How can I prevent boost log to duplicate message

Question

I have code where I log different categories of information. Each piece of code has a specific tag that has been made for them. (e.g : database.message, database.odbc, event.cevent ...)

Moreover, I've made a function that reads a json file and their value which corresponds to their according severity filter

{
    "database" :
    {
        "message": "INFO",
        "odbc": "DEBUG",
    },
    "event" :
    {
        "cevent" : "INFO",
    },
}

My problem is, I want to set a "basic filter", (for instance only log messages that are "INFO" or above) for all items whose tags were not set in this file.

Right now, I'm adding filters this way:

logging::core::get()->add_global_attribute("Tag", attrs::constant<std::string>(""));
logging::add_common_attributes();
std::vector<std::string> tags; // Suppose it's already filled with the tags and values from the json

...
for (const auto& tag :tags)
{
    boost::shared_ptr<text_sink> sink(new text_sink(backend)); // same as the doc
    auto level = logging::trivial::info; // just an example for more clarity 
    sink->set_filter(expr::attr<std::string>("Tag") == tag && expr::attr<logging::trivial::severity_level>("Severity") >= level);
    logging::core::get()->add_sink(sink);
}

This piece of code works and correctly reads and sets filter according to the json file.

So to add this "basic filter", I also added this once every filter has been set:

boost::shared_ptr<text_sink> basic_sink(new text_sink(backend));
auto filter = logging::trivial::severity >= logging::trivial::info;
for (const auto& tag : tags)
{
    filter = filter && expr::attr<std::string>("Tag") != tag;
}
basic_sink->set_filter(filter);
logging::core::get()->add_sink(basic_sink);

But it duplicates messages that are defined in the json, when I thought this would filter out tags stored. Do you have any ideas on how to avoid such duplication or do I have to implement such a sink as mentionned in this post

Answer 1

You weren't duplicating messages. You were adding duplicate sinks.

As someone else posted, you want to combine into one filter instead of duplicating your sinks.

However, since the filter expression is a compile-time static template expression that describes a deferred invocation, you need a deferred function to work with it.

I'd use boost::phoenix::function to make it simple:

boost::phoenix::function matching = [tags](logging::value_ref<std::string> actual) {
    return tags.contains(actual.get());
};

Now you can use the single filter expression:

auto filter = matching(_tag) && (logging::trivial::severity >= level);

C++ 11

The above assumed C++17, but you can spell it out for older beasts:

struct match_impl {
    std::set<std::string> target_tags;

    using result_type = bool;
    result_type operator()(logging::value_ref<std::string> actual_tag) const {
        return 0 < target_tags.count(actual_tag.get());
    }
};
boost::phoenix::function<match_impl> matching{match_impl{std::move(tags)}};

If Boost Log supports C++03 I know how to get that done too, but hopefully that's not required.

Full Demo

Live On Coliru

#include <boost/log/sinks/text_file_backend.hpp>
#include <boost/log/sources/logger.hpp>

#include <boost/log/attributes/scoped_attribute.hpp>
#include <boost/log/trivial.hpp>
#include <boost/log/utility/setup.hpp>

#include <boost/phoenix.hpp>
#include <set>

namespace logging = boost::log;
namespace sinks   = logging::sinks;
namespace attrs   = logging::attributes;

void init_logging(logging::trivial::severity_level level, std::set<std::string> tags) {

    auto core = logging::core::get();
    using text_sink = sinks::synchronous_sink<sinks::text_ostream_backend>;

    core->add_global_attribute("Tag", attrs::constant<std::string>(""));
    logging::add_common_attributes();

    auto backend = boost::make_shared<sinks::text_ostream_backend>();
    auto sink    = boost::make_shared<text_sink>(backend);
    sink->locked_backend()->add_stream(
        boost::shared_ptr<std::ostream>(&std::clog, boost::null_deleter{}));

#if __cplusplus < 201703L
    struct match_impl {
        std::set<std::string> target_tags;

        using result_type = bool;
        result_type operator()(logging::value_ref<std::string> actual_tag) const {
            return 0 < target_tags.count(actual_tag.get());
        }
    };
    boost::phoenix::function<match_impl> matching{match_impl{std::move(tags)}};
#else
    boost::phoenix::function matching = [tags](logging::value_ref<std::string> actual) {
        return tags.contains(actual.get());
    };
#endif

    auto _tag   = boost::log::expressions::attr<std::string>("Tag");
    auto filter = matching(_tag) && (logging::trivial::severity >= level);

    sink->set_filter(filter);
    core->add_sink(sink);
}

int main() {
    init_logging(logging::trivial::severity_level::error, {"foo", "bar", "qux"});

    for (std::string tag : {"foo", "bogus", "bar"}) {
        BOOST_LOG_SCOPED_THREAD_ATTR("Tag", attrs::constant<std::string>(tag));
        BOOST_LOG_TRIVIAL(debug)   << "debug   tagged with " << tag;
        BOOST_LOG_TRIVIAL(error)   << "error   tagged with " << tag;
        BOOST_LOG_TRIVIAL(fatal)   << "fatal   tagged with " << tag;
        BOOST_LOG_TRIVIAL(info)    << "info    tagged with " << tag;
        BOOST_LOG_TRIVIAL(trace)   << "trace   tagged with " << tag;
        BOOST_LOG_TRIVIAL(warning) << "warning tagged with " << tag;
    }
}

Prints the expected

error   tagged with foo
fatal   tagged with foo
error   tagged with bar
fatal   tagged with bar