Close main window with ESC Key

[h3xl3y]

Hi
i am very new to Qt and programming, sorry for trouble but i searched for 2 days for a answer and found a few things but it didn't work

running Qt Creator 2.8., compiler MinGW 4.8

I am trying to make the Program Close when press on ESC_Key

using this code in main window.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtGui>
#include <QObject>
#include <QEvent>
#include <QKeyEvent>


MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
installEventFilter(this);
}

MainWindow::~MainWindow()
{
delete ui;
}

bool MainWindow::eventFilter(QObject *target, QEvent *event)
{
if (event->type() == QEvent::KeyPress)
{
QKeyEvent *keyEvent = static_cast<QKeyEvent*>(event);

if (keyEvent->key() == Qt::Key_Escape)
{
this->close();
return QMainWindow::eventFilter(target,event);
}
}
return QMainWindow::eventFilter(target,event);
}


it gaves an error: no 'bool MainWindow::eventFilter(QObject*,QEvent*) member function declared in class 'MainWindow'

Thanks

[Infinity]

Show you header file. Maybe the declaration of eventFilter is missing there.

[h3xl3y]

here is the Header

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QKeyEvent>
#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private:
Ui::MainWindow *ui;

protected:
void keyPressEvent(QKeyEvent *event);
};

#endif // MAINWINDOW_H

[Infinity]

Add the declaration of the method

bool MainWindow::eventFilter(QObject *target, QEvent *event);
to the public section of the header file.

[h3xl3y]

it gives me this error:

extra qualification 'MainWindow::' on member 'eventFilter' [-fpermissive]

[Infinity]

You can't simply copy & paste the code I've posted. Of course you have to remove the "MainWindow::"-prefix, because you use declare the method inside "the class block".

I recommend you, to lean the basics of C++ before using complex libraries like Qt.

[h3xl3y]

if it should be like this then it doesn't work

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QKeyEvent>
#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
bool eventFilter(QObject *parent, QEvent *event);
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private:
Ui::MainWindow *ui;

protected:
void keyPressEvent(QKeyEvent *event);
};

#endif // MAINWINDOW_H

error: undefined reference to 'MainWindow::keyPressEvent(QKeyEvent*)'
file not found: moc_mainwindow.cpp

and you know that i'm trying to learn as i go but there is much info to absorb and not very clear when search for errors

[Infinity]

You also declare the protected method keyPressEvent, but it seems like the definition is missing. That's why you get the error. (There's only a definition for eventFilter, which had no declaration at the first time).


I am trying to make the Program Close when press on ESC_Key
I think, overwriting keyPressEvent (and not eventFilter) is the right way to solve your problem (so don't use the eventFilter method for that).

The following code should work.


class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private:
Ui::MainWindow *ui;

protected:
void keyPressEvent(QKeyEvent *event); // declaration
};

void MainWindow::keyPressEvent(QKeyEvent *event) // definition
{
switch(event->key())
{
case Qt::Key_Escape:
close();
break;
default:
QMainWindow::keyPressEvent(event);
}
}

[Gokulnathvc]

Use this
bool MainWindow::keypress(QKeyEvent *keyevent)
{
if (keyevent->key()==Qt::Key_W) // Your key here.
{
qApp::Quit();
}
}

[h3xl3y]

Sorry could not answer sooner

@Infinity
with the last code it gives this error: undeterminated #ifndef MAINWINDOW_H

@Gokulnathvc
with that code it still gives bool error: no 'bool MainWindow::keypress(QKeyEvent*) member function declared in class 'MainWindow'
and if i put this in header

bool keypress(QKeyEvent *keyevent)
i get this error: expected ';' at end of member declaration
if add ; then expected ';' before '::' token

[wysota]

None of the code posted so far in this thread has any chance to work because the main window object doesn't accept focus so it will receive no key events.

[h3xl3y]

Ok... Then, if its possible, can you give me a hand ?

[wysota]

You could install the event filter on the applicaytion object to intercept all events passing through the program.

[Gokulnathvc]

Just declare
void keyPressEvent(QKeyEvent *); in header file. And use it as
void MainWindow::keyPressEvent(QKeyEvent *event)
{
if(event->key() == Qt::Key_Escape)
{
myLabel->setText("You pressed ESC");
}
}

[h3xl3y]

Just declare
void keyPressEvent(QKeyEvent *); in header file. And use it as
void MainWindow::keyPressEvent(QKeyEvent *event)
{
if(event->key() == Qt::Key_Escape)
{
myLabel->setText("You pressed ESC");
}
}

Ok THanks. That help me a lot
i finally used this code

void MainWindow::keyPressEvent(QKeyEvent *event)
{
if(event->key() == Qt::Key_Escape)
qApp->quit();
}

[Infinity]

None of the code posted so far in this thread has any chance to work because the main window object doesn't accept focus so it will receive no key events.
I've tested my example code and it worked (using Qt 5.1.0 with QGtkTheme on Linux Mint).


with the last code it gives this error: undeterminated #ifndef MAINWINDOW_H
I don't know how you used the code, but if you use it properly there shouldn't occur errors.

[wysota]

I've tested my example code and it worked (using Qt 5.1.0 with QGtkTheme on Linux Mint).

Only if the widget currently with focus does not handle the particular key event.


#include <QtWidgets>

class MainWindow : public QMainWindow {
public:
MainWindow(QWidget *parent = 0) : QMainWindow(parent) {}
protected:
void keyPressEvent(QKeyEvent *ke) {
qDebug() << ke->key();
}
};

class Widget : public QWidget {
public:
Widget(QWidget *parent = 0) : QWidget(parent) {
setFocusPolicy(Qt::StrongFocus);
}
protected:
void keyPressEvent(QKeyEvent *ke) {}
};

int main(int argc, char **argv) {
QApplication app(argc, argv);
MainWindow mw;
Widget *w = new Widget;
mw.setCentralWidget(w);
mw.show();
return app.exec();
}

[tsuibin]

usethis function
void keyPressEvent(QKeyEvent *e)