QWidget: Must construct a QApplication before a QWidget
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@Chaki
That cannot be your "whole code", as it does not even have amain()
....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 fromw.show();
toreturn a.exec();
. And preferably do not link withqwt_plot
stuff at all. Does it work now? -
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(); }
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@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 ownQwtPlot
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. -
@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(); }
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@Chaki
Sorry, you are saying:-
With the code as shown --- the
#include
s included, but theQwtPlot 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.... :) -
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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
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@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 someqwt.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 ofqwt.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.... -