Boost set_filter не работает

Я учусь Boost. Следуя инструкциям, я пытаюсь установить фильтр для приемника, отправив ссылку на метод onlyWarnings.

Коротко:

sink->set_filter(&onlyWarnings);

Только в предупреждениях:

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

Я явно что-то упускаю в своем коде и важной части учебника.

ЗАГОЛОВОК:

#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;
}