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Trivial logging with filters

While severity levels can be used for informative purposes, you will normally want to apply filters to output only significant records and ignore the rest. It is easy to do so by setting a global filter in the library core, like this:

void init()
        logging::trivial::severity >= logging::trivial::info

int main(int, char*[])

    BOOST_LOG_TRIVIAL(trace) << "A trace severity message";
    BOOST_LOG_TRIVIAL(debug) << "A debug severity message";
    BOOST_LOG_TRIVIAL(info) << "An informational severity message";
    BOOST_LOG_TRIVIAL(warning) << "A warning severity message";
    BOOST_LOG_TRIVIAL(error) << "An error severity message";
    BOOST_LOG_TRIVIAL(fatal) << "A fatal severity message";

    return 0;

See the complete code.

Now, if we run this code sample, the first two log records will be ignored, while the remaining four will pass on to the console.

[Important] Important

Remember that the streaming expression is only executed if the record passed filtering. Don't specify business-critical calls in the streaming expression, as these calls may not get invoked if the record is filtered away.

A few words must be said about the filter setup expression. Since we're setting up a global filter, we have to acquire the logging core instance. This is what logging::core::get() does - it returns a pointer to the core singleton. The set_filter method of the logging core sets the global filtering function.

The filter in this example is built as a Boost.Phoenix lambda expression. In our case, this expression consists of a single logical predicate, whose left argument is a placeholder that describes the attribute to be checked, and the right argument is the value to be checked against. The severity keyword is a placeholder provided by the library. This placeholder identifies the severity attribute value in the template expressions; this value is expected to have name "Severity" and type severity_level. This attribute is automatically provided by the library in case of trivial logging; the user only has to supply its value in logging statements. The placeholder along with the ordering operator creates a function object that will be called by the logging core to filter log records. As a result, only log records with severity level not less than info will pass the filter and end up on the console.

It is possible to build more complex filters, combining logical predicates like this with each other, or even define your own function (including a C++11 lambda function) that would act as a filter. We will return to filtering in the following sections.