Got an error running the following problem:
#include <QMessageBox>
#include <QApplication>
int main() {
QApplication app();
QMessageBox msgBox(QMessageBox::Critical,
QObject::tr("text1"),
QObject::tr("text2"),
QMessageBox::Ok);
msgBox.exec();
return 0;
}
The error is:
The program breaks at QMessageBox msgBox(...);
The call stack is:
EDIT: Even after i have added QApplication instance in XTerm window named qtcreator_process_stub i see the following:
QWidget: Must construct QApplication before a QWidget
The line
QApplication app();
doesn't create a QApplication object - it actually declares a function taking no arguments and returning a QApplication! This is sometimes known as the "most vexing parse".
To actually construct the application object, you need to provide the program's arguments:
QApplication app(argc, argv);
The full program is then
#include <QMessageBox>
#include <QApplication>
int main(int argc, char **argv) {
QApplication app(argc, argv);
QMessageBox msgBox(QMessageBox::Critical,
QObject::tr("text1"),
QObject::tr("text2"),
QMessageBox::Ok);
msgBox.exec();
return 0;
}
Related
I have got FFmpeg compiled using MSYS2. I have a strange issue that appeared only recently.the app crashes.
This is my main file:
#include "mainwindow.h"
#include <QApplication>
#include <QDebug>
extern "C" {
#include <libavdevice/avdevice.h>
#include <libavformat/avformat.h>
#include <libavutil/avutil.h>
#include <libavcodec/avcodec.h>
}
int main(int argc, char *argv[])
{
//qDebug() << av_version_info();
// avdevice_register_all();
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
The program runs but crashes if I call 'avdevice_register_all()' and others FFmpeg functions，but only call 'av_version_info()' is runs, not crashes.
I don't really know how to solve this?
I have already tried to use google solving the problem,but no result.
I have following main.cpp:
#include <QtQml>
#include <QApplication>
#include <QQmlApplicationEngine>
#include "database/uepeoplemodel.h"
#include "core/uestatus.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QQmlApplicationEngine engine;
UeStatus* ueApplicationStatus=new UeStatus(qApp);
UePeopleModel* uePeopleModel=new UePeopleModel(qApp);
QObject::connect(uePeopleModel,
SIGNAL(ueSignalDatabaseConnectionChanged(UeTypeDatabaseConnectionStatus)),
ueApplicationStatus,
SLOT(ueSignalDatabaseConnectionChanged(UeTypeDatabaseConnectionStatus)));
engine.rootContext()->setContextProperty("uePeopleModel",
uePeopleModel);
engine.rootContext()->setContextProperty("ueApplicationStatus",
ueApplicationStatus);
engine.addImageProvider(QLatin1String("uePeopleModel"),
uePeopleModel);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}
Now, inside main.cpp I create two objects from classes:
UeStatus* ueApplicationStatus=new UeStatus(qApp);
UePeopleModel* uePeopleModel=new UePeopleModel(qApp);
and I connect signal from UePeopleModel to slot in ueApplicationStatus:
QObject::connect(uePeopleModel,
SIGNAL(ueSignalDatabaseConnectionChanged(UeTypeDatabaseConnectionStatus)),
ueApplicationStatus,
SLOT(ueSignalDatabaseConnectionChanged(UeTypeDatabaseConnectionStatus)));
The problem is when uePeopleModel is created, the signal is emmited inside constructor:
UePeopleModel::UePeopleModel(QObject* parent)
: QSqlQueryModel(parent),
QQuickImageProvider(QQmlImageProviderBase::Image,
QQmlImageProviderBase::ForceAsynchronousImageLoading)
{
if(!QSqlDatabase::connectionNames().contains(UePosDatabase::UeDatabaseConnectionNames::DATABASE_CONNECTION_NAME_PEOPLE,
Qt::CaseInsensitive))
{
this->ueSetDatabase(QSqlDatabase::addDatabase(UePosDatabase::DATABASE_DRIVER,
UePosDatabase::UeDatabaseConnectionNames::DATABASE_CONNECTION_NAME_PEOPLE));
} // if
this->ueDatabase().setHostName(/*this->uePosSettings()->ueDbHostname()*/UePosDatabase::UeDatabaseConnectionParameters::DATABASE_HOSTNAME);
this->ueDatabase().setDatabaseName(/*this->uePosSettings()->ueDbName()*/UePosDatabase::UeDatabaseConnectionParameters::DATABASE_NAME);
this->ueDatabase().setUserName(/*this->uePosSettings()->ueDbUser()*/UePosDatabase::UeDatabaseConnectionParameters::DATABASE_USERNAME);
this->ueDatabase().setPassword(/*this->uePosSettings()->ueDbPassword()*/UePosDatabase::UeDatabaseConnectionParameters::DATABASE_PASSWORD);
if(this->ueDatabase().open())
{
emit this->ueSignalDatabaseConnectionChanged(CONNECTED);
this->setQuery(UePosDatabase::UeSqlQueries::UeTablePeople::SQL_QUERY_GET_ALL_PEOPLE,
this->ueDatabase());
/*
if(this->lastError().isValid())
qDebug() << this->lastError();
*/
}
else
{
emit this->ueSignalDatabaseConnectionChanged(NOT_CONNECTED);
// qDebug() << this->ueDatabase().lastError();
}
// qDebug() << this->ueDatabase().connectionNames();
} // default constructor
before connect is issued and therfore ueApplicationStatus object does not catch is with slot. Does anyone has idea how to get rid of this situation?
You shouldn't use the constructor to open the connection with the database.
Use the constructor only to instantiate other classes, initialize variables, allocate memory and so on.
