File based logging in Qt for debugging

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Article

Created: skumar_rao (29 Mar 2010)

Last edited: hamishwillee (11 Oct 2012)

ID		Creation date	28th Mar 2010
Platform	S60 5th Edition	Tested on devices	Nokia N97 Mini
Category	Qt	Subcategory

Keywords (APIs, classes, methods, functions): qInstallMsgHandler

Overview

File Based Logging in Qt For Debugging.

I agree that there are ways to debug application using IDE, but in long run every developers requires a way to print debugging info in to the file. In case of Qt there are standard ways to use file logging as shown below.

• Using Log4Qt
• Using QxtLogger

But the above 2 requires developers either to include a heavier static lib or have to convert to a specific data format (like QVariants or QVariantList) before logging .

Here we will see a simple file logging methods which is very easy to use and requires not more then 25 lines. Before you start logging still it provides same level of flexibility to log debug/info/warning etc.

Step 1.

We will create a logging handler.

#include <fstream>
 
using namespace std;
ofstream logfile;
 
void SimpleLoggingHandler(QtMsgType type, const char *msg) {
    switch (type) {
        case QtDebugMsg:
            logfile << QTime::currentTime().toString().toAscii().data() << " Debug: " << msg << "\n";
            break;
        case QtCriticalMsg:
            logfile << QTime::currentTime().toString().toAscii().data() << " Critical: " << msg << "\n";
            break;
        case QtWarningMsg:
            logfile << QTime::currentTime().toString().toAscii().data() << " Warning: " << msg << "\n";
            break;
        case QtFatalMsg:
            logfile << QTime::currentTime().toString().toAscii().data() <<  " Fatal: " << msg << "\n";
            abort();
        }
    }

Step 2.

Open A file to be where application will be logging for example

    logfile.open("c:\\data\\logfile.txt", ios::app);

Step 3.

Install our handler as a qInstallMsgHandler.

    qInstallMsgHandler(SimpleLoggingHandler);

That is it now we can use the normal qCritical(), qDebug(), qFatal() and qWarning() to direct messages to file. for example

#include <QTime>
 
#define PREFIX "[myDebugPrint]" << QTime::currentTime().toString("hh:mm:ss.zzz")
 
void MyClass::myFunction(int variable)
{
    qDebug() << PREFIX << "MyClass::myFunction() begin";    
    qDebug("code is not activated");
    qDebug() << variable;
    qDebug() << PREFIX << "MyClass::myFunction() end";
}

Reference links

• Qt - cross-platform application and UI framework
• SDK help

--skumar_rao 16:52, 29 March 2010 (UTC)