Access and inheritance
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@mrjj
Hi,
So could we have QMainWindow as a subclass of MainWindow without calling its constructor in the constructor of MainWindow?By the way, the book says MainWindow as a subclass of QMainWinodw!
It seems that both are subclasses of each other! :)@tomy
hi
well its MainWindow as a subclass of QMainWindow
The names are so close its sometimes confusing.
The truth is, if a base class do not do anything in its constructor, nothing bad will happen not calling it.
Which seems why they didnt bother here for MainWindow -
@tomy
hi
well its MainWindow as a subclass of QMainWindow
The names are so close its sometimes confusing.
The truth is, if a base class do not do anything in its constructor, nothing bad will happen not calling it.
Which seems why they didnt bother here for MainWindow@mrjj
Hi, thanks, I got it.One other question. In page 70 of that book, about context menu, the following is written:
"A more sophisticated way of providing context menus is to reimplement the QWidget::contextMenuEvent() function,
create a QMenu widget, populate it with the desired actions, and call exec() on it."If I want that sophisticated context menu, I need to reimplement the QWidget::contextMenuEvent() function. I found that here, but how to know its code to be able to change that and use it for the application please?
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@mrjj
Hi, thanks, I got it.One other question. In page 70 of that book, about context menu, the following is written:
"A more sophisticated way of providing context menus is to reimplement the QWidget::contextMenuEvent() function,
create a QMenu widget, populate it with the desired actions, and call exec() on it."If I want that sophisticated context menu, I need to reimplement the QWidget::contextMenuEvent() function. I found that here, but how to know its code to be able to change that and use it for the application please?
Hi
You would create a new subclass.
However, it seldom useful for just a context menu and a plain QWidget.
So where do you want to use the context menu ?anyway for pure learning.
class MyWidget : public QWidget { Q_OBJECT public: explicit MyWidget(QWidget*parent = nullptr) : QWidget(parent) { } protected: // here we then override the defualt one we get from base class to have our own. virtual void contextMenuEvent(QContextMenuEvent *event) override { .....make menu.... } };To use it you would either
create from code
MyWidget *wid=new MyWidget(this)
and insert into layout / form.or use Creators promotion feature to allow to use in Designer.
https://doc.qt.io/qt-5/designer-using-custom-widgets.htmlNote that we here inherit from QWidget
However, it could have been from any widget. Like QPushButton etc. -
Hi
You would create a new subclass.
However, it seldom useful for just a context menu and a plain QWidget.
So where do you want to use the context menu ?anyway for pure learning.
class MyWidget : public QWidget { Q_OBJECT public: explicit MyWidget(QWidget*parent = nullptr) : QWidget(parent) { } protected: // here we then override the defualt one we get from base class to have our own. virtual void contextMenuEvent(QContextMenuEvent *event) override { .....make menu.... } };To use it you would either
create from code
MyWidget *wid=new MyWidget(this)
and insert into layout / form.or use Creators promotion feature to allow to use in Designer.
https://doc.qt.io/qt-5/designer-using-custom-widgets.htmlNote that we here inherit from QWidget
However, it could have been from any widget. Like QPushButton etc.@mrjj
Hi, thanks.I forgot to say that I declared it inside the protected modifier area of MainWindow this way:
... protected: void closeEvent(QCloseEvent*); // Previousely added void contextMenuEvent(QContextMenuEvent*); // I, too, added this one ...and it's pure code.
I want to use it into the spreadsheet program we talked about, to manipulate the code to have a better context menu for that program.
One question as well, what will we achieve by re-implementing that virtual protected method please? -
@mrjj
Hi, thanks.I forgot to say that I declared it inside the protected modifier area of MainWindow this way:
... protected: void closeEvent(QCloseEvent*); // Previousely added void contextMenuEvent(QContextMenuEvent*); // I, too, added this one ...and it's pure code.
I want to use it into the spreadsheet program we talked about, to manipulate the code to have a better context menu for that program.
One question as well, what will we achieve by re-implementing that virtual protected method please?Hi
If you add contextMenuEvent to MainWindow, then you need
to right click the actual MainWindow, to trigger it.
It wont be triggered by right clicking on the spreadSheet widget.