In your case, your constructor should be used to initialize the database, but you could create another method with:
if(this->ueDatabase().open())
{
emit this->ueSignalDatabaseConnectionChanged(CONNECTED);
this->setQuery(UePosDatabase::UeSqlQueries::UeTablePeople::SQL_QUERY_GET_ALL_PEOPLE,
this->ueDatabase());
}
else
{
emit this->ueSignalDatabaseConnectionChanged(NOT_CONNECTED);
}
This method would be called after the connect you have in your main.cpp.
I've solved the problem by adding method void ueConnectToDatabase() and moved the content of constructor into it and redefined main.cpp as:
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QQmlApplicationEngine engine;
UeStatus* ueApplicationStatus=new UeStatus(qApp);
UePeopleModel* uePeopleModel=new UePeopleModel(qApp);
QObject::connect(uePeopleModel,
SIGNAL(ueSignalDatabaseConnectionChanged(UeTypeDatabaseConnectionStatus)),
ueApplicationStatus,
SLOT(ueSignalDatabaseConnectionChanged(UeTypeDatabaseConnectionStatus)));
uePeopleModel->ueConnectToDatabase();
engine.rootContext()->setContextProperty("uePeopleModel",
uePeopleModel);
engine.rootContext()->setContextProperty("ueApplicationStatus",
ueApplicationStatus);
engine.addImageProvider(QLatin1String("uePeopleModel"),
uePeopleModel);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
return app.exec();
}
Now the object is created, signal and slot connected and AFTER this the connection to database occurs.
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QDialog dlg;
dlg.exec();
return a.exec();
}
That's all my code, but when I close the window, The process isn't exit, it seems that drop in the loop a.exec().
Generally speaking, calling any exec is a bad idea, other than QCoreApplication::exec() or QDrag::exec(). The presence of exec() and waitForXxx() methods is an enticing trap for the unwary. Those methods are "easy" to use, but that ease comes at a price of hard to track bugs. Don't use them.
You should simply show the dialog:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QMessageBox msg;
msg.setText("Hello");
msg.addButton(QMessageBox::Close);
msg.show();
return a.exec();
}
If you wish to wait for the dialog to be accepted or rejected, you should use the dialog's clickedButton slot. QMessageBox has a long-standing bug that makes the accepted and rejected signals useless :(
// https://github.com/KubaO/stackoverflown/tree/master/questions/messagebox-show-25545652
#include <QtGui>
#if QT_VERSION >= QT_VERSION_CHECK(5,0,0)
#include <QtWidgets>
#endif
#include <functional>
[...]
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QMessageBox msg;
msg.setText("Continue?");
msg.addButton(QMessageBox::Yes);
msg.addButton(QMessageBox::No);
auto onClick = [&msg]() {
auto role = msg.buttonRole(msg.clickedButton());
if (role == QMessageBox::NoRole)
QApplication::quit();
if (role == QMessageBox::YesRole) {
auto label = new QLabel("I'm running");
label->setAttribute(Qt::WA_DeleteOnClose);
label->show();
}
};
#if QT_VERSION >= QT_VERSION_CHECK(5,0,0)
QObject::connect(&msg, &QMessageBox::buttonClicked, onClick);
#else
QObject::connect(&msg, SIGNAL(buttonClicked(QAbstractButton*)),
new FunctorSlot{onClick, &msg}, SLOT(call()));
#endif
msg.show();
return app.exec();
}
#include "main.moc"
For Qt 4, you need the following helper:
// Qt 4 only
struct FunctorSlot : public QObject {
Q_OBJECT
public:
std::function<void()> callable;
template <typename Fun>
FunctorSlot(Fun && fun, QObject * parent = {}) :
QObject{parent}, callable{std::forward<Fun>(fun)} {}
Q_SLOT void call() {
callable();
}
};
Possible solution:
QApplication a(argc, argv);
QDialog dlg;
QTimer::singleShot( &dlg, 0, SLOT(exec()) );
return a.exec();
It will work well. First - application event loop will be started. Then dialog event loop will be executed. After closing of dialog, both dialog and application loop will be finished. Application loop will be terminated automatically (by default), when last window is closed.
But, as noted by #thuga - there are no reason to call exec(). It is enough to call show() method.
The program name is GoToCell in the form I created a label and two push buttons and I wrote the code in main.cpp as follows:
#include <QtGui/QApplication>
#include<QDialog>
#include "ui_GoToCell.h"
#include "GoToCell.h"
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
Ui::GoToCell ui;
QDialog *dialog=new QDialog;
ui.setupUi(dialog);
dialog->show();
return a.exec();
}
While running it I'm getting the following errors:
GoToCell is not a member of ui
What should I do?
I think you have misspelled object name in objectName property in the GoToCell.ui form.
Change it to GoToCell. It will then execute.
I'm trying to create an exit button that correctly closes the GUI I have made in QT. I have tried doing this in the following way:
#include <QtGui/QApplication>
#include "mainwindow.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
int window_width = QApplication::desktop()->width();
int window_height = QApplication::desktop()->height();
MainWindow w;
QPushButton * quit_btn = new QPushButton;
quit_btn->setParent(w.centralWidget());
quit_btn->setGeometry(window_width-50,12,32,32);
QObject::connect(quit_btn,SIGNAL(clicked()),qApp,SLOT(quit()));
w.resize(window_width,window_height);
w.show();
return a.exec();
}
Unfortunately when I push the button, the debugger gives an error:
Invalid address specified to RtlFreeHeap( 003E0000, 0028F950 )
Can anybody point me in the right direction?
Connect the button's clicked() signal to your main window's close() slot. That way things are cleaned up properly.