How to print to console when using Qt

Question

If it is good enough to print to stderr, you can use the following streams originally intended for debugging:

#include<QDebug>

//qInfo is qt5.5+ only.
qInfo() << "C++ Style Info Message";
qInfo( "C Style Info Message" );

qDebug() << "C++ Style Debug Message";
qDebug( "C Style Debug Message" );

qWarning() << "C++ Style Warning Message";
qWarning( "C Style Warning Message" );

qCritical() << "C++ Style Critical Error Message";
qCritical( "C Style Critical Error Message" );

// qFatal does not have a C++ style method.
qFatal( "C Style Fatal Error Message" );

Though as pointed out in the comments, bear in mind qDebug messages are removed if QT_NO_DEBUG_OUTPUT is defined

If you need stdout you could try something like this (as Kyle Strand has pointed out):

QTextStream& qStdOut()
{
    static QTextStream ts( stdout );
    return ts;
}

You could then call as follows:

qStdOut() << "std out!";

I found this most useful:

#include <QTextStream>

QTextStream out(stdout);
foreach(QString x, strings)
    out << x << endl;

Writing to stdout

If you want something that, like std::cout, writes to your application's standard output, you can simply do the following (credit to CapelliC):

QTextStream(stdout) << "string to print" << endl;

If you want to avoid creating a temporary QTextStream object, follow Yakk's suggestion in the comments below of creating a function to return a static handle for stdout:

inline QTextStream& qStdout()
{
    static QTextStream r{stdout};
    return r;
}

...

foreach(QString x, strings)
    qStdout() << x << endl;

---

Remember to flush the stream periodically to ensure the output is actually printed.

Writing to stderr

Note that the above technique can also be used for other outputs. However, there are more readable ways to write to stderr (credit to Goz and the comments below his answer):

qDebug() << "Debug Message";    // CAN BE REMOVED AT COMPILE TIME!
qWarning() << "Warning Message";
qCritical() << "Critical Error Message";
qFatal("Fatal Error Message");  // WILL KILL THE PROGRAM!

qDebug() is closed if QT_NO_DEBUG_OUTPUT is turned on at compile-time.

(Goz notes in a comment that for non-console apps, these can print to a different stream than stderr.)

---

NOTE: All of the Qt print methods assume that const char* arguments are ISO-8859-1 encoded strings with terminating \0 characters.

Add this to your project file:

CONFIG += console

What variables do you want to print? If you mean QStrings, those need to be converted to c-Strings. Try:

std::cout << myString.toAscii().data();