常用formatting

默认不设置formatter，所有添加的attribut都不会在log record中出现

添加时间戳的formating

void init()
{
    logging::add_file_log  // 等价于设置了file_sink，也可以手工分别建frontend_sink和backen_sink
    (
        keywords::file_name = "sample_%N.log",
        keywords::rotation_size = 10 * 1024 * 1024,
        keywords::time_based_rotation = sinks::file::rotation_at_time_point(0, 0, 0),
        keywords::format = "[%TimeStamp%]: %Message%"  /*TimeStamp是之前设的placeholder,Message是log record内容*/
    );

    logging::core::get()->set_filter
    (
        logging::trivial::severity >= logging::trivial::info
    );
}

添加行号和log等级

void init()
{
    logging::add_file_log
    (
        keywords::file_name = "sample_%N.log",
        // This makes the sink to write log records that look like this:
        // 1: <normal> A normal severity message
        // 2: <error> An error severity message
        keywords::format =
        (
            expr::stream
                << expr::attr< unsigned int >("LineID")
                << ": <" << logging::trivial::severity
                << "> " << expr::smessage
        )
    );
}

add_file_log函数等价于

void init()
{
    typedef sinks::synchronous_sink< sinks::text_ostream_backend > text_sink;
    boost::shared_ptr< text_sink > sink = boost::make_shared< text_sink >();

    sink->locked_backend()->add_stream(
        boost::make_shared< std::ofstream >("sample.log"));

    sink->set_formatter
    (
        expr::stream
               // line id will be written in hex, 8-digits, zero-filled
            << std::hex << std::setw(8) << std::setfill('0') << expr::attr< unsigned int >("LineID")
            << ": <" << logging::trivial::severity
            << "> " << expr::smessage
    );

    logging::core::get()->add_sink(sink);
}

借助Boost.Format设置formating

expr::format("%1%: <%2%> %3%")
            % expr::attr< unsigned int >("LineID")
            % logging::trivial::severity
            % expr::smessage

借助Boost.DateTime设置时间格式

expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%Y-%m-%d %H:%M:%S")

为backend_sink添加formatting

借助void (logging::record_view const& rec, logging::basic_formatting_ostream< CharT >& strm);
record_view是在frontend_sink中format后的log record，是只读的，可被不同的backend_sink用作最终输出数据

void my_formatter(logging::record_view const& rec, logging::formatting_ostream& strm)
{
    // Get the LineID attribute value and put it into the stream
    strm << logging::extract< unsigned int >("LineID", rec) << ": ";

    // The same for the severity level.
    // The simplified syntax is possible if attribute keywords are used.
    strm << "<" << rec[logging::trivial::severity] << "> ";

    // Finally, put the record message to the stream
    strm << rec[expr::smessage];
}

void init()
{
    typedef sinks::synchronous_sink< sinks::text_ostream_backend > text_sink;
    boost::shared_ptr< text_sink > sink = boost::make_shared< text_sink >();

    sink->locked_backend()->add_stream(
        boost::make_shared< std::ofstream >("sample.log"));

    sink->set_formatter(&my_formatter);

    logging::core::get()->add_sink(sink);
}

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