I've made LONG research on the web but can't find anything clear.
I think the answer is obvious but i'm beginer in Qt.
Why this code doesn't work ? My windows just popup realy fast.
Main.cpp
#include <QApplication>
#include "test.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Test test;
return a.exec();
}
Test.cpp
#include "test.h"
Test::Test()
{
MainWindow w;
w.show();
}
And this one work (the window keep open) :
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
Thank you !
The window closes because it is local variable of the Test constructor, and when the constructor exits, its destructor gets called, which closes it. You need to make the window object a member variable of the Test class.
Related
I am using vtk QT, i've installed VTK, PCL libraries well, after run the project the realese mode; i found the following error.
QWidget: Must construct a QApplication before a QWidget
As the error message says, you will need to create a QApplication before you can instantiate and use a QWidget. For example:
#include <QApplication>
#include <QDialog>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QDialog dialog;
dialog.setModal(true);
dialog.show();
return app.exec();
}
I am running a project that uses Qt. I have installed version 5.10.1 (msvc2017_64). I receive an error that the QSettings is undefined. I cannot find the reference in the libraries installed.
#include "mainwindow.h"
#include <QtWidgets/QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QCoreApplication::setApplicationName("Circuit Editor");
QSettings settings;
MainWindow w;
w.setWindowTitle("Circuit Editor");
w.show();
return a.exec();
}
My two cents:
1) You forgot to include the header:
#include <QSettings>
2) You declared a QSetting object but you are not doing anything with it. Is that what you want to do?
Never use #include <QtModule/QClass> format of includes. They hide project misconfiguration. Use #include <QClass> instead.
If you're unsure of what to include, include the entire module and deal with details later: #include <QtModule>.
Thus:
#include "mainwindow.h"
#if 1
// entire module
#include <QtWidgets>
#else
// needed classes
#include <QApplication>
#include <QSettings>
#endif
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QCoreApplication::setApplicationName("Circuit Editor");
QSettings settings;
MainWindow w;
w.setWindowTitle("Circuit Editor");
w.show();
return a.exec();
}
I just did a fresh install of Qt Creator so I have the latest version. I opened my project that I was working on from the previous install of Qt Creator. It complies without errors but seams to hang on this line:
QApplication a(argc, argv);
Here is the full main.cpp file:
#include "mainwindow.h"
#include <QApplication>
#include <QMouseEvent>
#include<QTextStream>
#include<QString>
int main(int argc, char *argv[]){
qDebug()<<1;
QApplication a(argc, argv);
qDebug()<<2;
MainWindow w;
w.show();
return a.exec();
}
When it runs my application will open displaying the window but nothing works.
Here is the applciation's output:
Starting /home/bandito/programming/build-etf-ipo-Unnamed-Debug/etf-ipo...
1
As you can see qDebug()<<1; is called but the application never makes it to qDebug()<<2;
I also tried replacing QApplication with QGuiApplication and QCoreApplication but both request the QApplication be called also.
I have problems to create a QMainWindow object from a subroutine. I need this to create a different number of windows on the user's request.
The problem lies in the fact that if I create it from a subroutine, it is created as a local variable and only shows for a split second. Usually I would need to return a pointer, but I have not managed to do it with this object. Here is the code:
#include <QApplication>
#include <QMainWindow>
QMainWindow runGUI(){
QMainWindow window;
window.show();
window.resize(340,260);
return window;
}
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
/*
QMainWindow window;
window.show();
window.resize(340,260);
*/
runGUI();
return app.exec();
}
How do I do this correctly?
Returning your QMainWindow from runGUI() calls the copy constructor: Qt objects are not designed to be copied. You should indeed return a pointer, and dynamically allocate the window:
#include <QApplication>
#include <QMainWindow>
QMainWindow* runGUI(){
QMainWindow* window = new QMainWindow;
window->setAttribute(Qt::WA_DeleteOnClose);
window->show();
window->resize(340,260);
return window;
}
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QMainWindow* window = runGUI();
return app.exec();
}
Note the use of Qt::WA_DeleteOnClose to ensure the window will be deleted when it is closed, avoiding memory leaks.
I would like to ask if you could help me with my app.
I have an application in C++ using Qt (and Qwt) in MCVS 2010. I want to open QDialog window with QwtPlot on button click from Main Window (QMainWindow). Here is some code:
MainWindow.cpp
void MainWindow::on_pushButton_1_clicked ()
{
Dialog_plot dp;
dp.setModal(true);
dp.exec();
}
Dialog_plot.cpp:
#include <qwt_plot.h>
#include <qwt_plot_canvas.h>
#include <qwt_plot_curve.h>
#include "Dialog_plot.h"
Dialog_plot::Dialog_plot(QWidget *parent)
{
plot = new QwtPlot();
//more code...
main.cpp:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
This code is compiling, but when I click on that pushbutton_1 in my application I'm getting error:
Must construct a QApplication before a QPaintDevice
I know that such error was discussed many times and I was reading a lot about it but I can't see solution for my problem.
One more thing I would like to mention - I have similar application with Qwt plot written by somebody else and his application compiles and works without any problems in my MCVS. I was trying to compare Linker/libraries included but it seems to be the same. So I guess there is a problem with my application, I just can't solve it. I really need some help!