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QWidget: Must construct a QApplication before a QWidget

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  • Christian EhrlicherC Offline
    Christian EhrlicherC Offline
    Christian Ehrlicher
    Lifetime Qt Champion
    wrote on last edited by
    #2

    You have a global static QWidget somewhere in your code.

    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
    Visit the Qt Academy at https://academy.qt.io/catalog

    1 Reply Last reply
    3
    • C Offline
      C Offline
      Chaki
      wrote on last edited by
      #3

      Hello and Thanks for your respone,

      but i could not find the problem. Here is the whole code:

      mainwindow.cpp

      #include "mainwindow.h"
      #include "ui_mainwindow.h"
      
      MainWindow::MainWindow(QWidget *parent)
          : QMainWindow(parent)
          , ui(new Ui::MainWindow)
      {
          ui->setupUi(this);
      }
      
      MainWindow::~MainWindow()
      {
          delete ui;
      }
      
      

      mainwindow.h

      #ifndef MAINWINDOW_H
      #define MAINWINDOW_H
      
      #include <QMainWindow>
      
      QT_BEGIN_NAMESPACE
      namespace Ui { class MainWindow; }
      QT_END_NAMESPACE
      
      class MainWindow : public QMainWindow
      {
          Q_OBJECT
      
      public:
          MainWindow(QWidget *parent = nullptr);
          ~MainWindow();
      
      private:
          Ui::MainWindow *ui;
      };
      #endif // MAINWINDOW_H
      
      

      I would appreciate your help!

      JonBJ 1 Reply Last reply
      0
      • C Chaki

        Hello and Thanks for your respone,

        but i could not find the problem. Here is the whole code:

        mainwindow.cpp

        #include "mainwindow.h"
        #include "ui_mainwindow.h"
        
        MainWindow::MainWindow(QWidget *parent)
            : QMainWindow(parent)
            , ui(new Ui::MainWindow)
        {
            ui->setupUi(this);
        }
        
        MainWindow::~MainWindow()
        {
            delete ui;
        }
        
        

        mainwindow.h

        #ifndef MAINWINDOW_H
        #define MAINWINDOW_H
        
        #include <QMainWindow>
        
        QT_BEGIN_NAMESPACE
        namespace Ui { class MainWindow; }
        QT_END_NAMESPACE
        
        class MainWindow : public QMainWindow
        {
            Q_OBJECT
        
        public:
            MainWindow(QWidget *parent = nullptr);
            ~MainWindow();
        
        private:
            Ui::MainWindow *ui;
        };
        #endif // MAINWINDOW_H
        
        

        I would appreciate your help!

        JonBJ Offline
        JonBJ Offline
        JonB
        wrote on last edited by JonB
        #4

        @Chaki
        That cannot be your "whole code", as it does not even have a main()....

        Oh, I see, you gave that earlier? You are not linking with any other files, right? In your main.cpp, try commenting out both the #include <qwt_plot... and all of the code from w.show(); to return a.exec();. And preferably do not link with qwt_plot stuff at all. Does it work now?

        C 1 Reply Last reply
        0
        • C Offline
          C Offline
          Chaki
          wrote on last edited by
          #5

          Sorry, forgot about main.cpp:

          #include "mainwindow.h"
          
          #include <QApplication>
          
          #include <qwt_plot_curve.h>
          #include <qwt_plot.h>
          
          int main(int argc, char *argv[])
          {
              QApplication a(argc, argv);
              MainWindow w;
              w.show();
          
              QwtPlot plot;
              plot.setTitle( "Plot Demo" );
              plot.setCanvasBackground( Qt::white );
          
              QwtPlotCurve *curve = new QwtPlotCurve();
                  curve->setTitle( "Some Points" ); // will later be used in legend
                  curve->setPen( Qt::blue, 4 ), // color and thickness in pixels
                  curve->setRenderHint( QwtPlotItem::RenderAntialiased, true ); // use antialiasing
          
                  QPolygonF points;
                      points << QPointF( 0.0, 4.4 ) << QPointF( 1.0, 3.0 )
                          << QPointF( 2.0, 4.5 ) << QPointF( 3.0, 6.8 )
                          << QPointF( 4.0, 7.9 ) << QPointF( 5.0, 7.1 );
               curve->setSamples( points );
          
               curve->attach( &plot );
               plot.resize( 600, 400 );
               plot.show();
              return a.exec();
          }
          
