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async_connect (3 of 4 overloads)

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

    typename Protocol,
    typename SocketService,
    typename Iterator,
    typename ConnectCondition,
    typename ComposedConnectHandler>
void async_connect(
    basic_socket< Protocol, SocketService > & s,
    Iterator begin,
    ConnectCondition connect_condition,
    ComposedConnectHandler handler);

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.



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


An iterator pointing to the start of a sequence of endpoints.


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

Iterator connect_condition(
    const boost::system::error_code& ec,
    Iterator 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 an iterator pointing to the next endpoint to be tried. The function object should return the next iterator, but is permitted to return a different iterator so that endpoints may be skipped. The implementation guarantees that the function object will never be called with the end iterator.


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, an iterator denoting the successfully
  // connected endpoint. Otherwise, the end iterator.
  Iterator iterator

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_service::post().


This overload assumes that a default constructed object of type Iterator represents the end of the sequence. This is a valid assumption for iterator types such as boost::asio::ip::tcp::resolver::iterator.


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

struct my_connect_condition
  template <typename Iterator>
  Iterator operator()(
      const boost::system::error_code& ec,
      Iterator next)
    if (ec) std::cout << "Error: " << ec.message() << std::endl;
    std::cout << "Trying: " << next->endpoint() << std::endl;
    return next;

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

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

// ...

r.async_resolve(q, resolve_handler);

// ...

void resolve_handler(
    const boost::system::error_code& ec,
    tcp::resolver::iterator i)
  if (!ec)
    boost::asio::async_connect(s, i,

// ...

void connect_handler(
    const boost::system::error_code& ec,
    tcp::resolver::iterator i)
  if (ec)
    // An error occurred.
    std::cout << "Connected to: " << i->endpoint() << std::endl;