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!
Related
I have applied a dark style to my application, so good until then, my question is why the style is not applied to the title bar of my application, and the rest of the forms that open in me application, as you can see, it stays white and looks very bad, any suggestions would be appreciated.
#include "mainwindow.h"
#include <QApplication>
#include <QFile>
#include <QTextStream>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QFile f(":/qdarkstyle/style.qss");
f.open(QIODevice::ReadOnly|QIODevice::Text);
QTextStream ts(&f);
a.setStyleSheet(ts.readAll());
MainWindow w;
w.show();
return a.exec();
}
I got the subject from here.
https://github.com/ColinDuquesnoy/QDarkStyleSheet
[Works on Qt 6.2.2, not sure about Qt5]
There might be a configuration file qt.conf in the same folder as your executable file. If it does not exist, create. And add the lines
[Platforms]
WindowsArguments = darkmode=1
Use darkmode=2 if you also want to change the title bar colour on the fly.
I am developing a GUI application, but whenever I am trying to close the application, it throws an error that "Must construct QGuiapplication first". My main is not returning exit code 0, so it's not exiting normally. I think some destructor is getting called twice but need some help here. I am attaching main.cpp code here for reference.
#include <QGuiApplication>
#include <QFontDatabase>
#include <QtWebEngine>
#include "ApplicationManager.h"
#include "AppLogger.h"
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QCoreApplication::setAttribute(Qt::AA_UseOpenGLES);
QGuiApplication app(argc, argv);
QtWebEngine::initialize();
app.setApplicationName("MCS3.0");
QFontDatabase::addApplicationFont(":/Fonts/Roboto.ttf");
#ifdef VERSION
app.setApplicationVersion(QString("%1").arg(VERSION));
logInfoMessage(app.applicationName()+app.applicationVersion()+" Started");
#endif
ApplicationManager::instance().run();
return app.exec();
}
The relevant part of the problem is inside ApplicationManager.h which was not exposed by OP.
I bet that it makes another instance of QApplication (or QGUIApplication or QCoreApplication).
How can I know this? It's partly a guess (as the name looks like) and partly result of the following test:
testQApp.cc:
#include <QtWidgets>
int main(int argc, char **argv)
{
QApplication app(argc, argv);
{ QApplication app(argc, argv);
QLabel qLbl("The app in app");
qLbl.show();
app.exec();
}
return app.exec();
}
testQApp.pro:
SOURCES = testQApp.cc
QT = widgets
Compiled and tested in cygwin64 on Windows 10:
$ qmake-qt5 testQApp.pro
$ make
$ ./testQApp
When I quit the application, the issue occurs:
QApplication::exec: Please instantiate the QApplication object first
Segmentation fault (core dumped)
$
To make this complete, the relevant paragraph of doc. about QApplication:
For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any given time. For non-QWidget based Qt applications, use QGuiApplication instead, as it does not depend on the QtWidgets library.
Please, note that the emphasize is not done by me.
I have the following piece of code:
#include <QtWidgets/QtWidgets>
#include <QtMultimedia/QCamera>
#include <QtMultimedia/QMediaPlayer>
int main(int argc, char * argv[])
{
QApplication testQt(argc, argv);
QMainWindow w;
QWidget videoContainer(&w);
w.setCentralWidget(&videoContainer);
QVideoWidget videoWidget(&videoContainer);
QCamera cam(&w);
cam.setViewfinder(&videoWidget);
cam.start();
w.show();
return testQt.exec();
}
in which I am trying to create a main window, create a container widget to display video, create a videowidget in that container, and then finally set the viewfinder of the camera to that videowidget. However, when I try to do this I get the error
Variable has incomplete type 'QVideoWidget'
Why am I getting this error?
You need to include the corresponding header as follows:
#include <QVideoWidget>
You may also need to add this to your project file:
QT += multimediawidgets
I was trying to create a simple console application to try out Qt's XML parser. I started a project in VS2008 and got this template:
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
return a.exec();
}
Since I don't need event processing, I was wondering whether I may get into trouble if I neglect to create a QCoreApplication and running the event loop. The docs state that it's recommended in most cases.
For the sake of curiosity however, I am wondering how could I make some generic task execute on the event loop and then terminate the application. I was unable to google a relevant example.