          
          JonBJ 1 Reply Last reply
          0
          • C Chaki

            Sorry, forgot about main.cpp:

            #include "mainwindow.h"
            
            #include <QApplication>
            
            #include <qwt_plot_curve.h>
            #include <qwt_plot.h>
            
            int main(int argc, char *argv[])
            {
                QApplication a(argc, argv);
                MainWindow w;
                w.show();
            
                QwtPlot plot;
                plot.setTitle( "Plot Demo" );
                plot.setCanvasBackground( Qt::white );
            
                QwtPlotCurve *curve = new QwtPlotCurve();
                    curve->setTitle( "Some Points" ); // will later be used in legend
                    curve->setPen( Qt::blue, 4 ), // color and thickness in pixels
                    curve->setRenderHint( QwtPlotItem::RenderAntialiased, true ); // use antialiasing
            
                    QPolygonF points;
                        points << QPointF( 0.0, 4.4 ) << QPointF( 1.0, 3.0 )
                            << QPointF( 2.0, 4.5 ) << QPointF( 3.0, 6.8 )
                            << QPointF( 4.0, 7.9 ) << QPointF( 5.0, 7.1 );
                 curve->setSamples( points );
            
                 curve->attach( &plot );
                 plot.resize( 600, 400 );
                 plot.show();
                return a.exec();
            }
            
            
            JonBJ Offline
            JonBJ Offline
            JonB
            wrote on last edited by
            #6

            @Chaki
            I have re-edited my post once I realized that. Please help yourself to try to spot the problem area by trying the commenting-out I now suggest you do.

            1 Reply Last reply
            0
            • JonBJ JonB

              @Chaki
              That cannot be your "whole code", as it does not even have a main()....

              Oh, I see, you gave that earlier? You are not linking with any other files, right? In your main.cpp, try commenting out both the #include <qwt_plot... and all of the code from w.show(); to return a.exec();. And preferably do not link with qwt_plot stuff at all. Does it work now?

              C Offline
              C Offline
              Chaki
              wrote on last edited by
              #7

              @JonB Yes, with commenting out those lines, it works - does that mean that the problem is with Qwt?

              JonBJ 1 Reply Last reply
              0
              • C Chaki

                @JonB Yes, with commenting out those lines, it works - does that mean that the problem is with Qwt?

                JonBJ Offline
                JonBJ Offline
                JonB
                wrote on last edited by
                #8

                @Chaki
                Well, yes, obviously! :)

                As @Christian-Ehrlicher observed, this error would be most likely if there is a static QWidget somewhere.

                Now re-introduce the 2 #include <qwt_plot... lines, but leave your own QwtPlot commented out. I assume it goes wrong again? Now try commenting out separately each one of the two include lines. Do you find it works with one commented out but not the other? We are trying to spot just where this problem stems from, before we figure what to do about it.

                C 1 Reply Last reply
                1
                • JonBJ JonB

                  @Chaki
                  Well, yes, obviously! :)

                  As @Christian-Ehrlicher observed, this error would be most likely if there is a static QWidget somewhere.

                  Now re-introduce the 2 #include <qwt_plot... lines, but leave your own QwtPlot commented out. I assume it goes wrong again? Now try commenting out separately each one of the two include lines. Do you find it works with one commented out but not the other? We are trying to spot just where this problem stems from, before we figure what to do about it.

                  C Offline
                  C Offline
                  Chaki
                  wrote on last edited by
                  #9

                  @JonB firstly, Thank you for your Effort!

                  yes it works with both of the #include lines.

                  I have shortened the main.cpp:

                  #include "mainwindow.h"
                  
                  #include <QApplication>
                  #include <qwt_plot.h>
                  #include <qwt_plot_curve.h>
                  
                  int main(int argc, char *argv[])
                  {
                      QApplication a(argc, argv);
                      MainWindow w;
                      w.show();
                  
                      //QwtPlot plot;
                      //plot.show();
                  
                      return a.exec();
                  }
                  
                  
                  JonBJ 1 Reply Last reply
                  0
                  • C Offline
                    C Offline
                    Chaki
                    wrote on last edited by
                    #10

                    This is the first time, when I get an Error:

                    #include "mainwindow.h"
                    
                    #include <QApplication>
                    #include <qwt_plot.h>
                    
                    int main(int argc, char *argv[])
                    {
                        QApplication a(argc, argv);
                        MainWindow w;
                        w.show();
                    
                        QwtPlot plot;
                        //plot.show();
                    
                        return a.exec();
                    }
                    
                    
                    JonBJ 1 Reply Last reply
                    0
                    • C Chaki

                      @JonB firstly, Thank you for your Effort!

                      yes it works with both of the #include lines.

