Confusion between the usage of 'this' and 'parent'
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@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.@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 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
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?
“ In order to be irreplaceable, one must always be different” – Coco Chanel
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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?
@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. -
@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_Hlayoutexample.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.
@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
parentwasnullptr. 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?Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches
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@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
parentwasnullptr. 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?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?
“ In order to be irreplaceable, one must always be different” – Coco Chanel
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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?
@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->... } -
@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.@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
parentin the child at all? 99% of the time children should never need to look at/accessparent, 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 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
parentwasnullptr. 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?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.
“ In order to be irreplaceable, one must always be different” – Coco Chanel
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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.
@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 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@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.htmlYes, 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.
“ In order to be irreplaceable, one must always be different” – Coco Chanel
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@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.htmlYes, 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.
@Swati777999 I upvoted your last post. Now you should be able to answer faster :-)
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@Swati777999 I upvoted your last post. Now you should be able to answer faster :-)
@jsulm
So kind of you! :) -
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?
@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,
wis the parent ofquit. In other words,quitis a child ofw.Tell me: Who is the parent of
w?Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches
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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.
@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
parentin the child at all?That's not the intention, I believe. OP just has a big misunderstanding that "
wis the parent object in this code, therefore the pointerparentshould point tow.@Swati777999, note that in your code
parent->setLayout(gridLay);is the same asthis->parentWidget()->setLayout(gridLay);Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches
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@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,
wis the parent ofquit. In other words,quitis a child ofw.Tell me: Who is the parent of
w?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.
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@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
parentin the child at all?That's not the intention, I believe. OP just has a big misunderstanding that "
wis the parent object in this code, therefore the pointerparentshould point tow.@Swati777999, note that in your code
parent->setLayout(gridLay);is the same asthis->parentWidget()->setLayout(gridLay);On running the above code! Layout_Example.PNG, I get the following result.
This is the result that I'm getting because of the code that I wrote in main.cpp .
The codes are written in layoutExample.cpp , is not making any change in the main window, even if I write like below..
LayoutExample::LayoutExample(QWidget *parent) : QMainWindow(parent) { box = new QWidget(parent); button1=new QPushButton("Click Here",parent); button2 = new QPushButton("Sign up",parent); gridLay = new QGridLayout(parent); box->setFixedSize(100,100); ................. ................. ...............All I want to know, is how can I add widgets in this mainWindow where you can see a pushbutton?
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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.
@Swati777999 said in Confusion between the usage of 'this' and 'parent':
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.
Yes, that's correct:
wis at the top of this parent-child-ladder.So the next question is: Which part of your code in main.cpp set
wasquit's parent? Or, asked in a different way: How does main.cpp know thatquitis not the the "primary parent"?All I want to know, is how can I add widgets in this mainWindow where you can see a pushbutton?
To answer this question, there are actually 2 different things you need to know:
- How to set widget parents
- How to use layouts
These are different different topics.
- #1 is handled by my questions above. Try to answer those questions.
- #2 can be studied from the Basic Layouts Example: https://doc.qt.io/qt-5/qtwidgets-layouts-basiclayouts-example.html Run the example and see.
Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches
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@Swati777999 said in Confusion between the usage of 'this' and 'parent':
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.
Yes, that's correct:
wis at the top of this parent-child-ladder.So the next question is: Which part of your code in main.cpp set
wasquit's parent? Or, asked in a different way: How does main.cpp know thatquitis not the the "primary parent"?All I want to know, is how can I add widgets in this mainWindow where you can see a pushbutton?
To answer this question, there are actually 2 different things you need to know:
- How to set widget parents
- How to use layouts
These are different different topics.
- #1 is handled by my questions above. Try to answer those questions.
- #2 can be studied from the Basic Layouts Example: https://doc.qt.io/qt-5/qtwidgets-layouts-basiclayouts-example.html Run the example and see.
@JKSH said in Confusion between the usage of 'this' and 'parent':
So the next question is: Which part of your code in main.cpp set
wasquit's parent? Or, asked in a different way: How does main.cpp know thatquitis not the the "primary parent"?Doesn't LayoutExample w in main.cpp declare w as the parent object?
QPushButton quit("Quit",&w);This sentence sets the parent of quit to be w.
“ In order to be irreplaceable, one must always be different” – Coco Chanel
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@JKSH said in Confusion between the usage of 'this' and 'parent':
So the next question is: Which part of your code in main.cpp set
wasquit's parent? Or, asked in a different way: How does main.cpp know thatquitis not the the "primary parent"?Doesn't LayoutExample w in main.cpp declare w as the parent object?
QPushButton quit("Quit",&w);This sentence sets the parent of quit to be w.
@Swati777999 said in Confusion between the usage of 'this' and 'parent':
QPushButton quit("Quit",&w);This sentence sets the parent of quit to be w.
Correct.
This line constructs a QPushButton called
quit, and setswas the parent of the QPushButton object. This happens because the address ofwis passed into the QPushButton constructor.Doesn't LayoutExample w in main.cpp declare w as the parent object?
Sort of... I understand what you mean and you are correct, but please be careful with how you use the word "parent".
This line constructs a LayoutExample object called
w, and sets nothing as the parent of the LayoutExample object. This happens becausenullptris passed into the LayoutExample constructor.button1=new QPushButton("Click Here",parent);Please fill in the blanks (see above for examples):
This line constructs a QPushButton called
button1, and sets ________ as the parent of the QPushButton object. This happens because ________ is passed into the QPushButton constructor.Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches
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@Swati777999 said in Confusion between the usage of 'this' and 'parent':
QPushButton quit("Quit",&w);This sentence sets the parent of quit to be w.
Correct.
This line constructs a QPushButton called
quit, and setswas the parent of the QPushButton object. This happens because the address ofwis passed into the QPushButton constructor.Doesn't LayoutExample w in main.cpp declare w as the parent object?
Sort of... I understand what you mean and you are correct, but please be careful with how you use the word "parent".
This line constructs a LayoutExample object called
w, and sets nothing as the parent of the LayoutExample object. This happens becausenullptris passed into the LayoutExample constructor.button1=new QPushButton("Click Here",parent);Please fill in the blanks (see above for examples):
This line constructs a QPushButton called
button1, and sets ________ as the parent of the QPushButton object. This happens because ________ is passed into the QPushButton constructor.@JKSH said in Confusion between the usage of 'this' and 'parent':
@Swati777999 said in Confusion between the usage of 'this' and 'parent':
QPushButton quit("Quit",&w);This line constructs a LayoutExample object called
w, and sets nothing as the parent of the LayoutExample object. This happens becausenullptris passed into the LayoutExample constructor.button1=new QPushButton("Click Here",parent);Please fill in the blanks (see above for examples):
This line constructs a QPushButton called
button1, and sets ________ as the parent of the QPushButton object. This happens because ________ is passed into the QPushButton constructor.Answer to above fill in the blanks: parent, parent (which is set to null pointer)
