1

I have a question with some of the code for a tutorial I'm following for Qt Creator. My first question is how is MyRect() (in file "main.cpp" on line MyRect * rect = new MyRect();) was created? I thought it was only a class and not a function. Could someone explain that?

I'm also having trouble understanding how (under "myrect.cpp") the keyPressEvent function is called whenever I press an arrow key. How does the function know to respond without me calling it? If it has been called and I've just missed it, could someone specify where, thanks.

main.cpp:

#include <QApplication>
#include <QGraphicsScene>
#include "myrect.h"
#include <QGraphicsView>

using namespace std;

int main(int argc, char *argv[]){

    QApplication a(argc, argv);

    // create a scene
    QGraphicsScene * scene = new QGraphicsScene();

    // create an item to put into the scene
    MyRect * rect = new MyRect();
    rect->setRect(0,0,100,100);

    // add the item to the scene
    scene->addItem(rect);

    //Make item focusable
    rect->setFlag(QGraphicsItem::ItemIsFocusable);
    rect->setFocus();
    // add a view to visualize the scene
    QGraphicsView * view = new QGraphicsView(scene);

    view->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    view->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    view->show();
    return a.exec();
}

myrect.cpp:

#include "MyRect.h"
#include <QGraphicsScene>
#include <QKeyEvent>
#include "bullet.h"
#include <QDebug>

void MyRect::keyPressEvent(QKeyEvent *event){
    if(event->key() == Qt::Key_Left){
        setPos(x()-10,y());
    }
    else if(event->key() == Qt::Key_Right){
        setPos(x()+10,y());
    }
    else if(event->key() == Qt::Key_Up){
        setPos(x(),y()-10);
    }
    else if(event->key() == Qt::Key_Down){
        setPos(x(),y()+10);
    }
    else if(event->key() == Qt::Key_Space){
        //creates a bullet
        Bullet* bullet = new Bullet();
        bullet->setPos(x(),y());
        scene()->addItem(bullet);
    }
}

myrect.h:

#ifndef MYRECT_H
#define MYRECT_H
#include <QGraphicsRectItem>

    class MyRect: public QGraphicsRectItem{
    public:
        void keyPressEvent(QKeyEvent* event);
    };

#endif // MYRECT_H

bullet.cpp:

#include "Bullet.h"
#include <QTimer>

Bullet::Bullet(){
    //draw rect
    setRect(0,0,10,50);

    //connect
    QTimer * timer = new QTimer();
    connect(timer,SIGNAL(timeout()),this,SLOT(move()));
    timer->start(50);
}

void Bullet::move(){
    //move bullet up
    setPos(x(),y()-10);
}

bullet.h:

#ifndef BULLET_H
#define BULLET_H
#include <QGraphicsRectItem>
#include <QObject>

class Bullet: public QObject, public QGraphicsRectItem{
    Q_OBJECT
public:
  Bullet();
public slots:
    void move();
};

#endif // BULLET_H
cbuchart
  • 10,847
  • 9
  • 53
  • 93
Johnathan Maxwell
  • 35
  • 3
  • 1
    [A constructor is a special member function with the name of the class that initializes an instance of the class.](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) – user4581301 May 11 '17 at 07:04
  • The keypressEvent function likely has been installed as a handler for a key press event by the QT IDE's GUI builder tool. Go digging in the property panels for the UI components and you should be abe to find it. Whenever the event occurs, code provided by the GUI framework that handles the user interface calls the function for you. – user4581301 May 11 '17 at 07:10
  • @user4581301 thank you so much for the answers. When "rect" equals "MyRect()" what happens when MyRect() is called and what does the initializer do? – Johnathan Maxwell May 11 '17 at 07:30
  • 1
    few more details: application's event loop (launched by a.exec() ) cathes key press event and pass it to correct QWidget (QGraphicsView here). it in its own virtual keyEventHandler passes it to QGraphicsScene to which view is connected, QgraphicsScene pass it to correct QGraphicsItem to handle ( object of MyRect class here ) and so Myrect::keyPressEvent is called. – Andrew Kashpur May 11 '17 at 07:59
  • These questions have little to do with Qt. I think you'd need to start with a C++ book, like Strostrup's the C++ Programming Language, 4th ed. It would clear everything up in just a few chapters. – Kuba hasn't forgotten Monica May 11 '17 at 12:23

0 Answers0