async_connect (6 of 6 overloads)

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

template<
    typename Protocol,
    typename Iterator,
    typename ConnectCondition,
    typename IteratorConnectHandler>
DEDUCED async_connect(
    basic_socket< Protocol > & s,
    Iterator begin,
    Iterator end,
    ConnectCondition connect_condition,
    IteratorConnectHandler && 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.

Parameters

s

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

begin

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

end

An iterator pointing to the end of 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 boost::system::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.

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, 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_context::post().

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 boost::system::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 boost::asio::connect function as follows:

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::iterator i)
{
  if (!ec)
  {
    tcp::resolver::iterator end;
    boost::asio::async_connect(s, i, end,
        my_connect_condition(),
        connect_handler);
  }
}

// ...

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