                      I have shortened the main.cpp:

                      #include "mainwindow.h"
                      
                      #include <QApplication>
                      #include <qwt_plot.h>
                      #include <qwt_plot_curve.h>
                      
                      int main(int argc, char *argv[])
                      {
                          QApplication a(argc, argv);
                          MainWindow w;
                          w.show();
                      
                          //QwtPlot plot;
                          //plot.show();
                      
                          return a.exec();
                      }
                      
                      
                      JonBJ Offline
                      JonBJ Offline
                      JonB
                      wrote on last edited by JonB
                      #11

                      @Chaki
                      Sorry, you are saying:

                      • With the code as shown --- the #includes included, but the QwtPlot plot of yours commented out --- now it does work, with no error?

                      • And if you just uncomment your QwtPlot plot it then reverts to error "QWidget: Must construct a QApplication before a QWidget".

                      Is that right?

                      EDIT OK, I think your latest post has crossed with this. You have isolated just the QwtPlot plot; line alone to be the cause of the error, is that correct? And the message is indeed still "QWidget: Must construct a QApplication before a QWidget".? I'm having a think.... :)

                      1 Reply Last reply
                      0
                      • C Chaki

                        This is the first time, when I get an Error:

                        #include "mainwindow.h"
                        
                        #include <QApplication>
                        #include <qwt_plot.h>
                        
                        int main(int argc, char *argv[])
                        {
                            QApplication a(argc, argv);
                            MainWindow w;
                            w.show();
                        
                            QwtPlot plot;
                            //plot.show();
                        
                            return a.exec();
                        }
                        
                        
                        JonBJ Offline
                        JonBJ Offline
                        JonB
                        wrote on last edited by
                        #12

                        @Chaki
                        While I'm thinking, can you replace just line QwtPlot plot; by QWidget plot;, now what? Good or bad?

                        C 1 Reply Last reply
                        0
                        • JonBJ JonB

                          @Chaki
                          While I'm thinking, can you replace just line QwtPlot plot; by QWidget plot;, now what? Good or bad?

                          C Offline
                          C Offline
                          Chaki
                          wrote on last edited by
                          #13

                          @JonB Yes Sir, it works with QWidget plot;

                          QWidget plot;
                          plot.show();
                          
                          ``
                          JonBJ 1 Reply Last reply
                          0
                          • C Chaki

                            @JonB Yes Sir, it works with QWidget plot;

                            QWidget plot;
                            plot.show();
                            
                            ``
                            JonBJ Offline
                            JonBJ Offline
                            JonB
                            wrote on last edited by
                            #14

                            @Chaki
                            How big is the #include <qwt_plot.h> file (you'll need to go find where it is)? Is it small enough to be postable here, or is it large and/or #includes a bunch of further QWT header files?

                            1 Reply Last reply
                            0
                            • C Offline
                              C Offline
                              Chaki
                              wrote on last edited by
                              #15

                              It is quit large:

                              /* -*- mode: C++ ; c-file-style: "stroustrup" -*- *****************************
                               * Qwt Widget Library
                               * Copyright (C) 1997   Josef Wilgen
                               * Copyright (C) 2002   Uwe Rathmann
                               *
                               * This library is free software; you can redistribute it and/or
                               * modify it under the terms of the Qwt License, Version 1.0
                               *****************************************************************************/
                              
                              #ifndef QWT_PLOT_H
                              #define QWT_PLOT_H
                              
                              #include "qwt_global.h"
                              #include "qwt_text.h"
                              #include "qwt_plot_dict.h"
                              #include "qwt_scale_map.h"
                              #include "qwt_interval.h"
                              #include <qframe.h>
                              #include <qlist.h>
                              #include <qvariant.h>
                              
                              class QwtPlotLayout;
                              class QwtAbstractLegend;
                              class QwtScaleWidget;
                              class QwtScaleEngine;
                              class QwtScaleDiv;
                              class QwtScaleDraw;
                              class QwtTextLabel;
                              
                              /*!
                                \brief A 2-D plotting widget
                              
                                QwtPlot is a widget for plotting two-dimensional graphs.
                                An unlimited number of plot items can be displayed on
                                its canvas. Plot items might be curves (QwtPlotCurve), markers
                                (QwtPlotMarker), the grid (QwtPlotGrid), or anything else derived
                                from QwtPlotItem.
                                A plot can have up to four axes, with each plot item attached to an x- and
                                a y axis. The scales at the axes can be explicitly set (QwtScaleDiv), or
                                are calculated from the plot items, using algorithms (QwtScaleEngine) which
                                can be configured separately for each axis.
                              
