dlib C++ Library - logger_ex_2.cpp

    // The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt

    /*

        This is a somewhat complex example illustrating the use of the logger object
        from the dlib C++ Library.  It will demonstrate using multiple loggers and threads.

        The output of this program looks like this:
        0 INFO  [0] example: This is an informational message.
        0 WARN  [0] example: The variable is bigger than 4!  Its value is 8
        0 INFO  [0] example: make two threads
        0 WARN  [0] example.test_class: warning!  someone called warning()!
        0 INFO  [0] example: we are going to sleep for half a second.
        0 INFO  [1] example.thread: entering our thread
        0 WARN  [1] example.test_class: warning!  someone called warning()!
        0 INFO  [2] example.thread: entering our thread
        0 WARN  [2] example.test_class: warning!  someone called warning()!
      203 INFO  [1] example.thread: exiting our thread
      203 INFO  [2] example.thread: exiting our thread
      503 INFO  [0] example: we just woke up
      503 INFO  [0] example: program ending

    */

    #include <dlib/logger.h>
    #include <dlib/misc_api.h>
    #include <dlib/threads.h>

    using namespace dlib;

    /*
        Here we create three loggers.  Note that it is the case that:
            - logp.is_child_of(logp) == true
            - logt.is_child_of(logp) == true
            - logc.is_child_of(logp) == true

        logp is the child of itself because all loggers are their own children :)  But the other
        two are child loggers of logp because their names start with logp.name() + "." which means
        that whenever you set a property on a logger it will also set that same property on all of
        the logger's children.
    */
    logger logp("example");
    logger logt("example.thread");
    logger logc("example.test_class");

    class test
    {
    public:
        test ()
        {
            // this message won't get logged because LINFO is too low
            logc << LINFO << "constructed a test object";
        }

        ~test ()
        {
            // this message won't get logged because LINFO is too low
            logc << LINFO << "destructed a test object";
        }

        void warning ()
        {
            logc << LWARN << "warning!  someone called warning()!";
        }
    };

    void thread (void*)
    {
        logt << LINFO << "entering our thread";

        test mytest;
        mytest.warning();

        dlib::sleep(200);

        logt << LINFO << "exiting our thread";
    }

    void setup_loggers (
    )
    {
        // Create a logger that has the same name as our root logger logp.  This isn't very useful in
        // this example program but if you had loggers defined in other files then you might not have
        // easy access to them when starting up your program and setting log levels.  This mechanism
        // allows you to manipulate the properties of any logger so long as you know its name.
        logger temp_log("example");

        // For this example I don't want to log debug messages so I'm setting the logging level of
        // All our loggers to LINFO.  Note that this statement sets all three of our loggers to this
        // logging level because they are all children of temp_log.
        temp_log.set_level(LINFO);

        // In addition I only want the example.test_class to print LWARN or higher messages so I'm going
        // to set that here too.  Note that we set this value after calling temp_log.set_level(). If we
        // did it the other way around the set_level() call on temp_log would set logc_temp.level() and
        // logc.level() back to LINFO since temp_log is a parent of logc_temp.
        logger logc_temp("example.test_class");
        logc_temp.set_level(LWARN);

        // Finally, note that you can also configure your loggers from a text config file.
        // See the documentation for the configure_loggers_from_file() function for details.
    }

    int main()
    {
        setup_loggers();

        // print our first message.  It will go to cout because that is the default.
        logp << LINFO << "This is an informational message.";

        int variable = 8;

        // Here is a debug message.  It won't print though because its log level is too low (it is below LINFO).
        logp << LDEBUG << "The integer variable is set to " << variable;

        if (variable > 4)
            logp << LWARN << "The variable is bigger than 4!  Its value is " << variable;

        logp << LINFO << "make two threads";
        create_new_thread(thread,0);
        create_new_thread(thread,0);

        test mytest;
        mytest.warning();

        logp << LINFO << "we are going to sleep for half a second.";
        // sleep for half a second
        dlib::sleep(500);
        logp << LINFO << "we just woke up";

        logp << LINFO << "program ending";

        // It is also worth pointing out that the logger messages are atomic.  This means, for example, that
        // in the above log statements that involve a string literal and a variable, no other thread can
        // come in and print a log message in-between the literal string and the variable.  This is good
        // because it means your messages don't get corrupted.  However, this also means that you shouldn't
        // make any function calls inside a logging statement if those calls might try to log a message
        // themselves since the atomic nature of the logger would cause your application to deadlock.
    }