Boost trivial log - custom severity name

Question

I am using the boost trivial log with the default severity levels and I would like to map custom text for those default severity levels (e.g. instead of warning use warn).

In my formatter I use << boost::log::trivial::severity << " " << boost::log::expressions::smessage;

Logging a warning would be something like this:

BOOST_LOG_TRIVIAL(warning) << "Some log";

// What I would like to get logged
warn Some log

Here I saw an example of defining a custom severity_level (which did not work for me either) but in would prefer to keep using the default severity levels and just be able to define an operator<< for it in order to change the severity level text.

I tried to define an custom operator<< for the trivial severity_level but I did not manage to make it work, not sure if that would even be possible.

Answer 1

In order to customize log record formatting you will have to configure a sink that will process log records. For the purpose of this answer I will assume you have created and added a sink that fits your use case.

You can use your own function for converting severity_level values to strings by injecting it into the formatter expression. Given that the expression is implemented using Boost.Phoenix, you can use boost::phoenix::bind for that:

std::string format_severity_level(
    boost::log::value_ref<boost::log::trivial::severity_level,
        boost::log::trivial::tag::severity> const& level)
{
    std::string str;

    // Check if the log record has a severity level attribute
    if (level)
    {
        switch (*level)
        {
        case boost::log::trivial::severity_level::info:
            str = "info";
            break;

        // ...
        }
    }

    return str;
}

// Set the formatter in the sink
sink->set_formatter
(
    boost::log::expressions::stream
        << "["
        << boost::phoenix::bind(
               &format_severity_level, boost::log::trivial::severity.or_none())
        << "] " << boost::log::expressions::smessage
);

Here, boost::phoenix::bind creates a function object that is injected into the formatter expression and gets called when the formatter is invoked. The boost::log::trivial::severity placeholder is used to extract the severity level attribute value from the log record and pass it, wrapped in value_ref, to your format_severity_level function that would convert it to a string. The .or_none() call indicates that in case if no such attribute is attached to the log record, the call should proceed with an empty value_ref, which format_severity_level is supposed to test for.