successfully established. It is an initiating function for an asynchronous operation, and always returns immediately.

Parameters

s

The socket to be connected. If the socket is already open, it will be closed.

endpoints

A sequence of endpoints.

token

The completion token that will be used to produce a completion handler, which will be called when the connect 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(
  // Result of operation. if the sequence is empty, set to
  // boost::asio::error::not_found. Otherwise, contains the
  // error from the last connection attempt.
  const boost::system::error_code& error,

  // On success, the successfully connected endpoint.
  // Otherwise, a default-constructed endpoint.
  const typename Protocol::endpoint& endpoint
);

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(boost::system::error_code, typename Protocol::endpoint)

Example

tcp::resolver r(my_context);
tcp::resolver::query q("host", "service");
tcp::socket s(my_context);

// ...

r.async_resolve(q, resolve_handler);

// ...

void resolve_handler(
    const boost::system::error_code& ec,
    tcp::resolver::results_type results)
{
  if (!ec)
  {
    boost::asio::async_connect(s, results, connect_handler);
  }
}

// ...

void connect_handler(
    const boost::system::error_code& ec,
    const tcp::endpoint& endpoint)
{
  // ...
}

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 socket's async_connect operation.