Confusion between the usage of 'this' and 'parent'

[Swati777999]

@Christian-Ehrlicher
There's only one example in the documentation.

The following code is something I tried on my own.

layoutexample.h

#ifndef LAYOUTEXAMPLE_H
#define LAYOUTEXAMPLE_H

#include <QMainWindow>
#include <QWidget>
#include <QTableWidget>
#include <QGridLayout>
#include <QPushButton>
#include <QMenuBar>

class LayoutExample : public QMainWindow
{
    Q_OBJECT
    QWidget *box;
    QGridLayout *gridLay;
    QPushButton *button1;
    QPushButton *button2;

public:
    LayoutExample(QWidget *parent=nullptr);
    ~LayoutExample();
};
#endif // LAYOUTEXAMPLE_H

layoutexample.cpp

#include "layoutexample.h"
#include <QGridLayout>
#include <QWidget>
#include <QTableWidget>
#include <QPushButton>
#include <QMenuBar>

LayoutExample::LayoutExample(QWidget *parent)
    : QMainWindow(parent)
{
    box = new QWidget();
    button1=new QPushButton("Click Here");
    button2 = new QPushButton("Sign up");
    gridLay = new QGridLayout();
    box->setFixedSize(100,100);

    gridLay->setRowMinimumHeight(5,4);
    gridLay->setHorizontalSpacing(20);
    gridLay->addWidget(box);
    gridLay->addWidget(button1);
    gridLay->addWidget(button2);

    this->setLayout(gridLay);
    //this->parentWidget()->setLayout(gridLay);
}

LayoutExample::~LayoutExample()
{
}

In the above code, in place of 'this' when I write parent , my program crashes. This code does not add widgets to the main window. Only the effects of codes of main.cpp are reflected in the main window.

How can I use w parent Qobject from main.cpp in layout.cpp? [I am so confused about the structure of writing codes in these 3 files and parent-child implementation]

Does parent in layout.cpp refer to w ?
main.cpp

#include "layoutexample.h"

#include <QApplication>
#include <QWidget>
#include <QTableWidget>
#include <QMenuBar>


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

    LayoutExample w;  // w: parent QObject
    QPushButton quit("Quit",&w);      // quit: child QObject
    quit.move(100,100); // x and y coordinates of widgets wrt to the mainwindow
    w.setWindowTitle("Layout Example");
    quit.setBaseSize(100,100);
    quit.setAutoFillBackground("yes");

    QGridLayout grid(&w);    
   //w.setLayout(grid);

    w.show();
    return a.exec();
}

}

Edit: I've changed the name of the project from layout to layoutExample to avoid confusion.

[Rozhin_so]

@jsulm Yes you are right. But for getting the main idea of what parent means , she can read that at first.

[jsulm]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

In the above code, in place of 'this' when I write parent , my program crashes

Do you mean this line:

this->setLayout(gridLay);

?
Of course it will crash if you replace this with parent and parent is a nullptr (which it is by default, unless you pass a pointer to a valid object), C++ basics.
But why would you want to do so?
Child should not change its parent, else it is bad design.

[Christian Ehrlicher]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

In the above code, in place of 'this' when I write parent , my program crashes.

I would be surprised if not.

If you want to know about layouts then read https://doc.qt.io/qt-5/layout.html

And calling the MainWindow 'layout' is somewhat... strange

[Swati777999]

@Rozhin_so
I am acquainted with the concept of inheritance of C++ but in this case, I am more concerned about accessing the parent object in child class via parent or this keyword.

[jsulm]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

accessing the parent object in child class via parent or this keyword

Via "this" you access your object, not parent.
And again: you should NOT manipulate parent in child! It is really bad design.
If you want to know what parent is used for please read https://doc.qt.io/qt-5/objecttrees.html

[Swati777999]

@Christian-Ehrlicher

I named mainwindow as the name of the project;layout while creating it. Later, I found it to be confusing.

