[SOLVED]QWidget: Cannot create a QWidget when no GUI is being used

[nimingzhe2008]

When I ran my code,no widget appeared and IDE showed me the message "QWidget: Cannot create a QWidget when no GUI is being used" . I don't know why.Here's my code

tooltipzone_mine.pro
@#-------------------------------------------------

Project created by QtCreator 2012-11-03T21:39:57

#-------------------------------------------------

QT += core
QT += gui

TARGET = tooltipzone_mine

TEMPLATE = app

SOURCES += main.cpp
tipzone.cpp

HEADERS +=
tipzone.h
@

tipzone.h
@#ifndef TIPZONE_H
#define TIPZONE_H
#include<QWidget>

class tipzone:public QWidget
{
public:
tipzone();
void paintEvent( QPaintEvent* );
};

#endif // TIPZONE_H@

tipzone.cpp
@#include "tipzone.h"

#include<QPainter>
#include<QPaintEvent>
#include<QRect>

tipzone::tipzone():QWidget()
{
}

void tipzone::paintEvent(QPaintEvent *)
{
QRect redRect, greenRect, blueRect, yellowRect;

redRect = QRect( 0, 0, width()/2, height()/2 );
greenRect = QRect( width()/2, 0, width()/2, height()/2 );
blueRect = QRect( 0, height()/2, width()/2, height()/2 );
yellowRect = QRect( width()/2, height()/2, width()/2, height()/2 );

QPainter p( this );

p.setPen( Qt::black );

p.setBrush( Qt::red );
p.drawRect( redRect );

p.setBrush( Qt::green );
p.drawRect( greenRect );

p.setBrush( Qt::blue );
p.drawRect( blueRect );

p.setBrush( Qt::yellow );
p.drawRect( yellowRect );

}

@

main.cpp
@#include <QtCore/QCoreApplication>
#include"tipzone.h"

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
tipzone t;
t.show();
return a.exec();
}
@

[Code_ReaQtor]

I think the problem with your code was that

@void paintEvent( QPaintEvent* );@

is a [virtual protected] function, "QWidget":http://qt-project.org/doc/qt-4.8/qwidget.html#paintEvent

You may try this to your header file:

@#ifndef TIPZONE_H
#define TIPZONE_H
#include<QWidget>

class tipzone:public QWidget
{
public:
tipzone();

protected:
void paintEvent( QPaintEvent* );
};

#endif // TIPZONE_H@

[nimingzhe2008]

[quote author="Code_ReaQtor" date="1352037059"]I think the problem with your code was that

@void paintEvent( QPaintEvent* );@

is a [virtual protected] function, "QWidget":http://qt-project.org/doc/qt-4.8/qwidget.html#paintEvent

You may try this to your header file:

@#ifndef TIPZONE_H
#define TIPZONE_H
#include<QWidget>

class tipzone:public QWidget
{
public:
tipzone();

protected:
void paintEvent( QPaintEvent* );
};

#endif // TIPZONE_H@[/quote]

Thank you,but it doesn't work.

[nimingzhe2008]

I got it.I should use QApplication instead of QCoreApplication in main.cpp.

[MuhamedAuda]

Dear nmingzhe2008
it easy to build this project if u used Qt Creator wizard

just start new project.

type class name : tipzone

and base class: Qwidget

and then add ur own code

@#-------------------------------------------------

Project created by QtCreator 2012-11-04T17:03:28

#-------------------------------------------------

QT += core gui

TARGET = untitled3
TEMPLATE = app

SOURCES += main.cpp
tipzone.cpp

HEADERS += tipzone.h
@

@//// tipzone.h
#ifndef TIPZONE_H
#define TIPZONE_H

#include <QtGui/QWidget>

class tipzone : public QWidget
{
Q_OBJECT

public:
tipzone(QWidget *parent = 0);
// just i add paintEvent only
void paintEvent(QPaintEvent *);
~tipzone();
};

#endif // TIPZONE_H
@

@////////////// tipzone.cpp

#include "tipzone.h"
// and added ur includefiles
#include<QPainter>
#include<QPaintEvent>
#include<QRect>

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

tipzone::~tipzone()
{
}
// now just implementation for pain ...
void tipzone::paintEvent(QPaintEvent *e)
{
// now i added ur code
QRect redRect, greenRect, blueRect, yellowRect;

redRect = QRect( 0, 0, width()/2, height()/2 );
greenRect = QRect( width()/2, 0, width()/2, height()/2 );
blueRect = QRect( 0, height()/2, width()/2, height()/2 );
yellowRect = QRect( width()/2, height()/2, width()/2, height()/2 );
QPainter p( this );
p.setPen( Qt::black );
p.setBrush( Qt::red );
p.drawRect( redRect );
p.setBrush( Qt::green );
p.drawRect( greenRect );
p.setBrush( Qt::blue );
p.drawRect( blueRect );
p.setBrush( Qt::yellow );
p.drawRect( yellowRect );

}
@

@/////////main.cpp

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

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
tipzone w;
w.show();
return a.exec();
}
@

the out put divided the main windown into four rectangle colored in Red, Green, Blue and Yallow colos nice code
Muhamed Auda

[Macro]

[quote author="nimingzhe2008" date="1352040548"]I got it.I should use QApplication instead of QCoreApplication in main.cpp.[/quote]

Yeah That's Correct. QCoreApplication is used for non GUI Applications. For more details check this "QCoreApplication Class Reference.":http://qt-project.org/doc/qt-4.8/qcoreapplication.html#details