Question

The following page provides a nice simple solution for file based logging in Qt for debugging without using a larger logging framework like the many that are suggested in other SO questions.

I’m writing a library and would like to instantiate a logger that the classes in the library can use (mostly for debugging purposes). There is no int main() function since it’s a library. So would the best approach be to add the instantiation into a file like logger.h and have any classes include logger.h if it would like to do qDebug() << PREFIX << "Bla" as the link above suggests?

Answer 1

I pretty much agree with OrcunC but I’d recommend making that ofstream a little more accessible and capable of handling the Qt value types.

Here’s my recommended process:

1. Create a global QIODevice that to which everything will be written. This will probably be a QFile.
2. Create a QTextStream wrapper around that QIODevice that you’ll then use for all the logging.
3. If you want something slightly more complicated, create methods that do the filtering based on log level info.

For example:

// setup the global logger somewhere appropriate
QFile *file = new QFile("your.log");
file->open(QIODevice::ReadOnly);
QTextStream *qlogger = new QTextStream(file);

And once the global logger is initialized, you could reference it as a global:

#include "qlogger.h"
//... and within some method
*qlogger << "your log" << aQtValueType;

But you might want some filtering:

#include "qlogger.h"
// lower number = higher priority
void setCurrentLogLevel(int level) {
   globalLogLevel = level;
}
QTextStream* qLog(int level) {
   if (level <= globalLogLevel) {
       return qlogger;
   }
   return getNullLogger(); // implementation left to reader
}

And then you’d likely create an enum that represented the LogLevel and do something like this:

#include "qlogger.h"
//...
setCurrentLogLevel(LogLevel::Warning);
*qLog(LogLevel::Debug) << "this will be filtered" << yourQMap;
*qLog(LogLevel::Critical) << "not filtered" << yourQString;

As you’d be dealing with globals, carefully consider memory management issues.