Then how can I add widgets to the mainwindow; the object of layout which is w that I've created?

[jsulm]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

Then how can I add widgets to the mainwindow

You do it in mainwindow, not in its children.
In mainwindow you create its children and add them to the layout.

[JKSH]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

In the above code, in place of 'this' when I write parent , my program crashes.

Your code crashes because parent was nullptr. You must not operate on null pointers.

layout w;

The line of code above is the same as layout w(nullptr); Can you tell me why?

[Swati777999]

@JKSH

ohh , I see, you're correct , in the constructor the parent has been set to null pointer but is it not the standard structure of any Qt project?

[jsulm]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

but is it not the standard structure of any Qt project?

parent can be nullptr if the object does not have a parent.
So, you should always check the parent pointer before using it!

if (parent) {
    parent->...
}

[JonB]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

I am more concerned about accessing the parent object in child class via parent

Then how can I add widgets to the mainwindow

You are asking/worrying about the wrong thing! Please re-read the replies from @jsulm above and think about them

But why would you want to do so?

Child should not change its parent, else it is bad design.

And again: you should NOT manipulate parent in child! It is really bad design.

Why are you trying to access the parent in the child at all? 99% of the time children should never need to look at/access parent, and you will be in that 99%! The correct place to manipulate things inside the parent/main window is inside that parent/main window, and not in any child!

[Swati777999]

@JKSH

Now, I am somehow able to connect the dots and understand the connectivity well.

In the constructor implementation [layout.cpp] , *parent points to the null pointer [ done in its declaration] intentionally so that we can independently create objects of parent and child in layout.cpp for simplicity.

[jsulm]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

intentionally so that we can independently create objects of parent and child in layout.cpp for simplicity.

No.
parent pointer can be nullptr simply because not every object has a parent.
I really hope you will spend some minutes to read the documentation I gave you...

[Swati777999]

@jsulm said in Confusion between the usage of 'this' and 'parent':

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

accessing the parent object in child class via parent or this keyword

Via "this" you access your object, not parent.
And again: you should NOT manipulate parent in child! It is really bad design.
If you want to know what parent is used for please read https://doc.qt.io/qt-5/objecttrees.html

Yes, I read this document but when it comes to implementing concepts that I read, I make a lot of blunders(please bear with me ) and red underlines are enough to scare off a beginner like me.

The sequence of my non-instantaneous reply in this forum is adding woes to the confusion. I can't post a reply within 10 minutes of my last reply and by the time, I am eligible to post my next reply, I had already received responses by other members. It's lil frustrating for not being able to reply immediately.

[jsulm]

@Swati777999 I upvoted your last post. Now you should be able to answer faster :-)

[Swati777999]

@jsulm
So kind of you! :)

[JKSH]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

is it not the standard structure of any Qt project?

A widget can have 0 or 1 parent, and it can have many children. This is what https://doc.qt.io/qt-5/objecttrees.html says.

In your code, w is the parent of quit. In other words, quit is a child of w.

Tell me: Who is the parent of w?

[JKSH]

@Swati777999 said in Confusion between the usage of 'this' and 'parent':

*parent points to the null pointer [ done in its declaration] intentionally

This is wrong. The declaration contains the default value. Please see https://www.programiz.com/cpp-programming/default-argument

Compare the following constructor declarations:

// layout.h
layout(QWidget *parent = nullptr)

// qpushbutton.h
QPushButton(const QString &text, QWidget *parent = nullptr)

@JonB said in Confusion between the usage of 'this' and 'parent':

Why are you trying to access the parent in the child at all?

That's not the intention, I believe. OP just has a big misunderstanding that "w is the parent object in this code, therefore the pointer parent should point to w.

@Swati777999, note that in your code parent->setLayout(gridLay); is the same as this->parentWidget()->setLayout(gridLay);

[Swati777999]

Tell me: Who is the parent of w?

I think w is the primary parent of this parent-child-ladder. It's from where this hierarchy of parent-child originates, I think.