asio C++ library


connect (7 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,
    Iterator end,
    ConnectCondition connect_condition);

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.


An iterator pointing to the end 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 asio::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.

Return Value

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



Thrown on failure. If the sequence is empty, the associated error_code is asio::error::not_found. Otherwise, contains the error from the last connection attempt.


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 asio::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 asio::connect function as follows:

tcp::resolver r(io_service);
tcp::resolver::query q("host", "service");
tcp::resolver::iterator i = r.resolve(q), end;
tcp::socket s(io_service);
i = asio::connect(s, i, end, my_connect_condition());
std::cout << "Connected to: " << i->endpoint() << std::endl;