connect (8 of 12 overloads)

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

template<
    typename Protocol,
    typename EndpointSequence,
    typename ConnectCondition>
Protocol::endpoint connect(
    basic_socket< Protocol > & s,
    const EndpointSequence & endpoints,
    ConnectCondition connect_condition,
    asio::error_code & ec,
    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 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.

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.

ec

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

Return Value

On success, the successfully connected endpoint. Otherwise, a default-constructed 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(io_context);
tcp::resolver::query q("host", "service");
tcp::socket s(io_context);
asio::error_code ec;
tcp::endpoint e = asio::connect(s,
    r.resolve(q), my_connect_condition(), ec);
if (ec)
{
  // An error occurred.
}
else
{
  std::cout << "Connected to: " << e << std::endl;
}