Strange multiple keys pressed problem - 2D Game Qt !

[SpicyPaper]

Hi all,

I am currently developing a small 2D game with Qt C++.
I was just trying some little things before starting dev. the game.

The problem is that I can't catch more than 3 key pressed at the same time.
I need this because I want to play with 2 - 4 players on the same PC and on the same keyboard.
If I don't find a solution my players won't be able to play at the same time, and that's a problem because it's a real time game.

I am using the basic keyPressEvent to catch key pressed.
Then I store the keys in a QMap(int, bool), to know witch key is pressed and not pressed.
I am also using a timer to treat the QMap values and do some actions when one or more keys are pressed.
The problem is when 3 key are pressed and held down and I press a 4th key.
The 4th key is not catch, actually the program don't call any more the keyPressEvent.

My code is following :

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QtWidgets>

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);
    ~Widget();

private:
    QTimer *timer;
    QMap<int, bool> keys;
    void keyPressEvent(QKeyEvent *event);
    void keyReleaseEvent(QKeyEvent *e);

private slots:
    void timerOutEvent();
};

#endif // WIDGET_H

Widget.cpp

#include "widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    timer = new QTimer();
    timer->setInterval(1000/60);
    timer->start();

    connect(timer, &QTimer::timeout, this, &Widget::timerOutEvent);
}

Widget::~Widget()
{

}

void Widget::timerOutEvent()
{
    QString txt = "";

    if(keys[Qt::Key_Up])
    {
        txt += "u";
    }
    if(keys[Qt::Key_Down])
    {
        txt += "d";
    }
    if(keys[Qt::Key_Left])
    {
        txt += "l";
    }
    if(keys[Qt::Key_Right])
    {
        txt += "r";
    }

    qDebug() << txt;
}

void Widget::keyReleaseEvent(QKeyEvent *event)
{
    keys[event->key()] = false;
    QWidget::keyReleaseEvent(event);
}

void Widget::keyPressEvent(QKeyEvent *event)
{
    keys[event->key()] = true;
    QWidget::keyPressEvent(event);
}

main.cpp

#include "widget.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.show();

    return a.exec();
}

Thank you in advance for your help !
Alex

[SpicyPaper]

I found the answer, and I post it on stackoverflow :
https://stackoverflow.com/questions/46949341/qt-multiple-keys-pressed-at-once/

[Bondrusiek]

I found also a great solution. You can add as class member:
QSet<int> pressedKeys;
And you can catch the key events in an event filter:

bool MyWidget::eventFilter(QObject * obj, QEvent * event)
{

    if(event->type()==QEvent::KeyPress) {

        pressedKeys += ((QKeyEvent*)event)->key();

        if( pressedKeys.contains(Qt::Key_Up) && pressedKeys.contains(Qt::Key_Left) )
        {
            // up and left is pressed
        }

    }
    else if(event->type()==QEvent::KeyRelease)
    {

        pressedKeys -= ((QKeyEvent*)event)->key();
    }


    return false;
}

and install the event filter in the constructor:
this->installEventFilter(this);
Reference: https://stackoverflow.com/questions/23816380/using-multiple-keys-in-qt-c
Project where I use it:
https://youtu.be/YuSc8oCXHxk