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connect (6 of 8 overloads)

Establishes a socket connection by trying each endpoint in a sequence.

    typename Protocol,
    typename SocketService,
    typename Iterator,
    typename ConnectCondition>
Iterator connect(
    basic_socket< Protocol, SocketService > & s,
    Iterator begin,
    ConnectCondition connect_condition,
    boost::system::error_code & ec);

This function attempts to connect a socket to one of a sequence of endpoints. It does this by repeated calls to the socket's 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.


Set to indicate what error occurred, if any. If the sequence is empty, set to boost::asio::error::not_found. Otherwise, contains the error from the last connection attempt.

Return Value

On success, an iterator denoting the successfully connected endpoint. Otherwise, the end iterator.


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);
boost::system::error_code ec;
tcp::resolver::iterator i = boost::asio::connect(
    s, r.resolve(q), my_connect_condition(), ec);
if (ec)
  // An error occurred.
  std::cout << "Connected to: " << i->endpoint() << std::endl;