async_connect (1 of 6 overloads)

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

template<
    typename Protocol,
    typename EndpointSequence,
    typename RangeConnectHandler>
DEDUCED async_connect(
    basic_socket< Protocol > & s,
    const EndpointSequence & endpoints,
    RangeConnectHandler && handler,
    typename enable_if< 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.

Parameters

s

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

endpoints

A sequence of endpoints.

handler

The handler to be called when the connect operation completes. Copies will be made of the handler as required. The function signature of the 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 handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using boost::asio::io_context::post().

Example

tcp::resolver r(io_context);
tcp::resolver::query q("host", "service");
tcp::socket s(io_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)
{
  // ...
}