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This option is set by default when the +program is running in the <tt>gdb</tt> debugger under Linux. +<tr bgcolor="#d0d0d0"> <td valign="top">-dograb +<td valign="top">Ignore any implicit or explicit -nograb. -dograb wins over +-nograb even when -nograb is last on the command line. +<tr bgcolor="#f0f0f0"> <td valign="top">-sync +<td valign="top">Runs the application in X synchronous mode. Synchronous mode +forces the X server to perform each X client request +immediately and not use buffer optimization. It makes the +program easier to debug and often much slower. The -sync +option is only valid for the X11 version of TQt. +</table></center> +<p> <h2> Warning and Debugging Messages +</h2> +<a name="2"></a><p> TQt includes three global functions for writing out warning and debug +text. +<ul> +<li> <a href="qapplication.html#qDebug">qDebug()</a> for writing debug output for testing etc. +<li> <a href="qapplication.html#qWarning">qWarning()</a> for writing warning output when program +errors occur. +<li> <a href="qapplication.html#qFatal">qFatal()</a> for writing fatal error messages +and exiting. +</ul> +<p> The TQt implementation of these functions prints the text to the <tt>stderr</tt> +output under Unix/X11 and to the debugger under Windows. You can +take over these functions by installing a message handler; +<a href="qapplication.html#qInstallMsgHandler">qInstallMsgHandler()</a>. +<p> The debugging functions <a href="qobject.html#dumpObjectTree">TQObject::dumpObjectTree</a>() and <a href="qobject.html#dumpObjectInfo">TQObject::dumpObjectInfo</a>() 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. +<p> <h2> Debugging Macros +</h2> +<a name="3"></a><p> The header file <a href="qglobal-h.html">qglobal.h</a> contains many debugging macros and +<tt>#define</tt>s. +<p> Two important macros are: +<ul> +<li> <a href="qapplication.html#Q_ASSERT">Q_ASSERT(b)</a> where b is a boolean +expression, writes the warning: "ASSERT: 'b' in file file.cpp (234)" +if b is FALSE. +<li> <a href="qapplication.html#Q_CHECK_PTR">Q_CHECK_PTR(p)</a> where p is a pointer. +Writes the warning "In file file.cpp, line 234: Out of memory" if p is +0. +</ul> +<p> These macros are useful for detecting program errors, e.g. like this: +<pre> + char *alloc( int size ) + { + <a href="qapplication.html#Q_ASSERT">Q_ASSERT</a>( size > 0 ); + char *p = new char[size]; + <a href="qapplication.html#Q_CHECK_PTR">Q_CHECK_PTR</a>( p ); + return p; + } +</pre> + +<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. +<p> Note that the Q_ASSERT macro is a null expression if <tt>QT_CHECK_STATE</tt> (see +below) is not defined. Any code in it will simply not be +executed. Similarly Q_CHECK_PTR is a null expression if <tt>QT_CHECK_NULL</tt> is +not defined. Here is an example of how you should <em>not</em> use Q_ASSERT and +Q_CHECK_PTR: +<p> <pre> + char *alloc( int size ) + { + char *p; + <a href="qapplication.html#Q_CHECK_PTR">Q_CHECK_PTR</a>( p = new char[size] ); // WRONG! + return p; + } +</pre> + +<p> The problem is tricky: <em>p</em> 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 <em>alloc</em> +returns a wild pointer. +<p> The TQt library contains hundreds of internal checks that will print +warning messages when some error is detected. +<p> The tests for sanity and the resulting warning messages inside TQt are +conditional, based on the state of various debugging flags: +<center><table cellpadding="4" cellspacing="2" border="0"> +<tr bgcolor="#a2c511"> <th valign="top">Flag <th valign="top">Meaning +<tr bgcolor="#d0d0d0"> <td valign="top">QT_CHECK_STATE <td valign="top">Check for consistent/expected object state +<tr bgcolor="#f0f0f0"> <td valign="top">QT_CHECK_RANGE <td valign="top">Check for variable range errors +<tr bgcolor="#d0d0d0"> <td valign="top">QT_CHECK_NULL <td valign="top">Check for dangerous null pointers +<tr bgcolor="#f0f0f0"> <td valign="top">QT_CHECK_MATH <td valign="top">Check for dangerous math, e.g. division by 0 +<tr bgcolor="#d0d0d0"> <td valign="top">QT_NO_CHECK <td valign="top">Turn off all QT_CHECK_... flags +<tr bgcolor="#f0f0f0"> <td valign="top">QT_DEBUG <td valign="top">Enable debugging code +<tr bgcolor="#d0d0d0"> <td valign="top">QT_NO_DEBUG <td valign="top">Turn off QT_DEBUG flag +</table></center> +<p> 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. +<p> Example: +<pre> + void f( char *p, int i ) + { + #if defined(QT_CHECK_NULL) + if ( p == 0 ) + <a href="qapplication.html#qWarning">qWarning</a>( "f: Null pointer not allowed" ); + #endif + + #if defined(QT_CHECK_RANGE) + if ( i < 0 ) + <a href="qapplication.html#qWarning">qWarning</a>( "f: The index cannot be negative" ); + #endif + } +</pre> + +<p> <h2> Common bugs +</h2> +<a name="4"></a><p> There is one bug that is so common that it deserves mention here: If +you include the <a href="metaobjects.html#Q_OBJECT">Q_OBJECT</a> macro in a class declaration and run the +<a href="moc.html">moc</a>, but forget to link the <a href="moc.html#moc">moc</a>-generated +object code into your executable, you will get very confusing error +messages. Any link error complaining about a lack of <tt>vtbl</tt>, +<tt>_vtbl</tt>, <tt>__vtbl</tt> or similar is likely to be a result of this +problem. +<p> +<!-- eof --> +<p><address><hr><div align=center> +<table width=100% cellspacing=0 border=0><tr> +<td>Copyright © 2007 +<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a> +<td align=right><div align=right>TQt 3.3.8</div> +</table></div></address></body> +</html> |