Debugging Techniques

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

Two important macros are:

• Q_ASSERT(b) where b is a boolean expression, writes the warning: "ASSERT: 'b' in file file.cpp (234)" if b is FALSE.
• Q_CHECK_PTR(p) where p is a pointer. Writes the warning "In file file.cpp, line 234: Out of memory" if p is 0.

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

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

If you define the flag QT_FATAL_ASSERT, Q_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 Q_ASSERT macro is a null expression if QT_CHECK_STATE (see below) is not defined. Any code in it will simply not be executed. Similarly Q_CHECK_PTR is a null expression if QT_CHECK_NULL is not defined. Here is an example of how you should not use Q_ASSERT and Q_CHECK_PTR:

  char *alloc( int size )
  {
      char *p;
      Q_CHECK_PTR( p = new char[size] ); // WRONG!
      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 QT_CHECK_NULL flag defined, the code in the Q_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 QT_DEBUG and all the QT_CHECK flags are on. To turn off QT_DEBUG, define QT_NO_DEBUG. To turn off the QT_CHECK flags, define QT_NO_CHECK.

Example:

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

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

Common bugs