async_connect (4 of 6 overloads)

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

template<
    typename Protocol,
    typename Executor,
    typename EndpointSequence,
    typename ConnectCondition,
    typename RangeConnectToken = DEFAULT>
DEDUCED async_connect(
    basic_socket< Protocol, Executor > & s,
    const EndpointSequence & endpoints,
    ConnectCondition connect_condition,
    RangeConnectToken && token = DEFAULT,
    typename constraint< is_endpoint_sequence< EndpointSequence >::value >::type  = 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.

connect_condition

A function object that is called prior to each connection attempt. The signature of the function object must be:

bool connect_condition(
    const asio::error_code& ec,
    const typename Protocol::endpoint& next);

The ec parameter contains the result from the most recent connect operation. Before the first connection attempt, ec is always set to indicate success. The next parameter is the next endpoint to be tried. The function object should return true if the next endpoint should be tried, and false if it should be skipped.

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
  // asio::error::not_found. Otherwise, contains the
  // error from the last connection attempt.
  const asio::error_code& error,

  // On success, an iterator denoting the successfully
  // connected endpoint. Otherwise, the end iterator.
  Iterator iterator
);

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, typename Protocol::endpoint)

Example

The following connect condition function object can be used to output information about the individual connection attempts:

struct my_connect_condition
{
  bool operator()(
      const asio::error_code& ec,
      const::tcp::endpoint& next)
  {
    if (ec) std::cout << "Error: " << ec.message() << std::endl;
    std::cout << "Trying: " << next << std::endl;
    return true;
  }
};

It would be used with the asio::connect function as follows:

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 asio::error_code& ec,
    tcp::resolver::results_type results)
{
  if (!ec)
  {
    asio::async_connect(s, results,
        my_connect_condition(),
        connect_handler);
  }
}

// ...

void connect_handler(
    const asio::error_code& ec,
    const tcp::endpoint& endpoint)
{
  if (ec)
  {
    // An error occurred.
  }
  else
  {
    std::cout << "Connected to: " << endpoint << std::endl;
  }
}

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.