Asynchronously establishes a socket connection by trying each endpoint in a sequence.
template< typename Protocol, typename Executor, typename Iterator, typename ConnectCondition, typename IteratorConnectToken = DEFAULT> DEDUCED async_connect( basic_socket< Protocol, Executor > & s, Iterator begin, Iterator end, ConnectCondition connect_condition, IteratorConnectToken && token = DEFAULT);
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. It is an initiating function for an asynchronous operation,
and always returns immediately.
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:
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.
The completion
token that will be used to produce a completion handler, which
will be called when the connect completes. Potential completion tokens
include use_future
,
use_awaitable
,
yield_context
,
or a function object with the correct completion signature. The function
signature of the completion handler must be:
void handler( // Result of operation. if the sequence is empty, set to // asio::error::not_found. Otherwise, contains the // error from the last connection attempt. const asio::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 completion handler will not be invoked from within this
function. On immediate completion, invocation of the handler will
be performed in a manner equivalent to using post
.
void(asio::error_code, Iterator)
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(my_context); tcp::resolver::query q("host", "service"); tcp::socket s(my_context); // ... r.async_resolve(q, resolve_handler); // ... void resolve_handler( const asio::error_code& ec, tcp::resolver::iterator i) { if (!ec) { tcp::resolver::iterator end; asio::async_connect(s, i, end, my_connect_condition(), connect_handler); } } // ... void connect_handler( const asio::error_code& ec, tcp::resolver::iterator i) { if (ec) { // An error occurred. } else { std::cout << "Connected to: " << i->endpoint() << std::endl; } }
This asynchronous operation supports cancellation for the following cancellation_type
values:
cancellation_type::terminal
cancellation_type::partial
if they are also supported by the socket's async_connect
operation.