How to print to console when using Qt

Question

I m using Qt4 and C   for making some programs in computer graphics  I need to be able to print some variables in my console at run-time  not debugging  but cout doesn t seem to work even if I add the libraries  Is there a way to do this

User · Answer

build  amp  run    Default for  Run in terminal  --  Enable  to flush the buffer use this command --  fflush stdout   you can also use   n  in printf or cout

User · Answer

Well  after studying several examples on the Internet describing how to output messages from a GUI in Qt to stdout  I have refined a working stand-alone example on redirecting messages to a console  via qDebug   and installing qInstallMessageHandler    The console will be showed at the same time as the GUI and can be hidden if deemed necessary  The code is easy to integrate with existing code in your project  Here is the full sample and feel free to use it in any way as you like  as long as you adhere to the License GNU GPL v2  You have to use a form of some sort and a MainWindow I think - otherwise the sample will run  but probably crash when forced to quit  Note  there is no way to quit via a close button or a menu close because I have tested those alternatives and the application will crash eventually every now and then  Without the close button the application will be stable and you can close it down from the main window  Enjoy    include  mainwindow h   include  lt QApplication gt     GNU GPL V2  2015-02-07  include  lt QMessageBox gt   include  lt windows h gt   define CONSOLE COLUMNS 80  define CONSOLE ROWS    5000  define YOURCONSOLETITLE  Your Console Title   typedef struct       CONSOLE SCREEN BUFFER INFOEX conScreenBuffInfoEX       HANDLE con screenbuf      HWND hwndConsole      HMENU consoleMenu       QString consoleTitle       QMessageBox mBox      QString localMsg      QString errorMessage      WINBOOL errorCode     consoleT   static consoleT  console   BOOL WINAPI catchCTRL  DWORD ctrlMsg             if  ctrlMsg    CTRL C EVENT                 HWND hwndWin   GetConsoleWindow                   ShowWindow hwndWin SW FORCEMINIMIZE                  return TRUE     void removeCloseMenu         int i       for  i   0  i  lt  10  i              console- gt hwndConsole   FindWindowW  NULL  console- gt consoleTitle toStdWString   data              if console- gt hwndConsole    NULL              break             if    console- gt errorCode   0   amp  amp   console- gt hwndConsole    NULL               console- gt errorMessage    QString   nFindWindowW error   1  n   arg console- gt errorCode        if    console- gt errorCode   0   amp  amp     console- gt consoleMenu   GetSystemMenu  console- gt hwndConsole  FALSE              console- gt errorMessage    QString  GetSystemMenu error   1  n   arg console- gt errorCode        if   console- gt errorCode   DeleteMenu  console- gt consoleMenu  SC CLOSE  MF BYCOMMAND                console- gt errorMessage    QString  DeleteMenu error   1  n   arg console- gt errorCode      void initialiseConsole         console- gt conScreenBuffInfoEX cbSize   sizeof CONSOLE SCREEN BUFFER INFOEX       console- gt consoleMenu   NULL      console- gt consoleTitle   YOURCONSOLETITLE      console- gt con screenbuf   INVALID HANDLE VALUE      console- gt errorCode   0      console- gt errorMessage           console- gt hwndConsole   NULL      console- gt localMsg            if   console- gt errorCode   FreeConsole             console- gt errorMessage    QString   nFreeConsole error   1  n   arg console- gt errorCode        if   console- gt errorCode   AllocConsole             console- gt errorMessage    QString   nAllocConsole error   1  n   arg console- gt errorCode        if   console- gt errorCode   -1   amp  amp   INVALID HANDLE VALUE    console- gt con screenbuf   CreateConsoleScreenBuffer  GENERIC WRITE   GENERIC READ 0  NULL  CONSOLE TEXTMODE BUFFER  NULL             console- gt errorMessage    QString   nCreateConsoleScreenBuffer error   1  n   arg console- gt errorCode        if   console- gt errorCode   SetConsoleActiveScreenBuffer console- gt con screenbuf            console- gt errorMessage    QString   nSetConsoleActiveScreenBuffer error   1  n   arg console- gt errorCode        if   console- gt errorCode   GetConsoleScreenBufferInfoEx console- gt con screenbuf   amp console- gt conScreenBuffInfoEX            console- gt errorMessage    QString   nGetConsoleScreenBufferInfoEx error   1  n   arg console- gt errorCode        console- gt conScreenBuffInfoEX dwSize X   CONSOLE COLUMNS      console- gt conScreenBuffInfoEX dwSize Y   CONSOLE ROWS       if   console- gt errorCode   SetConsoleScreenBufferInfoEx console- gt con screenbuf   amp console- gt conScreenBuffInfoEX           console- gt errorMessage    QString   nSetConsoleScreenBufferInfoEx error   1  n   arg console- gt errorCode        if   console- gt errorCode   SetConsoleTitleW console- gt consoleTitle toStdWString   data              console- gt errorMessage    QString  SetConsoleTitle error   1  n   arg console- gt errorCode        SetConsoleCtrlHandler NULL  FALSE       SetConsoleCtrlHandler catchCTRL  TRUE        removeCloseMenu         if console- gt errorMessage length    gt  0           console- gt mBox setText console- gt errorMessage           console- gt mBox show              void messageHandler QtMsgType type  const QMessageLogContext  amp context  const QString  amp msg         if  console- gt con screenbuf    INVALID HANDLE VALUE             switch  type             case QtDebugMsg              console- gt localMsg   console- gt errorMessage    Debug      msg              WriteConsoleW console- gt con screenbuf  console- gt localMsg toStdWString   data    console- gt localMsg toStdWString   length    NULL  NULL                WriteConsoleA console- gt con screenbuf    n-- n   4  NULL  NULL                break           case QtWarningMsg              console- gt localMsg   console- gt errorMessage    Warning      msg              WriteConsoleW console- gt con screenbuf  console- gt localMsg toStdWString   data    console- gt localMsg toStdWString   length     NULL  NULL                WriteConsoleA console- gt con screenbuf    n-- n   4  NULL  NULL                break           case QtCriticalMsg              console- gt localMsg   console- gt errorMessage    Critical      msg              WriteConsoleW console- gt con screenbuf  console- gt localMsg toStdWString   data    console- gt localMsg toStdWString   length    NULL  NULL                WriteConsoleA console- gt con screenbuf    n-- n   4  NULL  NULL                break           case QtFatalMsg              console- gt localMsg   console- gt errorMessage    Fatal      msg              WriteConsoleW console- gt con screenbuf  console- gt localMsg toStdWString   data    console- gt localMsg toStdWString   length    NULL  NULL                WriteConsoleA console- gt con screenbuf    n-- n   4  NULL  NULL                abort                         int main int argc  char  argv           qInstallMessageHandler messageHandler        QApplication a argc  argv        console   new consoleT        initialiseConsole         qDebug    lt  lt   Hello World         MainWindow w      w show         return a exec

