Qwt and QMainWindow singleton instance sharing method across classes

[picobuntu]

Hi,

When I started using Qwt I used it in a single class (my only class), but getting bigger and bigger I decided to split it.
So now I have a class handling the mainWindow and another one using only Qwt for plotting curves.

In order to have access to my UI, I used a singleton technique (with getInstance() and static member) with methods in my mainwindow class (like getters or setters to use the ui pointer).

But with that, I have an error when it compile : "QWidget: Must construct a QApplication before a QPaintDevice". With Google I learned it was caused by my static instance of mainwindow.

How can I reconcile a nice way to access my Ui in other classes and Qwt ?

Thanks in advance
Have a nice day

[Santosh Reddy]

Show us your main(), and where do you create QApplication, QMainWindow and other Qwt objects?

[picobuntu]

Main.cpp

#include <QtGui/QApplication>
#include "viewer.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Viewer w;
    w.show();
 
    return a.exec();
}

#include <QtGui/QApplication>
#include "viewer.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Viewer w;
    w.show();

    return a.exec();
}

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Viewer.cpp == MainWindow

#include "viewer.h"
#include "ui_viewer.h"
 
Viewer* Viewer::instance = 0;
 
Viewer* Viewer::getInstance(){
    if(instance==0){
        instance = new Viewer;
    }
    return instance;
}
 
#include <QtGui>
#include <QDebug>
 
Viewer::Viewer(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::Viewer)
{
    ui->setupUi(this);
    //connects(...);
}
 
Viewer::~Viewer()
{
    delete ui;
}
 
void Viewer::setPlot(QwtPlot *plot, QString plotType){
    if(plotType=="dark") ui->darkPlot->addWidget(plot);
    else ui->lightPlot->addWidget(plot);
}
 
Various functions without QwtPlot () {...}

#include "viewer.h"
#include "ui_viewer.h"

Viewer* Viewer::instance = 0;

Viewer* Viewer::getInstance(){
    if(instance==0){
        instance = new Viewer;
    }
    return instance;
}

#include <QtGui>
#include <QDebug>

Viewer::Viewer(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::Viewer)
{
    ui->setupUi(this);
    //connects(...);
}

Viewer::~Viewer()
{
    delete ui;
}

void Viewer::setPlot(QwtPlot *plot, QString plotType){
    if(plotType=="dark") ui->darkPlot->addWidget(plot);
    else ui->lightPlot->addWidget(plot);
}

Various functions without QwtPlot () {...}

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viewer.h

class Viewer : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit Viewer(QWidget *parent = 0);
    static Viewer* getInstance();
    ~Viewer();
    void VariousFunctionsWithoutQwt(...);
 
private slots:
    void VariousFunctionsWithoutQwt(...);
 
private:
    Ui::Viewer *ui;
    static Viewer* instance;
 
};

class Viewer : public QMainWindow
{
    Q_OBJECT
    
public:
    explicit Viewer(QWidget *parent = 0);
    static Viewer* getInstance();
    ~Viewer();
    void VariousFunctionsWithoutQwt(...);

private slots:
    void VariousFunctionsWithoutQwt(...);
    
private:
    Ui::Viewer *ui;
    static Viewer* instance;

};

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and finally my plot.cpp and plot.h where there is Qwt used.
plot.cpp:

#include "plot.h"
#include "viewer.h"
 
QwtPlot *plotD;
QwtPlot *plotL;
 
Viewer* viewer = Viewer::getInstance();
 
Plot::Plot(QWidget *parent) : QWidget(parent)
{
}
 
void Plot::initPlot(){
    if(viewer->isPlotEmpty("dark")){
        plotD = new QwtPlot();
        ...
        viewer->setPlot(plotD,"dark");
    }
    if(viewer->isPlotEmpty("light")){
        plotL = new QwtPlot();
        ...
        viewer->setPlot(plotL,"light");
    }
 
}
 
void Plot::plot(){
    ...
    plotD->replot();
    plotL->replot();
}

#include "plot.h"
#include "viewer.h"

QwtPlot *plotD;
QwtPlot *plotL;

Viewer* viewer = Viewer::getInstance();

Plot::Plot(QWidget *parent) : QWidget(parent)
{
}

void Plot::initPlot(){
    if(viewer->isPlotEmpty("dark")){
        plotD = new QwtPlot();
        ...
        viewer->setPlot(plotD,"dark");
    }
    if(viewer->isPlotEmpty("light")){
        plotL = new QwtPlot();
        ...
        viewer->setPlot(plotL,"light");
    }

}

void Plot::plot(){
    ...
    plotD->replot();
    plotL->replot();
}

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plot.h

#ifndef PLOT_H
#define PLOT_H
 
#include <QWidget>
 
#include "qwt_plot.h"
#include "qwt_plot_curve.h"
#include "qwt_symbol.h"
#include "qwt_plot_marker.h"
#include "qwt_legend.h"
#include "qwt_scale_engine.h"
 
class Plot : public QWidget
{
    Q_OBJECT
public:
    explicit Plot(QWidget *parent);
 
private:
    void initPlot();
 
private slots:
    void plot();
 
};
 
#endif // PLOT_H

#ifndef PLOT_H
#define PLOT_H

#include <QWidget>

#include "qwt_plot.h"
#include "qwt_plot_curve.h"
#include "qwt_symbol.h"
#include "qwt_plot_marker.h"
#include "qwt_legend.h"
#include "qwt_scale_engine.h"

class Plot : public QWidget
{
    Q_OBJECT
public:
    explicit Plot(QWidget *parent);

private:
    void initPlot();
        
private slots:
    void plot();

};

#endif // PLOT_H

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Thanks

[Uwe]

"QWidget: Must construct a QApplication before a QPaintDevice".

AFAIR this is the result of a build issue even if it happens at runtime - but as I'm not using Windows myself I'm not 100% sure.
Do you have a properly installed version ( which one ? ) of Qwt and are you using qwt.prf included from a qmake project file ?

Uwe