filter
fileter添加条件

BOOST_LOG_ATTRIBUTE_KEYWORD(line_id, "LineID", unsigned int)
BOOST_LOG_ATTRIBUTE_KEYWORD(severity, "Severity", severity_level)
BOOST_LOG_ATTRIBUTE_KEYWORD(tag_attr, "Tag", std::string)

void init()
{
    // Setup the common formatter for all sinks
    logging::formatter fmt = expr::stream
        << std::setw(6) << std::setfill('0') << line_id << std::setfill(' ')
        << ": <" << severity << ">\t"
        << expr::if_(expr::has_attr(tag_attr))  // 判断是否包含tag_attr
           [
               expr::stream << "[" << tag_attr << "] "
           ]
        << expr::smessage;

    // Initialize sinks
    typedef sinks::synchronous_sink< sinks::text_ostream_backend > text_sink;
    boost::shared_ptr< text_sink > sink = boost::make_shared< text_sink >(); //创建第一个frontend_sink

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

    sink->set_formatter(fmt);

    logging::core::get()->add_sink(sink);   // 添加所有信息输出的sink，保存到 full.log

    sink = boost::make_shared< text_sink >(); //创建第一个frontend_sink

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

    sink->set_formatter(fmt);

    sink->set_filter(severity >= warning || (expr::has_attr(tag_attr) && tag_attr == "IMPORTANT_MESSAGE")); //设置filer

    logging::core::get()->add_sink(sink);  // 添加只输出关键信息的sink，保存到 important.log

    // Add attributes
    logging::add_common_attributes();
}

使用Boost.Phoenix

bool my_filter(logging::value_ref< severity_level, tag::severity > const& level,
               logging::value_ref< std::string, tag::tag_attr > const& tag)
{
    return level >= warning || tag == "IMPORTANT_MESSAGE";
}

void init()
{
    // ...

    namespace phoenix = boost::phoenix;
    sink->set_filter(phoenix::bind(&my_filter, severity.or_none(), tag_attr.or_none()));

    // ...
}

streaming expression 只在filter通过后才执行，所以至关重要的信息不要用steaming expression输出，防止被filter掉。

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