Debugging Techniques

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

Command Line Options

When you run a Qt program you can specify several command line options that can help with debugging.

Warning and Debugging Messages

Qt includes three global functions for writing out warning and debug text.

The Qt implementation of these functions prints the text to the stderr output under Unix/X11 and to the debugger under Windows. You can take over these functions by installing a message handler; qInstallMsgHandler().

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.

Debugging Macros

The header file qglobal.h contains many debugging macros and #defines.

Two important macros are:

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

  char *alloc( int size )
      ASSERT( size > 0 );
      char *p = new char[size];
      CHECK_PTR( p );
      return p;

If you define the flag QT_FATAL_ASSERT, ASSERT will call fatal() instead of warning(), so a failed assertion will cause the program to exit after printing the error message.

Note that the ASSERT macro is a null expression if CHECK_STATE (see below) is not defined. Any code in it will simply not be executed. Similarly CHECK_PTR is a null expression if CHECK_NULL is not defined. Here is an example of how you should NOT use ASSERT and CHECK_PTR:

  char *alloc( int size )
      char *p;
      CHECK_PTR( p = new char[size] );  // never do this!
      return p;

The problem is tricky: p is set to a sane value only as long as the correct checking flags are defined. If this code is compiled without the CHECK_NULL flag defined, the code in the CHECK_PTR expression is not executed (correctly, since it's only a debugging aid) and alloc returns a wild pointer.

The Qt library contains hundreds of internal checks that will print warning messages when some error is detected.

The tests for sanity and the resulting warning messages inside Qt are conditional, based on the state of various debugging flags:

By default, both DEBUG and all the CHECK flags are on. To turn off DEBUG, define NO_DEBUG. To turn off the CHECK flags, define NO_CHECK.


  void f( char *p, int i )
  #if defined(CHECK_NULL)
      if ( p == 0 )
          qWarning( "f: Null pointer not allowed" );

  #if defined(CHECK_RANGE)
      if ( i < 0 )
          qWarning( "f: The index cannot be negative" );

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 moc, but forget to link the moc-generated object code into your executable, you will get very confusing error message.

Any link error complaining about a lack of vtbl, _vtbl, __vtbl or similar is likely to be this problem.

Copyright 2000 TrolltechTrademarks
Qt version 2.2.1