Boost set_filter is not working

问题

I'm learning Boost. Following a tutorial, I try to set a filter on a sink by sending a reference to the method onlyWarnings.

Brief:

sink->set_filter(&onlyWarnings);

In onlyWarnings:

set["Severity"].extract<int>()  // is always 0

I'm obviously missing something in my code and an important part of the tutorial.

HEADER:

#ifndef ONEPRINT_LOGGER_H
#define ONEPRINT_LOGGER_H

#include <boost/log/core/core.hpp>
#include <boost/log/attributes/attribute_value_set.hpp>
#include <boost/log/trivial.hpp>
#include <boost/log/sources/severity_logger.hpp>
#include <boost/log/utility/setup/file.hpp>
#include <boost/log/utility/setup/console.hpp>
#include <boost/log/utility/setup/common_attributes.hpp>
#include <boost/log/sinks.hpp>
#include <boost/core/null_deleter.hpp>
#include <iostream>

namespace expr = boost::log::expressions;
namespace sources = boost::log::sources;
namespace sinks = boost::log::sinks;
namespace keywords = boost::log::keywords;

namespace ids {

    enum severity_level
    {
        normal,
        warning,
        error,
        critical
    };

    class Logger {
    public:
        Logger();
        ~Logger();
        void logIt(std::string msg);

    protected:
        typedef sinks::asynchronous_sink<sinks::text_ostream_backend> asynchronousSink;
        void setupLogging();
    };

}
#endif //ONEPRINT_LOGGER_H

CPP:

#include "Logger.h"

class counter;

using namespace ids;

namespace {  // NON-MEMBER METHODS
    bool onlyWarnings(const boost::log::attribute_value_set& set)
    {
        return set["Severity"].extract<int>() > 0;
    }

    void severity_and_message(const boost::log::record_view &view, boost::log::formatting_ostream &os)
    {
        os << view.attribute_values()["Severity"].extract<int>() << ": " <<
        view.attribute_values()["Message"].extract<std::string>();
    }
}

Logger::Logger() {
    setupLogging();
    logIt("Testing");
}

Logger::~Logger() {

}

void Logger::setupLogging()
{
    boost::shared_ptr< boost::log::core > core = boost::log::core::get();
    boost::shared_ptr<sinks::text_ostream_backend> backend = boost::make_shared<sinks::text_ostream_backend>();

    boost::shared_ptr<Logger::asynchronousSink> sink(new Logger::asynchronousSink(backend));
    boost::shared_ptr<std::ostream> stream{&std::clog, boost::null_deleter{}};
    sink->locked_backend()->add_stream(stream);

    sink->set_filter(&onlyWarnings);
    sink->set_formatter(&severity_and_message);

    core->add_sink(sink);
}

void Logger::logIt(std::string msg) {
    BOOST_LOG_TRIVIAL(warning) << msg;

    sources::severity_logger<severity_level> severityLogger;
    BOOST_LOG_SEV(severityLogger, critical) << msg;
}

回答1:

Are you sure that severity is just another attribute? I would recommend that you start from a working example, like this one:

http://www.boost.org/doc/libs/1_56_0/libs/log/doc/html/log/tutorial/advanced_filtering.html

http://boost-log.sourceforge.net/libs/log/example/doc/tutorial_filtering.cpp

Did you get them working before you started your own coding? The tag-severity logger has an example filter that looks very different from yours.

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";
}

Perhaps try something more like:

bool my_filter(logging::value_ref< severity_level, tag::severity > const& level)
{
    return level >= warning ;
}

来源：https://stackoverflow.com/questions/29707017/boost-set-filter-is-not-working