How to redirect qDebug, qWarning, qCritical etc output?

From here all credit goes to spirit.

#include <QApplication>
#include <QtDebug>
#include <QFile>
#include <QTextStream>

void myMessageHandler(QtMsgType type, const QMessageLogContext &, const QString & msg)
{
    QString txt;
    switch (type) {
    case QtDebugMsg:
        txt = QString("Debug: %1").arg(msg);
        break;
    case QtWarningMsg:
        txt = QString("Warning: %1").arg(msg);
        break;
    case QtCriticalMsg:
        txt = QString("Critical: %1").arg(msg);
        break;
    case QtFatalMsg:
        txt = QString("Fatal: %1").arg(msg);
        abort();
    }
    QFile outFile("log");
    outFile.open(QIODevice::WriteOnly | QIODevice::Append);
    QTextStream ts(&outFile);
    ts << txt << endl;
}

int main( int argc, char * argv[] )
{
    QApplication app( argc, argv );
    qInstallMessageHandler(myMessageHandler);   
    ...
    return app.exec();
}

Here is a working example of hooking the default message handler.

Thank you @Ross Rogers!

// -- main.cpp

// Get the default Qt message handler.
static const QtMessageHandler QT_DEFAULT_MESSAGE_HANDLER = qInstallMessageHandler(0);

void myCustomMessageHandler(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
    // Handle the messages!

    // Call the default handler.
    (*QT_DEFAULT_MESSAGE_HANDLER)(type, context, msg);
}

int main(int argc, char *argv[])
{
    qInstallMessageHandler(myCustomMessageHandler);

    QApplication a(argc, argv);

    qDebug() << "Wello Horld!";

    return 0;
}

You've to install a message handler using qInstallMsgHandler function, and then, you can use QTextStream to write the debug message to a file. Here is a sample example:

#include <QtGlobal>
#include <stdio.h>
#include <stdlib.h>

void myMessageOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
    QByteArray localMsg = msg.toLocal8Bit();
    switch (type) {
    case QtDebugMsg:
        fprintf(stderr, "Debug: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtInfoMsg:
        fprintf(stderr, "Info: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtWarningMsg:
        fprintf(stderr, "Warning: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtCriticalMsg:
        fprintf(stderr, "Critical: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtFatalMsg:
        fprintf(stderr, "Fatal: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        abort();
    }
}

int main(int argc, char **argv)
{
    qInstallMessageHandler(myMessageOutput); // Install the handler
    QApplication app(argc, argv);
    ...
    return app.exec();
}

Taken from the doc of qInstallMsgHandler (I only added the comments):

• QtMsgHandler qInstallMessageHandler ( QtMsgHandler handler )

In the above example, the function myMessageOutput uses stderr which you might want to replace with some other file stream, or completely re-write the function!

Once you write and install this function, all your qDebug (as well as qWarning, qCritical etc) messages would be redirected to the file you're writing to in the handler.