Console output in a Qt GUI app

Question

I have a Qt GUI application running on Windows that allows command-line options to be passed and under some circumstances I want to output a message to the console and then quit  for example   int main int argc  char  argv        QApplication a argc  argv      if  someCommandLineParam          std  cout  lt  lt   Hello  world        return 0         MainWindow w    w show       return a exec        However  the console messages do not appear when I run the app from a command-prompt   Does anyone know how I can get this to work

User · Answer

It may have been an oversight of other answers, or perhaps it is a requirement of the user to indeed need console output, but the obvious answer to me is to create a secondary window that can be shown or hidden (with a checkbox or button) that shows all messages by appending lines of text to a text box widget and use that as a console?

The benefits of such a solution are:

A simple solution (providing all it displays is a simple log).
The ability to dock the 'console' widget onto the main application window. (In Qt, anyhow).
The ability to create many consoles (if more than 1 thread, etc).
A pretty easy change from local console output to sending log over network to a client.

Hope this gives you food for thought, although I am not in any way yet qualified to postulate on how you should do this, I can imagine it is something very achievable by any one of us with a little searching / reading!

User · Answer

Something you may want to investigate  at least for windows  is the AllocConsole   function in the windows api  It calls GetStdHandle a few times to redirect stdout  stderr  etc   A quick test shows this doesn t entirely do what we want it to do  You do get a console window opened alongside your other Qt stuff  but you can t output to it  Presumably  because the console window is open  there is some way to access it  get a handle to it  or access and manipulate it somehow  Here s the MSDN documentation for those interested in figuring this out   AllocConsole    http   msdn microsoft com en-us library windows desktop ms681944 28v vs 85 29 aspx  GetStdHandle       http   msdn microsoft com en-us library windows desktop ms683231 28v vs 85 29 aspx   I d add this as a comment  but the rules prevent me from doing so

User · Answer

After a rather long struggle with exactly the same problem I found that simply  CONFIG      console   really does the trick  It won t work until you explicitly tell QtCreator to execute qmake on the project  right click on project  AND change something inside the source file  then rebuild  Otherwise compilation is skipped and you still won t see the output on the command line  Now my program works in both GUI and cmd line mode

User · Answer

I used this header below for my projects  Hope it helps    ifndef   DEBUG  H  define   DEBUG  H   include  lt QtGui gt       static void myMessageOutput bool debug  QtMsgType type  const QString  amp  msg         if   debug  return       QDateTime dateTime   QDateTime  currentDateTime        QString dateString   dateTime toString  yyyy MM dd hh mm ss zzz         switch  type             case QtDebugMsg              fprintf stderr   Debug   s n   msg toAscii   data                 break          case QtWarningMsg              fprintf stderr   Warning   s n   msg toAscii   data                 break          case QtCriticalMsg              fprintf stderr   Critical   s n   msg toAscii   data                 break          case QtFatalMsg              fprintf stderr   Fatal   s n   msg toAscii   data                 abort              endif   PS  you could add dateString to output if you want in future

User · Answer

void Console         AllocConsole        FILE  pFileCon   NULL      pFileCon   freopen  CONOUT     w   stdout        COORD coordInfo      coordInfo X   130      coordInfo Y   9000       SetConsoleScreenBufferSize GetStdHandle STD OUTPUT HANDLE   coordInfo       SetConsoleMode GetStdHandle STD OUTPUT HANDLE  ENABLE QUICK EDIT MODE  ENABLE EXTENDED FLAGS      int main int argc  char  argv          Console        std  cout lt  lt  start         qDebug   lt  lt  start      You can t use std  cout as others have said my way is perfect even for some code can t include  qdebug

User · Answer

No way to output a message to console when using  QT    gui   fprintf stderr        also can t print output   Use QMessageBox instead to show the message

User · Answer

Oh you can Output a message when using QT    gui and CONFIG    console   You need printf  foo bar   but cout  lt  lt   foo bar  doesn t works

User · Answer

Make sure Qt5Core dll is in the same directory with your application executable   I had a similar issue in Qt5 with a console application   if I start the application from Qt Creator  the output text is visible  if I open cmd exe and start the same application there  no output is visible  Very strange   I solved it by copying Qt5Core dll to the directory with the application executable    Here is my tiny console application    include  lt QCoreApplication gt   include  lt QDebug gt   int main int argc  char  argv          int x 343      QString str  Hello World        qDebug   lt  lt  str  lt  lt  x lt  lt  lalalaa        QTextStream out stdout       out  lt  lt   aldfjals alsdfajs

User · Answer

One solution is to run powershell and redirect the output to whatever stream you want   Below is an example of running powershell from cmd exe and redirecting my exec exe output to both the console and an output txt file   powershell    my exec exe   tee output txt

User · Answer

In your  pro add  CONFIG             console

User · Answer

I also played with this  discovering that redirecting output worked  but I never saw output to the console window  which is present for every windows application   This is my solution so far  until I find a Qt replacement for ShowWindow and GetConsoleWindow   Run this from a command prompt without parameters - get the window   Run from command prompt with parameters  eg    cmd aaa bbb ccc  - you get the text output on the command prompt window - just as you would expect for any Windows console app   Please excuse the lame example - it represents about 30 minutes of tinkering    include  mainwindow h   include  lt QTextStream gt   include  lt QCoreApplication gt   include  lt QApplication gt   include  lt QWidget gt   include  lt windows h gt   QT USE NAMESPACE  int main int argc  char  argv          if  argc  gt  1                 User has specified command-line arguments         QCoreApplication a argc  argv           QTextStream  out stdout           int     i           ShowWindow  GetConsoleWindow   SW NORMAL           for  i 1  i lt argc  i                 out  lt  lt  i  lt  lt       lt  lt  argv  i   lt  lt  endl          out  lt  lt  endl  lt  lt   Hello  World   lt  lt  endl          out  lt  lt   Application Directory Path    lt  lt  a applicationDirPath    lt  lt  endl          out  lt  lt   Application File Path    lt  lt  a applicationFilePath    lt  lt  endl          MessageBox  0  LPCWSTR  Continue    LPCWSTR  Silly Question  MB YESNO           return 0        else            QApplication a argc  argv           MainWindow w           w setWindowTitle  Simple example            w show            return a exec