                                The simpleplot example is a good starting point to see how to set up a
                                plot widget.
                              
                                \image html plot.png
                              
                                \par Example
                                  The following example shows (schematically) the most simple
                                  way to use QwtPlot. By default, only the left and bottom axes are
                                  visible and their scales are computed automatically.
                                  \code
                                    #include <qwt_plot.h>
                                    #include <qwt_plot_curve.h>
                              
                                    QwtPlot *myPlot = new QwtPlot( "Two Curves", parent );
                              
                                    // add curves
                                    QwtPlotCurve *curve1 = new QwtPlotCurve( "Curve 1" );
                                    QwtPlotCurve *curve2 = new QwtPlotCurve( "Curve 2" );
                              
                                    // connect or copy the data to the curves
                                    curve1->setData( ... );
                                    curve2->setData( ... );
                              
                                    curve1->attach( myPlot );
                                    curve2->attach( myPlot );
                              
                                    // finally, refresh the plot
                                    myPlot->replot();
                                  \endcode
                                \endpar
                              */
                              
                              class QWT_EXPORT QwtPlot: public QFrame, public QwtPlotDict
                              {
                                  Q_OBJECT
                              
                                  Q_PROPERTY( QBrush canvasBackground
                                      READ canvasBackground WRITE setCanvasBackground )
                                  Q_PROPERTY( bool autoReplot READ autoReplot WRITE setAutoReplot )
                              
                              #if 0
                                  // This property is intended to configure the plot
                                  // widget from a special dialog in the deigner plugin.
                                  // Disabled until such a dialog has been implemented.
                              
                                  Q_PROPERTY( QString propertiesDocument
                                      READ grabProperties WRITE applyProperties )
                              #endif
                              
                              public:
                                  //! \brief Axis index
                                  enum Axis
                                  {
                                      //! Y axis left of the canvas
                                      yLeft,
                              
                                      //! Y axis right of the canvas
                                      yRight,
                              
                                      //! X axis below the canvas
                                      xBottom,
                              
                                      //! X axis above the canvas
                                      xTop,
                              
                                      //! Number of axes
                                      axisCnt
                                  };
                              
                                  /*!
                                      Position of the legend, relative to the canvas.
                              
                                      \sa insertLegend()
                                   */
                                  enum LegendPosition
                                  {
                                      //! The legend will be left from the QwtPlot::yLeft axis.
                                      LeftLegend,
                              
                                      //! The legend will be right from the QwtPlot::yRight axis.
                                      RightLegend,
                              
                                      //! The legend will be below the footer
                                      BottomLegend,
                              
                                      //! The legend will be above the title
                                      TopLegend
                                  };
                              
                                  explicit QwtPlot( QWidget * = NULL );
                                  explicit QwtPlot( const QwtText &title, QWidget * = NULL );
                              
                                  virtual ~QwtPlot();
                              
                                  void applyProperties( const QString & );
                                  QString grabProperties() const;
                              
                                  void setAutoReplot( bool = true );
                                  bool autoReplot() const;
                              
                                  // Layout
                              
                                  void setPlotLayout( QwtPlotLayout * );
                              
                                  QwtPlotLayout *plotLayout();
                                  const QwtPlotLayout *plotLayout() const;
                              
                                  // Title
                              
                                  void setTitle( const QString & );
                                  void setTitle( const QwtText & );
                                  QwtText title() const;
                              
                                  QwtTextLabel *titleLabel();
                                  const QwtTextLabel *titleLabel() const;
                              
                                  // Footer
                              
                                  void setFooter( const QString & );
                                  void setFooter( const QwtText & );
                                  QwtText footer() const;
                              
                                  QwtTextLabel *footerLabel();
                                  const QwtTextLabel *footerLabel() const;
                              
                                  // Canvas
                              
                                  void setCanvas( QWidget * );
                              
