src/site/markdown/qt-support.md

Qt Support {#qt-support}
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When using Qt, messages from the Qt framework itself or other libraries
may use the `QDebug` classes.  By default, this will print to stderr,
thus bypassing the logger entirely.  In order to have these messages
routed to Log4cxx, a message handler for Qt must be installed.

Log4cxx provides a cmake build option `LOG4CXX_QT_SUPPORT=ON`
which adds the log4cxx::qt namespace methods
for directing Qt messages to Log4cxx and
using the Qt event loop to process a configuration file change.
Use the target `log4cxx-qt` instead of `log4cxx`
in your `target_link_libraries` cmake directive.
Also, including `log4cxx-qt/logger.h` allows you to use QString values
in the LOG4CXX_WARN, LOG4CXX_INFO, LOG4CXX_DEBUG etc. macros.

To install a message handler that will route the Qt logging messages
through Log4cxx, include the messagehandler.h and call
`qInstallMessageHandler` as follows:

```cpp
#include <log4cxx-qt/messagehandler.h>

...

qInstallMessageHandler( log4cxx::qt::messageHandler );
```

Note that by default, this message handler also calls `abort` upon a
fatal message.

For how to use the Qt event loop to monitor the configuration file,
see the \ref com/foo/config-qt.h and \ref com/foo/config-qt.cpp example files.

Note that when using the above technique
you *must* configure Log4cxx after creating your QCoreApplication instance
(see the \ref MyApp-qt.cpp file for an example of this).

\example MyApp-qt.cpp
This file is an example of how to configure Log4cxx in a Qt application.

\example com/foo/config-qt.h
This header file is for Log4cxx configuration in a Qt application.

\example com/foo/config-qt.cpp
This file is an example of how to use the Qt event loop to monitor the Log4cxx configuration file.