how to use qInstallMessageHandler to ignore specific debug messages?

[Ylvy]

How i could allow it to only print qDebug() messages?
In my attempt below its not printing qDebug().

void myMessageHandler(QtMsgType type, const QMessageLogContext& context, 
	const QString& message) 
{
	switch (type)
	{
	case QtDebugMsg: 
		qDebug() << message; // <- doesnt work
	break;

	// Stop these msg from being printed to debug.
	case QtWarningMsg:
	case QtCriticalMsg:
	case QtFatalMsg:
	case QtInfoMsg:
	default:
	break;
	}
}

[Chris Kawa]

@Ylvy You can't use qDebug inside the handler. That's recursive.
qInstallMessageHandler returns a previous handler (or the default one if you didn't set any yet). If you want to just filter out everything but debug you can store the old handler and call it from yours, e.g.

// put it somewhere accessible to your handler
QtMessageHandler defaultHandler {};

// install new and get the old one
defaultHandler = qInstallMessageHandler(myMessageHandler);

and in your handler:

void myMessageHandler(QtMsgType type, const QMessageLogContext& context, const QString& message) 
{
   if (type == QtDebugMsg)
      defaultHandler(type, context, message);
}

[JonB]

@Ylvy
If this is installed as the message handler, it gets called by qDebug(). But you issue a qDebug() in this case from inside it. Sounds dangerously like infinite recursion to me?!

Try something like printf() or fprintf(stderr or cerr << for the qDebug() you have, does that get fired OK?

What you probably ought do here is call the message handler you replaced by this in the case QtDebugMsg route.

[Ylvy]

@JonB said in how to use qInstallMessageHandler to ignore specific debug messages?:

@Ylvy
What you probably ought do here is call the message handler you replaced by this in the case QtDebugMsg route.

How?

[Chris Kawa]

@Ylvy You can't use qDebug inside the handler. That's recursive.
qInstallMessageHandler returns a previous handler (or the default one if you didn't set any yet). If you want to just filter out everything but debug you can store the old handler and call it from yours, e.g.

// put it somewhere accessible to your handler
QtMessageHandler defaultHandler {};

// install new and get the old one
defaultHandler = qInstallMessageHandler(myMessageHandler);

and in your handler:

void myMessageHandler(QtMsgType type, const QMessageLogContext& context, const QString& message) 
{
   if (type == QtDebugMsg)
      defaultHandler(type, context, message);
}