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basic_stream::async_connect (3 of 5 overloads)

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

Synopsis
template<
    class EndpointSequence,
    class ConnectCondition,
    class RangeConnectHandler = net::default_completion_token_t<executor_type>>
DEDUCED
async_connect(
    EndpointSequence const& endpoints,
    ConnectCondition connect_condition,
    RangeConnectHandler&& handler = net::default_completion_token_t< executor_type >{});
Description

This function attempts to connect the stream to one of a sequence of endpoints by trying each endpoint until a connection is successfully established. The underlying socket is automatically opened if needed. An automatically opened socket is not returned to the closed state upon failure. The algorithm, known as a composed asynchronous operation, is implemented in terms of calls to the underlying socket's async_connect function. If the timeout timer expires while the operation is outstanding, the current connection attempt will be canceled and the completion handler will be invoked with the error error::timeout.

Parameters

Name

Description

endpoints

A sequence of endpoints. This this object must meet the requirements of EndpointSequence.

connect_condition

A function object that is called prior to each connection attempt. The signature of the function object must be:

bool connect_condition(
    error_code const& ec,
    typename Protocol::endpoint const& 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 completion handler to invoke when the operation completes. The implementation takes ownership of the handler by performing a decay-copy. The equivalent function signature of the handler must be:

void handler(
    // Result of operation. if the sequence is empty, set to
    // net::error::not_found. Otherwise, contains the
    // error from the last connection attempt.
    error_code const& error,

    // On success, the successfully connected endpoint.
    // Otherwise, a default-constructed endpoint.
    typename Protocol::endpoint const& endpoint
);

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 net::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()(
        error_code const& ec,
        net::ip::tcp::endpoint const& next)
    {
        if (ec)
            std::cout << "Error: " << ec.message() << std::endl;
        std::cout << "Trying: " << next << std::endl;
        return true;
    }
};

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