So not sure having them in MainWindow is what you want ?- One question as well, what will we achieve by re-implementing that virtual protected method please?
Well it allows us to override a default contextMenuEvent so for any widget that can actually show a menu already( like QlineEdit)
it allows to to alter what should be shown ( a new menu )
Or it allows us to add a menu to a QWidget that dont already have such menu.BUT
Often the signal is used instead (of the Event ) as that requires no subclass.
https://doc.qt.io/qt-5/qwidget.html#customContextMenuRequested
That you can just connect to and do get the same effect without subclassing.However, WHAT widget do you want to add contextMenu to ?
The spreadsheet? -
Hi
If you add contextMenuEvent to MainWindow, then you need
to right click the actual MainWindow, to trigger it.
It wont be triggered by right clicking on the spreadSheet widget.
So not sure having them in MainWindow is what you want ?- One question as well, what will we achieve by re-implementing that virtual protected method please?
Well it allows us to override a default contextMenuEvent so for any widget that can actually show a menu already( like QlineEdit)
it allows to to alter what should be shown ( a new menu )
Or it allows us to add a menu to a QWidget that dont already have such menu.BUT
Often the signal is used instead (of the Event ) as that requires no subclass.
https://doc.qt.io/qt-5/qwidget.html#customContextMenuRequested
That you can just connect to and do get the same effect without subclassing.However, WHAT widget do you want to add contextMenu to ?
The spreadsheet? -
Hi I would use the SIGNAL then.
I assume it has not right click menu already?
To use it, you enable it with
ui->spreadsheet->setContextMenuPolicy(Qt::CustomContextMenu);
(ui->spreadsheet might not be actual name ;)then connect signal to a slot where you build and exec() the menu.
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Hi I would use the SIGNAL then.
I assume it has not right click menu already?
To use it, you enable it with
ui->spreadsheet->setContextMenuPolicy(Qt::CustomContextMenu);
(ui->spreadsheet might not be actual name ;)then connect signal to a slot where you build and exec() the menu.
I assume it has not right click menu already?
It has. The author said: "A more sophisticated way of providing context menus is to reimplement the QWidget::contextMenuEvent() function,
create a QMenu widget, populate it with the desired actions, and call exec() on it.", so I was motivated to re-implement that method to probably have a nicer or more advanced context menu for the cells!
By the way, I like to use pure code only (no design mode). :) -
I assume it has not right click menu already?
It has. The author said: "A more sophisticated way of providing context menus is to reimplement the QWidget::contextMenuEvent() function,
create a QMenu widget, populate it with the desired actions, and call exec() on it.", so I was motivated to re-implement that method to probably have a nicer or more advanced context menu for the cells!
By the way, I like to use pure code only (no design mode). :) -
@tomy
Well im not sure what he meant by that since its just a QMenu in any case.So what does it have now ?
Im not sure why using event would be more sophisticated. Its just other way
of having a context menu. -
@tomy
Hi
Is that build from code or a default one ?
I mean is there code to build it and show it ?@mrjj Hi,
Yes. The book also shows the code for that.
I know I can add more options to that, for instance, to have five buttons in the context menu rather than that three ones. But by the sentence the author said, I though there should be another way to have a more advanced context menu by re-implementing the virtual protected method when mentioned earlier.By the way, don't you have that book?
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@mrjj Hi,
Yes. The book also shows the code for that.
I know I can add more options to that, for instance, to have five buttons in the context menu rather than that three ones. But by the sentence the author said, I though there should be another way to have a more advanced context menu by re-implementing the virtual protected method when mentioned earlier.By the way, don't you have that book?
@tomy
Hi
Well the menu already have both shortcut and icon so not sure what else could be added.
There is no "more advanced" menu possible by overwriting contextMenuEvent as
you can just 100% the same just altering
void MainWindow::createActions()
to show what you want.yes, i have it as PDF. did read most of it way back.
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@tomy
Hi
Well the menu already have both shortcut and icon so not sure what else could be added.
There is no "more advanced" menu possible by overwriting contextMenuEvent as
you can just 100% the same just altering
void MainWindow::createActions()
to show what you want.yes, i have it as PDF. did read most of it way back.