                                  QWidget *canvas();
                                  const QWidget *canvas() const;
                              
                                  void setCanvasBackground( const QBrush & );
                                  QBrush canvasBackground() const;
                              
                                  virtual QwtScaleMap canvasMap( int axisId ) const;
                              
                                  double invTransform( int axisId, int pos ) const;
                                  double transform( int axisId, double value ) const;
                              
                                  // Axes
                              
                                  QwtScaleEngine *axisScaleEngine( int axisId );
                                  const QwtScaleEngine *axisScaleEngine( int axisId ) const;
                                  void setAxisScaleEngine( int axisId, QwtScaleEngine * );
                              
                                  void setAxisAutoScale( int axisId, bool on = true );
                                  bool axisAutoScale( int axisId ) const;
                              
                                  void enableAxis( int axisId, bool tf = true );
                                  bool axisEnabled( int axisId ) const;
                              
                                  void setAxisFont( int axisId, const QFont & );
                                  QFont axisFont( int axisId ) const;
                              
                                  void setAxisScale( int axisId, double min, double max, double stepSize = 0 );
                                  void setAxisScaleDiv( int axisId, const QwtScaleDiv & );
                                  void setAxisScaleDraw( int axisId, QwtScaleDraw * );
                              
                                  double axisStepSize( int axisId ) const;
                                  QwtInterval axisInterval( int axisId ) const;
                              
                                  const QwtScaleDiv &axisScaleDiv( int axisId ) const;
                              
                                  const QwtScaleDraw *axisScaleDraw( int axisId ) const;
                                  QwtScaleDraw *axisScaleDraw( int axisId );
                              
                                  const QwtScaleWidget *axisWidget( int axisId ) const;
                                  QwtScaleWidget *axisWidget( int axisId );
                              
                                  void setAxisLabelAlignment( int axisId, Qt::Alignment );
                                  void setAxisLabelRotation( int axisId, double rotation );
                              
                                  void setAxisTitle( int axisId, const QString & );
                                  void setAxisTitle( int axisId, const QwtText & );
                                  QwtText axisTitle( int axisId ) const;
                              
                                  void setAxisMaxMinor( int axisId, int maxMinor );
                                  int axisMaxMinor( int axisId ) const;
                              
                                  void setAxisMaxMajor( int axisId, int maxMajor );
                                  int axisMaxMajor( int axisId ) const;
                              
                                  // Legend
                              
                                  void insertLegend( QwtAbstractLegend *,
                                      LegendPosition = QwtPlot::RightLegend, double ratio = -1.0 );
                              
                                  QwtAbstractLegend *legend();
                                  const QwtAbstractLegend *legend() const;
                              
                                  void updateLegend();
                                  void updateLegend( const QwtPlotItem * );
                              
                                  // Misc
                              
                                  virtual QSize sizeHint() const;
                                  virtual QSize minimumSizeHint() const;
                              
                                  virtual void updateLayout();
                                  virtual void drawCanvas( QPainter * );
                              
                                  void updateAxes();
                                  void updateCanvasMargins();
                              
                                  virtual void getCanvasMarginsHint(
                                      const QwtScaleMap maps[], const QRectF &canvasRect,
                                      double &left, double &top, double &right, double &bottom) const;
                              
                                  virtual bool event( QEvent * );
                                  virtual bool eventFilter( QObject *, QEvent * );
                              
                                  virtual void drawItems( QPainter *, const QRectF &,
                                      const QwtScaleMap maps[axisCnt] ) const;
                              
                                  virtual QVariant itemToInfo( QwtPlotItem * ) const;
                                  virtual QwtPlotItem *infoToItem( const QVariant & ) const;
                              
                              Q_SIGNALS:
                                  /*!
                                    A signal indicating, that an item has been attached/detached
                              
                                    \param plotItem Plot item
                                    \param on Attached/Detached
                                   */
                                  void itemAttached( QwtPlotItem *plotItem, bool on );
                              
                                  /*!
                                    A signal with the attributes how to update
                                    the legend entries for a plot item.
                              
                                    \param itemInfo Info about a plot item, build from itemToInfo()
                                    \param data Attributes of the entries ( usually <= 1 ) for
                                                the plot item.
                              
