C++ – How to add logging to a library so it can integrate easily with the logging system of the program using the library

clogging

I'm writing a library that has a lot of information that may be of use in a log in the program which uses it, but I don't know the best way to expose it in such a way that the program using my library can integrate my library's logs with its own logs seemlessly (if desired).

Picking a specific logging library for my library adds to the list of dependencies for using my library as well as ties the main program to that library — and if multiple libraries used by the main program did this, there could each have selected a different library.

I've though about having the program register a C++ stream object with the library for it to use. That seems like it would be relatively general purpose, but I also thought about just having the main program register a callback function which would be called for with the contents and metadata when data is logged. Another option would just be to store the log data in the library in some sort of list for the main program to grab whenever it wants to deal with that data, letting the main program decide when it has time to deal with the data.

I'm looking for suggestions and pros/cons of different approaches so I can decide what is best in my situation.

Best Answer

You can expose multiple methods to receive the logging from your library, and wrap all but one in adaptors to the "real" one used in the library.

E.g. you decide to internally have a std::function<void(std::string)> collection that is each logger callback. You provide:

void registerLogCallback(std::function<void(std::string)> callback); 
// Main logging implemention

and also

registerLogStream(std::ostream stream) { 
    registerLogCallback([stream](std::string message){ stream << message; }); 
}

and also

template<typename OutputIterator>
registerLogOutputIterator(OutputIterator iter) { 
    registerLogCallback([iter](std::string message){ *iter++ = message; }); 
}

and any more variations of "recieve strings from somewhere" types you care to implement adaptors for.

Related Solutions

What to Do with Production Error Logs

If you are using them to eliminate the bugs being reported, then they are doing their job.

They are useful NOW in the identification of current bugs, but if they are all fixed then I'm not sure there is much value in keeping the old logs once issues from them have been dealt with.

Forcing logger/appender to output line logged at lower than its current level

So, you need different treshold within the same class. I think you use a different logger corresponding to a specific name. You can thus set the log level to INFO for this specific Logger in DispatchServlet class.

For me, the benefits would be:

the possibility to specificly reduce the treshold to INFO as you required (the goal).
if one day you realize that your specific logs become too verbose and finally useless because the functionnality is 100% stable and doesn't cause any issue, you still can reduce the treshold by configuration and without impacting the rest of the logs. Logging in WARNING level or do something based on forcedLog would imply you to modify the sources for such a purpose.

The only CON I see is that it might be "surprising" but a correct documentation solves everything.

For example in your log4j.xml file, you could have:

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
    <!-- general application log -->
    <appender name="MyLogFileAppender" class="org.apache.log4j.FileAppender">
        <param name="File" value="mylog.log" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d{yyyy MM dd - HH:mm:ss,SSS} %-5p %t [%-40.40c] %x - %m%n"/>
        </layout>
    </appender>
    <logger name="DispatchServletSpecific">
        <level value="INFO" />
        <appender-ref ref="MyLogFileAppender" />
    </logger>
    <root>
         <level value="WARNING" />
         <appender-ref ref="MyLogFileAppender" />
    </root>
</log4j:configuration>

And in the Java code you create a specific Logger

Logger specificLogger = Logger.getLogger("DispatchServletSpecific");

This way the log treshold should be set to INFO while the rest of your app should be log only WARNING logs. Hope it helps.

Best Answer

Related Solutions

What to Do with Production Error Logs

Forcing logger/appender to output line logged at lower than its current level

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