User · Answer

If it is good enough to print to stderr  you can use the following streams originally intended for debugging    include lt QDebug gt     qInfo is qt5 5  only  qInfo    lt  lt   C   Style Info Message   qInfo   C Style Info Message      qDebug    lt  lt   C   Style Debug Message   qDebug   C Style Debug Message      qWarning    lt  lt   C   Style Warning Message   qWarning   C Style Warning Message      qCritical    lt  lt   C   Style Critical Error Message   qCritical   C Style Critical Error Message         qFatal does not have a C   style method  qFatal   C Style Fatal Error Message       Though as pointed out in the comments  bear in mind qDebug messages are removed if QT NO DEBUG OUTPUT is defined  If you need stdout you could try something like this  as Kyle Strand has pointed out    QTextStream amp  qStdOut         static QTextStream ts  stdout        return ts      You could then call as follows   qStdOut    lt  lt   std out

User · Answer

Add this to your project file   CONFIG    console

User · Answer

Go the Project s Properties - gt  Linker- gt  System - gt  SubSystem  then set it to Console  S

User · Answer

Writing to stdout If you want something that  like std  cout  writes to your application s standard output  you can simply do the following  credit to CapelliC   QTextStream stdout   lt  lt   quot string to print quot   lt  lt  endl   If you want to avoid creating a temporary QTextStream object  follow Yakk s suggestion in the comments below of creating a function to return a static handle for stdout  inline QTextStream amp  qStdout         static QTextStream r stdout       return r          foreach QString x  strings      qStdout    lt  lt  x  lt  lt  endl    Remember to flush the stream periodically to ensure the output is actually printed  Writing to stderr Note that the above technique can also be used for other outputs  However  there are more readable ways to write to stderr  credit to Goz and the comments below his answer   qDebug    lt  lt   quot Debug Message quot         CAN BE REMOVED AT COMPILE TIME  qWarning    lt  lt   quot Warning Message quot   qCritical    lt  lt   quot Critical Error Message quot   qFatal  quot Fatal Error Message quot        WILL KILL THE PROGRAM   qDebug   is closed if QT NO DEBUG OUTPUT is turned on at compile-time   Goz notes in a comment that for non-console apps  these can print to a different stream than stderr    NOTE  All of the Qt print methods assume that const char  arguments are ISO-8859-1 encoded strings with terminating  0 characters

User · Answer

include  lt QTextStream gt      qDebug   lt  lt  Bla bla bla

User · Answer

What variables do you want to print  If you mean QStrings  those need to be converted to c-Strings  Try   std  cout  lt  lt  myString toAscii   data

User · Answer

It also has a syntax similar to prinft  e g    qDebug   message  d  says   s  num str      Very handy as well

User · Answer

I found this most useful    include  lt QTextStream gt   QTextStream out stdout   foreach QString x  strings      out  lt  lt  x  lt  lt  endl

User · Answer

If you are printing to stderr using the stdio library  a call to fflush stderr  should flush the buffer and get you real-time logging

User · Answer

What about including iostream library and precise that cout is an object of std like this     include  lt iostream gt   std  cout  lt  lt   Hello   lt  lt  std  endl

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