                                    \sa itemToInfo(), infoToItem(), QwtAbstractLegend::updateLegend()
                                   */
                                  void legendDataChanged( const QVariant &itemInfo,
                                      const QList<QwtLegendData> &data );
                              
                              public Q_SLOTS:
                                  virtual void replot();
                                  void autoRefresh();
                              
                              protected:
                                  static bool axisValid( int axisId );
                              
                                  virtual void resizeEvent( QResizeEvent *e );
                              
                              private Q_SLOTS:
                                  void updateLegendItems( const QVariant &itemInfo,
                                      const QList<QwtLegendData> &legendData );
                              
                              private:
                                  friend class QwtPlotItem;
                                  void attachItem( QwtPlotItem *, bool );
                              
                                  void initAxesData();
                                  void deleteAxesData();
                                  void updateScaleDiv();
                              
                                  void initPlot( const QwtText &title );
                              
                                  class AxisData;
                                  AxisData *d_axisData[axisCnt];
                              
                                  class PrivateData;
                                  PrivateData *d_data;
                              };
                              
                              #endif
                              
                              JonBJ 1 Reply Last reply
                              0
                              • C Chaki

                                It is quit large:

                                /* -*- mode: C++ ; c-file-style: "stroustrup" -*- *****************************
                                 * Qwt Widget Library
                                 * Copyright (C) 1997   Josef Wilgen
                                 * Copyright (C) 2002   Uwe Rathmann
                                 *
                                 * This library is free software; you can redistribute it and/or
                                 * modify it under the terms of the Qwt License, Version 1.0
                                 *****************************************************************************/
                                
                                #ifndef QWT_PLOT_H
                                #define QWT_PLOT_H
                                
                                #include "qwt_global.h"
                                #include "qwt_text.h"
                                #include "qwt_plot_dict.h"
                                #include "qwt_scale_map.h"
                                #include "qwt_interval.h"
                                #include <qframe.h>
                                #include <qlist.h>
                                #include <qvariant.h>
                                
                                class QwtPlotLayout;
                                class QwtAbstractLegend;
                                class QwtScaleWidget;
                                class QwtScaleEngine;
                                class QwtScaleDiv;
                                class QwtScaleDraw;
                                class QwtTextLabel;
                                
                                /*!
                                  \brief A 2-D plotting widget
                                
                                  QwtPlot is a widget for plotting two-dimensional graphs.
                                  An unlimited number of plot items can be displayed on
                                  its canvas. Plot items might be curves (QwtPlotCurve), markers
                                  (QwtPlotMarker), the grid (QwtPlotGrid), or anything else derived
                                  from QwtPlotItem.
                                  A plot can have up to four axes, with each plot item attached to an x- and
                                  a y axis. The scales at the axes can be explicitly set (QwtScaleDiv), or
                                  are calculated from the plot items, using algorithms (QwtScaleEngine) which
                                  can be configured separately for each axis.
                                
                                  The simpleplot example is a good starting point to see how to set up a
                                  plot widget.
                                
                                  \image html plot.png
                                
                                  \par Example
                                    The following example shows (schematically) the most simple
                                    way to use QwtPlot. By default, only the left and bottom axes are
                                    visible and their scales are computed automatically.
                                    \code
                                      #include <qwt_plot.h>
                                      #include <qwt_plot_curve.h>
                                
                                      QwtPlot *myPlot = new QwtPlot( "Two Curves", parent );
                                
                                      // add curves
                                      QwtPlotCurve *curve1 = new QwtPlotCurve( "Curve 1" );
                                      QwtPlotCurve *curve2 = new QwtPlotCurve( "Curve 2" );
                                
                                      // connect or copy the data to the curves
                                      curve1->setData( ... );
                                      curve2->setData( ... );
                                
                                      curve1->attach( myPlot );
                                      curve2->attach( myPlot );
                                
                                      // finally, refresh the plot
                                      myPlot->replot();
                                    \endcode
                                  \endpar
                                */
                                
                                class QWT_EXPORT QwtPlot: public QFrame, public QwtPlotDict
                                {
                                    Q_OBJECT
                                
                                    Q_PROPERTY( QBrush canvasBackground
                                        READ canvasBackground WRITE setCanvasBackground )
                                    Q_PROPERTY( bool autoReplot READ autoReplot WRITE setAutoReplot )
                                
                                #if 0
                                    // This property is intended to configure the plot
                                    // widget from a special dialog in the deigner plugin.
                                    // Disabled until such a dialog has been implemented.
                                