Thank you very much. I was too much stuck in a QML program's problem, sorry for the delay.
If you run the program, even by the latest version of Qt Creator and on a sophisticated and fancy operating system like Windows 10, you see the program still looks old, as though we are in 2000 using it.
Is it because of the code? If so, what parts can be updated, please? -
Thank you very much. I was too much stuck in a QML program's problem, sorry for the delay.
If you run the program, even by the latest version of Qt Creator and on a sophisticated and fancy operating system like Windows 10, you see the program still looks old, as though we are in 2000 using it.
Is it because of the code? If so, what parts can be updated, please?@tomy
Hi
Nope its just how the QWidgets look.
If you change the icons to flat style,
it will look more "modern"You could use something like
https://github.com/laserpants/qt-material-widgets
if you want to go all in for "modern" look. -
Thank you very much. I was too much stuck in a QML program's problem, sorry for the delay.
If you run the program, even by the latest version of Qt Creator and on a sophisticated and fancy operating system like Windows 10, you see the program still looks old, as though we are in 2000 using it.
Is it because of the code? If so, what parts can be updated, please?@tomy Do you use Qt4 or Qt5?
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@tomy
hi
well its MainWindow as a subclass of QMainWindow
The names are so close its sometimes confusing.
The truth is, if a base class do not do anything in its constructor, nothing bad will happen not calling it.
Which seems why they didnt bother here for MainWindow@mrjj said in Access and inheritance:
The truth is, if a base class do not do anything in its constructor, nothing bad will happen not calling it.
Just for completeness:
If the base class has a default constructor, and it's not called explicitly in the derived class' constructor the compiler is going to generate code to call it implicitly. Omitting it in the initializer list means nothing here, asQMainWindow()is going to be called either way. If the base class does not have a default constructor, and there's no call to the parent's constructor then this is going to generate a compile error, as the compiler has no idea what to do.Read and abide by the Qt Code of Conduct
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@mrjj said in Access and inheritance:
The truth is, if a base class do not do anything in its constructor, nothing bad will happen not calling it.
Just for completeness:
If the base class has a default constructor, and it's not called explicitly in the derived class' constructor the compiler is going to generate code to call it implicitly. Omitting it in the initializer list means nothing here, asQMainWindow()is going to be called either way. If the base class does not have a default constructor, and there's no call to the parent's constructor then this is going to generate a compile error, as the compiler has no idea what to do.@kshegunov
Hi,If the base class has a default constructor, and it's not called explicitly in the derived class' constructor the compiler is going to generate code to call it implicitly. Omitting it in the initializer list means nothing here, as
QMainWindow()is going to be called either way.Is it true for the Qt Creator compiler too? (I think so)
If the base class does not have a default constructor, and there's no call to the parent's constructor then this is going to generate a compile error, as the compiler has no idea what to do.
Can we conclude this way that, when the base class doesn't have a constructor, in either way, whether the subclass calls the parent's constructor (!) or it doesn't, we will get an error?
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@kshegunov
Hi,If the base class has a default constructor, and it's not called explicitly in the derived class' constructor the compiler is going to generate code to call it implicitly. Omitting it in the initializer list means nothing here, as
QMainWindow()is going to be called either way.Is it true for the Qt Creator compiler too? (I think so)
If the base class does not have a default constructor, and there's no call to the parent's constructor then this is going to generate a compile error, as the compiler has no idea what to do.
Can we conclude this way that, when the base class doesn't have a constructor, in either way, whether the subclass calls the parent's constructor (!) or it doesn't, we will get an error?
@tomy said in Access and inheritance:
Is it true for the Qt Creator compiler too? (I think so)
Qt Creator is not a compiler, it uses some compiler (whatever you've configured), but yes, it's true for all compiler, as this is standard (and required) behaviour for C++.
Can we conclude this way that, when the base class doesn't have a constructor, in either way, whether the subclass calls the parent's constructor (!) or it doesn't, we will get an error?
No. Not defining a class constructor means the compiler generates a default one for you (unless specifically told otherwise). If you haven't defined a constructor in the base class, then the derived class is going to implicitly call the automatically generated one.
Read and abide by the Qt Code of Conduct
