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    <p>You have to come up with error code and category in order to create <code>error_code</code> object. Here is an example, assuming that error is due to another host refusing connection:</p> <pre><code>error_code ec (errc::connection_refused, system_category()); return ec; </code></pre> <p>You can also pass <code>errno</code> value as error code when using system category. For example:</p> <pre><code>#include &lt;fstream&gt; #include &lt;cerrno&gt; #include &lt;boost/system/system_error.hpp&gt; void foo () { ifstream file ("test.txt"); if (!file.is_open ()) { int err_code = errno; boost::system::error_code ec (err_code , boost::system::system_category ()); throw boost::system::system_error (ec, "cannot open file"); } } </code></pre> <p>Unfortunately, this library is poorly <a href="http://www.boost.org/doc/libs/release/libs/system/doc/index.html" rel="nofollow noreferrer">documented</a>, so I can recommend you to look into <a href="http://www.boost.org/doc/libs/release/boost/system/" rel="nofollow noreferrer">header files</a> to figure things out. The code is fairly simple and straight forward there.</p> <p>Just in case your compiler supports C++11 and you are willing to use it, this functionality made it into standard. As far as I know gcc 4.6.1 has it already. Here is a simple example:</p> <pre><code>#include &lt;cerrno&gt; #include &lt;system_error&gt; std::error_code SystemError::getLastError () { int err_code = errno; return std::error_code (err_code, std::system_category ()); } void foo () { throw std::system_error (getLastError (), "something went wrong"); } </code></pre> <p>Generally, libraries pass <code>error_code</code> object around if there is no need to throw and use <code>system_error</code> to throw an exception describing system failures. Another reason to use <code>error_code</code> without exceptions is when you need to signal the error across different threads. But C++11 has <a href="https://stackoverflow.com/questions/233127/how-can-i-propagate-exceptions-between-threads">a solution for propagating exceptions across threads</a>.</p> <p>Hope it helps!</p>
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    1. POWhich `boost::system::error_code` value should be provided when `boost::asio::ip::tcp::resolver::resolve()` fails?
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