                                    Q_PROPERTY( QString propertiesDocument
                                        READ grabProperties WRITE applyProperties )
                                #endif
                                
                                public:
                                    //! \brief Axis index
                                    enum Axis
                                    {
                                        //! Y axis left of the canvas
                                        yLeft,
                                
                                        //! Y axis right of the canvas
                                        yRight,
                                
                                        //! X axis below the canvas
                                        xBottom,
                                
                                        //! X axis above the canvas
                                        xTop,
                                
                                        //! Number of axes
                                        axisCnt
                                    };
                                
                                    /*!
                                        Position of the legend, relative to the canvas.
                                
                                        \sa insertLegend()
                                     */
                                    enum LegendPosition
                                    {
                                        //! The legend will be left from the QwtPlot::yLeft axis.
                                        LeftLegend,
                                
                                        //! The legend will be right from the QwtPlot::yRight axis.
                                        RightLegend,
                                
                                        //! The legend will be below the footer
                                        BottomLegend,
                                
                                        //! The legend will be above the title
                                        TopLegend
                                    };
                                
                                    explicit QwtPlot( QWidget * = NULL );
                                    explicit QwtPlot( const QwtText &title, QWidget * = NULL );
                                
                                    virtual ~QwtPlot();
                                
                                    void applyProperties( const QString & );
                                    QString grabProperties() const;
                                
                                    void setAutoReplot( bool = true );
                                    bool autoReplot() const;
                                
                                    // Layout
                                
                                    void setPlotLayout( QwtPlotLayout * );
                                
                                    QwtPlotLayout *plotLayout();
                                    const QwtPlotLayout *plotLayout() const;
                                
                                    // Title
                                
                                    void setTitle( const QString & );
                                    void setTitle( const QwtText & );
                                    QwtText title() const;
                                
                                    QwtTextLabel *titleLabel();
                                    const QwtTextLabel *titleLabel() const;
                                
                                    // Footer
                                
                                    void setFooter( const QString & );
                                    void setFooter( const QwtText & );
                                    QwtText footer() const;
                                
                                    QwtTextLabel *footerLabel();
                                    const QwtTextLabel *footerLabel() const;
                                
                                    // Canvas
                                
                                    void setCanvas( QWidget * );
                                
                                    QWidget *canvas();
                                    const QWidget *canvas() const;
                                
                                    void setCanvasBackground( const QBrush & );
                                    QBrush canvasBackground() const;
                                
                                    virtual QwtScaleMap canvasMap( int axisId ) const;
                                
                                    double invTransform( int axisId, int pos ) const;
                                    double transform( int axisId, double value ) const;
                                
                                    // Axes
                                
                                    QwtScaleEngine *axisScaleEngine( int axisId );
                                    const QwtScaleEngine *axisScaleEngine( int axisId ) const;
                                    void setAxisScaleEngine( int axisId, QwtScaleEngine * );
                                
                                    void setAxisAutoScale( int axisId, bool on = true );
                                    bool axisAutoScale( int axisId ) const;
                                
                                    void enableAxis( int axisId, bool tf = true );
                                    bool axisEnabled( int axisId ) const;
                                
                                    void setAxisFont( int axisId, const QFont & );
                                    QFont axisFont( int axisId ) const;
                                
                                    void setAxisScale( int axisId, double min, double max, double stepSize = 0 );
                                    void setAxisScaleDiv( int axisId, const QwtScaleDiv & );
                                    void setAxisScaleDraw( int axisId, QwtScaleDraw * );
                                
                                    double axisStepSize( int axisId ) const;
                                    QwtInterval axisInterval( int axisId ) const;
                                
                                    const QwtScaleDiv &axisScaleDiv( int axisId ) const;
                                
                                    const QwtScaleDraw *axisScaleDraw( int axisId ) const;
                                    QwtScaleDraw *axisScaleDraw( int axisId );
                                
                                    const QwtScaleWidget *axisWidget( int axisId ) const;
                                    QwtScaleWidget *axisWidget( int axisId );
                                
                                    void setAxisLabelAlignment( int axisId, Qt::Alignment );
                                    void setAxisLabelRotation( int axisId, double rotation );
                                
                                    void setAxisTitle( int axisId, const QString & );
                                    void setAxisTitle( int axisId, const QwtText & );
                                    QwtText axisTitle( int axisId ) const;
                                
                                    void setAxisMaxMinor( int axisId, int maxMinor );
                                    int axisMaxMinor( int axisId ) const;
                                