User · Answer

Add    ifdef  WIN32 if  AttachConsole ATTACH PARENT PROCESS         freopen  CONOUT     w   stdout       freopen  CONOUT     w   stderr      endif   at the top of main    This will enable output to the console only if the program is started in a console  and won t pop up a console window in other situations  If you want to create a console window to display messages when you run the app outside a console you can change the condition to   if  AttachConsole ATTACH PARENT PROCESS     AllocConsole

User · Answer

Windows does not really support dual mode applications    To see console output you need to create a console application  CONFIG    console   However  if you double click on the program to start the GUI mode version then you will get a console window appearing  which is probably not what you want  To prevent the console window appearing you have to create a GUI mode application in which case you get no output in the console   One idea may be to create a second small application which is a console application and provides the output  This can call the second one to do the work    Or you could put all the functionality in a DLL then create two versions of the  exe file which have very simple main functions which call into the DLL  One is for the GUI and one is for the console

User · Answer

First of all you can try flushing the buffer   std  cout  lt  lt   Hello  world   lt  lt std  endl    For more Qt based logging you can try using qDebug

User · Answer

Easy  Step1  Create new project  Go File- New File or Project --  Other Project -- Empty Project  Step2  Use the below code   In  pro file  QT   widgets CONFIG    console TARGET   minimal SOURCES      main cpp   Step3   Create main cpp and copy the below code    include  lt QApplication gt   include  lt QtCore gt   using namespace  std   QTextStream in stdin   QTextStream out stdout    int main int argc  char  argv     QApplication app argc argv   qDebug    lt  lt   Please enter some text over here     lt  lt  endl  out flush    QString input  input   in readLine    out  lt  lt   The input is    lt  lt  input   lt  lt  endl  return app exec        I created necessary objects in the code for your understanding    Just Run It  If you want your program to get multiple inputs with some conditions  Then past the below code in Main cpp    include  lt QApplication gt   include  lt QtCore gt   using namespace  std   QTextStream in stdin   QTextStream out stdout    int main int argc  char  argv         QApplication app argc argv       qDebug    lt  lt   Please enter some text over here     lt  lt  endl      out flush        QString input      do          input   in readLine            if input size    6               out  lt  lt   The input is    lt  lt  input   lt  lt  endl                       else                       qDebug  Not the exact input man                   while  input size    0        qDebug   WE ARE AT THE END            endif     return app exec         end main   Hope it educates you    Good day

User · Answer

So many answers to this topic  0 0  So I tried it with Qt5 x from Win7 to Win10  It took me some hours to have a good working solution which doesn t produce any problems somewhere in the chain    include  mainwindow h    include  lt QApplication gt    include  lt windows h gt   include  lt stdio h gt   include  lt iostream gt         Add to project file     CONFIG    console     int main  int argc  char  argv           if  argc  lt  2              if defined  Q OS WIN             ShowWindow    GetConsoleWindow    SW HIDE      hide console window      endif         QApplication a  argc  argv            MainWindow  w   new MainWindow          w- gt show            int e   a exec            delete w    needed to execute deconstructor         exit  e      needed to exit the hidden console         return e            else               QCoreApplication a  argc  argv            std  string g          std  cout  lt  lt   Enter name             std  cin  gt  gt  g          std  cout  lt  lt   Name is     lt  lt  g  lt  lt  std  endl          exit  0            return a exec                  I tried it also without the  CONFIG    console   but then you need to redirect the streams and create the console on your own    ifdef  WIN32 if  AttachConsole ATTACH PARENT PROCESS     AllocConsole         freopen  CONOUT     w   stdout       freopen  CONOUT     w   stderr       freopen  CONIN     r   stdin      endif   BUT this only works if you start it through a debugger  otherwise all inputs are directed towards the system too  Means  if you type a name via std  cin the system tries to execute the name as a command   very strange   Two other warnings to this attempt would be  that you can t use   FreeConsole   it won t close it and if you start it through a console the app won t close       Last there is a Qt help section in QApplication to this topic  I tried the example there with an application and it doesn t work for the GUI  it stucked somewhere in an endless loop and the GUI won t be rendered or it simply crashes   QCoreApplication  createApplication int  amp argc  char  argv          for  int i   1  i  lt  argc    i          if   qstrcmp argv i    -no-gui                return new QCoreApplication argc  argv       return new QApplication argc  argv      int main int argc  char  argv          QScopedPointer lt QCoreApplication gt  app createApplication argc  argv         if  qobject cast lt QApplication   gt  app data                 start GUI version          else             start non-GUI version               return app- gt exec            So if you are using Windows and Qt simply use the console option  hide the console if you need the GUI and close it via exit

User · Answer

First of all  why would you need to output to console in a release mode build  Nobody will think to look there when there s a gui     Second  qDebug is fancy     Third  you can try adding console to your  pro s CONFIG  it might work

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