[SOLVED]Best coding practice
-
I am reasonably new to qt and c++ but am quite an experienced programmer.
My ( sort of ) problem is this I have been used to casting widgets ( gtk ) from the base widget to the specific via this sort of code:
@
combobox=(GtkCombBox*)widget
@But I have come across this sort of thing in the various QT docs:
@
QPushButton *button = parentWidget->findChild<QPushButton *>("button1");
@What is the better/best way to cast widgets in QT?
-
To perform that type off casts in C++ you usually use "dynamic_cast":http://en.cppreference.com/w/cpp/language/dynamic_cast , but with QObject subclasses (all widgets are derived indirectly from QObject) the recommended way is "qobject_cast ":http://qt-project.org/doc/qt-4.8/qobject.html#qobject_cast
LE: findChild is a way to get a child by name (if you have the parent), is not a casting method by itself, see the documentation "here":http://qt-project.org/doc/qt-4.8/qobject.html#findChild
-
[quote author="Zlatomir" date="1420548624"]...
LE: findChild is a way to get a child by name (if you have the parent), is not a casting method by itself, see the documentation "here":http://qt-project.org/doc/qt-4.8/qobject.html#findChild[/quote]Thanks for the links, yes I know what this function does it was just the first example I could easily find using "<QPushButton *>", sorry for any confusion.
-
Ok bit more info, I have an array of Qwidgets for the prefs some of which are boolean some are text etc so it is convenient to use a generic type and type cast the widget to get the value, all 3 ways of casting ( for instance a check box ), seem to work fine and consistently but give a few minor diffs in the size of the executable by just a few bytes, not worth worrying about.
So what is the best way:
@((QCheckBox*)prefsWidgets[widgnum])->setChecked(onoff);@
@dynamic_cast<QCheckBox *>(prefsWidgets[widgnum])->setChecked(onoff);@
@qobject_cast<QCheckBox *>(prefsWidgets[widgnum])->setChecked(onoff);@The first is slightly less typing!
-
Hi,
You should prefer the third option but with a pointer check:
@
QCheckBox checkBox = qobject_cast<QCheckBox>(prefsWidgets[widgnum]);
if (checkBox) checkBox->setChecked(onoff);
@Of course you can skip the null-ptr test if you are sure of the type.
First one is good, old but too powerful C-style cast which most of the C++ programmers would advise to avoid.
Dynamic cast is costly. That's the reason there is a qobject_cast.
-
In this instance I will probably go with the qobject_cast as I don't need to do any run time checking that I gather dynamic_cast does.
Thanks for the help guys.
-
Hi,
Indeed, don't do C style cast, there's no check at all with and you're in for a lot of problem. Also object_cast works across library boundaries.
Anyway, since it's only a matter of calling setChecked, you can even reduce your code by using the property system.
Just call:
@prefsWidgets[widgnum]->setProperty("checked", onoff);@
and you should be good to go
-
[quote author="SGaist" date="1420585048"]...Just call:
@prefsWidgets[widgnum]->setProperty("checked", onoff);@
...[/quote]
That's a much more elegant solution to the above bit of code, I didn't know I could access the property data directly, learned something new. Thanks!