                                    void setAxisMaxMajor( int axisId, int maxMajor );
                                    int axisMaxMajor( int axisId ) const;
                                
                                    // Legend
                                
                                    void insertLegend( QwtAbstractLegend *,
                                        LegendPosition = QwtPlot::RightLegend, double ratio = -1.0 );
                                
                                    QwtAbstractLegend *legend();
                                    const QwtAbstractLegend *legend() const;
                                
                                    void updateLegend();
                                    void updateLegend( const QwtPlotItem * );
                                
                                    // Misc
                                
                                    virtual QSize sizeHint() const;
                                    virtual QSize minimumSizeHint() const;
                                
                                    virtual void updateLayout();
                                    virtual void drawCanvas( QPainter * );
                                
                                    void updateAxes();
                                    void updateCanvasMargins();
                                
                                    virtual void getCanvasMarginsHint(
                                        const QwtScaleMap maps[], const QRectF &canvasRect,
                                        double &left, double &top, double &right, double &bottom) const;
                                
                                    virtual bool event( QEvent * );
                                    virtual bool eventFilter( QObject *, QEvent * );
                                
                                    virtual void drawItems( QPainter *, const QRectF &,
                                        const QwtScaleMap maps[axisCnt] ) const;
                                
                                    virtual QVariant itemToInfo( QwtPlotItem * ) const;
                                    virtual QwtPlotItem *infoToItem( const QVariant & ) const;
                                
                                Q_SIGNALS:
                                    /*!
                                      A signal indicating, that an item has been attached/detached
                                
                                      \param plotItem Plot item
                                      \param on Attached/Detached
                                     */
                                    void itemAttached( QwtPlotItem *plotItem, bool on );
                                
                                    /*!
                                      A signal with the attributes how to update
                                      the legend entries for a plot item.
                                
                                      \param itemInfo Info about a plot item, build from itemToInfo()
                                      \param data Attributes of the entries ( usually <= 1 ) for
                                                  the plot item.
                                
                                      \sa itemToInfo(), infoToItem(), QwtAbstractLegend::updateLegend()
                                     */
                                    void legendDataChanged( const QVariant &itemInfo,
                                        const QList<QwtLegendData> &data );
                                
                                public Q_SLOTS:
                                    virtual void replot();
                                    void autoRefresh();
                                
                                protected:
                                    static bool axisValid( int axisId );
                                
                                    virtual void resizeEvent( QResizeEvent *e );
                                
                                private Q_SLOTS:
                                    void updateLegendItems( const QVariant &itemInfo,
                                        const QList<QwtLegendData> &legendData );
                                
                                private:
                                    friend class QwtPlotItem;
                                    void attachItem( QwtPlotItem *, bool );
                                
                                    void initAxesData();
                                    void deleteAxesData();
                                    void updateScaleDiv();
                                
                                    void initPlot( const QwtText &title );
                                
                                    class AxisData;
                                    AxisData *d_axisData[axisCnt];
                                
                                    class PrivateData;
                                    PrivateData *d_data;
                                };
                                
                                #endif
                                
                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on last edited by JonB
                                #16

                                @Chaki
                                Yeah, not your fault, that doesn't help me.

                                Looking around, I came across a Python sample at https://pythonhosted.org/python-qwt/reference/plot.html.

                                I note this has:

                                from qwt.qt.QtGui import QApplication
                                ...
                                
                                app = QApplication([])
                                

                                Now, that is choosing to access the QApplication object not just from PyQt5 or PySide2 but from some qwt.qt.QtGui namespace. I'm wondering whether this is somehow different from the "normal" QApplication and is required for a Qwt app...? Can you remove your own #include <QApplication> and go find some equivalent of qwt.qt.QtGui to import it from?

                                Ah ha! Have a read of https://stackoverflow.com/questions/30185168/qt-with-qwt-runtime-error-qwidget-must-construct-a-qapplication-before-a-qwidg or https://stackoverflow.com/questions/27337630/error-with-qwt-qwidget-must-construct-a-qapplication-before-a-qpaintdevice. You are not alone! I Googled for qwtplot qapplication, I must now leave you to do similar and see what you can come up with from other peoples' problems/solutions....

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                                • C Offline
                                  C Offline
                                  Chaki
                                  wrote on last edited by
                                  #17

                                  Thanks for your effort Sir!

                                  1 Reply Last reply
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