Here is one simple way you could structure an application if you want an event loop running.
// main.cpp
#include <QtCore>
class Task : public QObject
{
Q_OBJECT
public:
Task(QObject *parent = 0) : QObject(parent) {}
public slots:
void run()
{
// Do processing here
emit finished();
}
signals:
void finished();
};
#include "main.moc"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
// Task parented to the application so that it
// will be deleted by the application.
Task *task = new Task(&a);
// This will cause the application to exit when
// the task signals finished.
QObject::connect(task, SIGNAL(finished()), &a, SLOT(quit()));
// This will run the task from the application event loop.
QTimer::singleShot(0, task, SLOT(run()));
return a.exec();
}
Don't forget to add the
CONFIG += console
flag in the qmake .pro file.
For the rest is just using some of Qt classes.
One way I use it is to spawn processes cross-platform.
You don't need the QCoreApplication at all, just include your Qt objects as you would other objects, for example:
#include <QtCore>
int main()
{
QVector<int> a; // Qt object
for (int i=0; i<10; i++)
{
a.append(i);
}
/* manipulate a here */
return 0;
}
I managed to create a simple console "hello world" with QT Creator
used creator 2.4.1 and QT 4.8.0 on windows 7
two ways to do this
Plain C++
do the following
File- new file project
under projects select : other Project
select "Plain C++ Project"
enter project name
5.Targets select Desktop 'tick it'
project managment just click next
you can use c++ commands as normal c++
or
QT Console
File- new file project
under projects select : other Project
select QT Console Application
Targets select Desktop 'tick it'
project managment just click next
add the following lines (all the C++ includes you need)
add "#include 'iostream' "
add "using namespace std; "
after QCoreApplication a(int argc, cghar *argv[])
10 add variables, and your program code..
example: for QT console "hello world"
file - new file project 'project name '
other projects - QT Console Application
Targets select 'Desktop'
project management - next
code:
#include <QtCore/QCoreApplication>
#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
cout<<" hello world";
return a.exec();
}
ctrl -R to run
compilers used for above MSVC 2010 (QT SDK) , and minGW(QT SDK)
hope this helps someone
As I have just started to use QT recently and also searched the Www for info and examples to get started with simple examples still searching...
You could fire an event into the quit() slot of your application even without connect().
This way, the event-loop does at least one turn and should process the events within your main()-logic:
#include <QCoreApplication>
#include <QTimer>
int main(int argc, char *argv[])
{
QCoreApplication app( argc, argv );
// do your thing, once
QTimer::singleShot( 0, &app, &QCoreApplication::quit );
return app.exec();
}
Don't forget to place CONFIG += console in your .pro-file, or set consoleApplication: true in your .qbs Project.CppApplication.
You can call QCoreApplication::exit(0) to exit with code 0
Had the same problem. found some videos on Youtube.
So here is an even simpler suggestion. This is all the code you need:
#include <QDebug>
int main(int argc, char *argv[])
{
qDebug() <<"Hello World"<< endl;
return 0;
}
The above code comes from
Qt5 Tutorial: Building a simple Console application by
Dominique Thiebaut
http://www.youtube.com/watch?v=1_aF6o6t-J4
I have experience with C++ but I've never really used Qt before. I'm trying to connect to a SQLite database, so I found a tutorial here and am going with that. In the QtCreator IDE, I went to Add New --> C++ Class and in the header file pasted in the header the header from that link and in the .cpp file I pasted the source. My main.cpp looks like this:
#include <QtGui/QApplication>
#include "mainwindow.h"
#include "databasemanager.h"
#include <qlabel.h>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
DatabaseManager db();
QLabel hello("nothing...");
if(db.openDB()){ // Line 13
hello.setText("Win!");
}
else{
hello.setText("Lame!");
}
hello.resize(100, 30);
hello.show();
return a.exec();
}
And I'm getting this error:
main.cpp:13: error: request for member 'openDB' in 'db', which is of non-class type 'DatabaseManager()'
Can anyone point me in the right direction? I know "copypaste" code isn't good, I just wanted to see if I could get DB connectivity working and I figured something like this would be simple... thanks for the help.
Change the DatabaseManager line to:
DatabaseManager db;
You're declaring a local function called db that takes no parameters and returns a DatabaseManager object when you provide the ();