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Debugging Techniques

Here we present some useful hints to help you with debugging your Qt-based software.

• Configuring Qt for Debugging
• Mac OS X and Xcode
• Command Line Options Recognized by Qt
• Warning and Debugging Messages
• Providing Support for the qDebug() Stream Operator
• Debugging Macros
• Common Bugs

Configuring Qt for Debugging

When configuring Qt for installation, it is possible to ensure that it is built to include debug symbols that can make it easier to track bugs in applications and libraries. However, on some platforms, building Qt in debug mode will cause applications to be larger than desirable.

Mac OS X and Xcode

The amount of space taken up by debug symbols generated by GCC can be excessively large. However, with the release of Xcode 2.3 it is now possible to use Dwarf symbols which take up a significantly smaller amount of space. To enable this feature when configuring Qt, pass the -dwarf-2 option to the configure script.

This is not enabled by default because previous versions of Xcode will not work with the compiler flag used to implement this feature. Mac OS X 10.5 will use dwarf-2 symbols by default.

dwarf-2 symbols contain references to source code, so the size of the final debug application should compare favorably to a release build.

Command Line Options Recognized by Qt

When you run a Qt application, you can specify several command-line options that can help with debugging. These are recognized by QApplication.

Warning and Debugging Messages

Qt includes four global functions for writing out warning and debug text. You can use them for the following purposes:

• qDebug() is used for writing custom debug output.
• qWarning() is used to report warnings and recoverable errors in your application.
• qCritical() is used for writing critical error mesages and reporting system errors.
• qFatal() is used for writing fatal error messages shortly before exiting.

If you include the <QtDebug> header file, the qDebug() function can also be used as an output stream. For example:

 qDebug() << "Widget" << widget << "at position" << widget->pos();

The Qt implementation of these functions prints the text to the stderr output under Unix/X11 and Mac OS X. With Windows, if it is a console application, the text is sent to console; otherwise, it is sent to the debugger. You can take over these functions by installing a message handler using qInstallMsgHandler().

If the QT_FATAL_WARNINGS environment variable is set, qWarning() exits after printing the warning message. This makes it easy to obtain a backtrace in the debugger.

Both qDebug() and qWarning() are debugging tools. They can be compiled away by defining QT_NO_DEBUG_OUTPUT and QT_NO_WARNING_OUTPUT during compilation.

The debugging functions QObject::dumpObjectTree() and QObject::dumpObjectInfo() are often useful when an application looks or acts strangely. More useful if you use object names than not, but often useful even without names.

Providing Support for the qDebug() Stream Operator

You can implement the stream operator used by qDebug() to provide debugging support for your classes. The class that implements the stream is QDebug. The functions you need to know about in QDebug are space() and nospace(). They both return a debug stream; the difference between them is whether a space is inserted between each item. Here is an example for a class that represents a 2D coordinate.

 QDebug operator<<(QDebug dbg, const Coordinate &c)
 {
     dbg.nospace() << "(" << c.x() << ", " << c.y() << ")";

     return dbg.space();
 }

Debugging Macros

The header file <QtGlobal> contains some debugging macros and #defines.

Three important macros are:

• Q_ASSERT(cond), where cond is a boolean expression, writes the warning "ASSERT: 'cond' in file xyz.cpp, line 234" and exits if cond is false.
• Q_ASSERT_X(cond, where, what), where cond is a boolean expression, where a location, and what a message, writes the warning: "ASSERT failure in where: 'what', file xyz.cpp, line 234" and exits if cond is false.
• Q_CHECK_PTR(ptr), where ptr is a pointer. Writes the warning "In file xyz.cpp, line 234: Out of memory" and exits if ptr is 0.

These macros are useful for detecting program errors, e.g. like this:

 char *alloc(int size)
 {
     Q_ASSERT(size > 0);
     char *ptr = new char[size];
     Q_CHECK_PTR(ptr);
     return ptr;
 }

Q_ASSERT(), Q_ASSERT_X(), and Q_CHECK_PTR() expand to nothing if QT_NO_DEBUG is defined during compilation. For this reason, the arguments to these macro should not have any side-effects. Here is an incorrect usage of Q_CHECK_PTR():

 char *alloc(int size)
 {
     char *ptr;
     Q_CHECK_PTR(ptr = new char[size]);  // WRONG
     return ptr;
 }

If this code is compiled with QT_NO_DEBUG defined, the code in the Q_CHECK_PTR() expression is not executed and alloc returns an unitialized pointer.

The Qt library contains hundreds of internal checks that will print warning messages when a programming error is detected. We therefore recommend that you use a debug version of Qt when developing Qt-based software.

Common Bugs

There is one bug that is so common that it deserves mention here: If you include the Q_OBJECT macro in a class declaration and run the meta-object compiler (moc), but forget to link the moc-generated object code into your executable, you will get very confusing error messages. Any link error complaining about a lack of vtbl, _vtbl, __vtbl or similar is likely to be a result of this problem.