boost::asio::async_connect

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

Synopsis

Declared in <boost/asio/connect.hpp>

template<
    typename Protocol,
    typename Executor,
    typename Iterator,
    boost::asio::completion_token_for<void(boost::system::error_code, Iterator)> IteratorConnectToken = default_completion_token_t<Executor>>
decltype(async_initiate<IteratorConnectToken,
      void (boost::system::error_code, Iterator)>(
        declval<detail::initiate_async_iterator_connect<Protocol, Executor>>(),
        token, begin, end, declval<detail::default_connect_condition>()))
async_connect(
    basic_socket<Protocol, Executor>& s,
    Iterator begin,
    Iterator end,
    IteratorConnectToken&& token = default_completion_token_t<Executor>(),
    int = 0);

Description

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.

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 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 boost::asio::async_immediate().

Completion Signature

void(boost::system::error_code, Iterator)

Example

std::vector<tcp::endpoint> endpoints = ...;
tcp::socket s(my_context);
boost::asio::async_connect(s,
    endpoints.begin(), endpoints.end(),
    connect_handler);

// ...

void connect_handler(
    const boost::system::error_code& ec,
    std::vector<tcp::endpoint>::iterator i)
{
  // ...
}

Per‐Operation Cancellation

This asynchronous operation supports cancellation for the following boost::asio::cancellation_type values:

cancellation_type::terminal
cancellation_type::partial

if they are also supported by the socket's async_connect operation.

Parameters

Name

Description

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.

token

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:

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