c++ - Qt with qwt runtime error: QWidget: Must construct a QApplication before a QWidget -

i trying make app using qt 5.4.1(and qwt 6.1.2).here environment:

• windows 7 x64
• visual studio 2012
• qt5.4.1 static
• qwt6.1.2

and have built qwt qt static libs successfully. creat widget class inherited qwtplot,and creat mainwindow has object of widget. build project. however,there runtime error:qwidget: must construct qapplication before qwidget.

---

this widget class inherited qwtplot:

#pragma once    #include <qwt/qwt_plot.h>  #include <qwt/qwt_plot_curve.h>        class drawwidget: public qwtplot  {  public:  	drawwidget(qwidget *parent );  	~drawwidget(void);  };

drawwidget::drawwidget(qwidget *parent )  	 :   qwtplot( parent ),  	 carve(null)  {  	  }

and follow mainwindow class:

#ifndef mainwindow_h  #define mainwindow_h      #include "drawwidget.h"    #include <qtwidgets/qmainwindow>    class mainwindow : public qmainwindow  {  	q_object      public:      mainwindow(qwidget *parent = 0);      ~mainwindow();       private:      drawwidget *drawwidget;  	    };    #endif // mainwindow_h

  mainwindow::mainwindow(qwidget *parent)  	  :qmainwindow(parent)    {       qwidget *widget = new qwidget(this);       this->setcentralwidget(widget);  	 qhboxlayout *mainlayout = new qhboxlayout(widget);    	 drawwidget = new drawwidget(widget);         mainlayout->addwidget(drawwidget);       centralwidget()->setlayout(mainlayout);      }

and main.cpp:

#include "mainwindow.h"  #include <qtwidgets/qapplication>  #include <qtplugin>  q_import_plugin(qwindowsintegrationplugin);    int main(int argc, char *argv[])  {      qapplication a(argc, argv);  	mainwindow w;  	w.resize(1000,600);  	w.show();  	return a.exec();  }

i build project in release version.any idea? thank you!

i had same problem few weeks ago in case problem additional library. make sure got builds of additional library debug debug building , release release building. add additionallibrarydebug/bin folder path environment variable when build debug version , additionallibraryrelease/bin when build release version (not both in same time)

good luck :) solved problem on way. :)